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Are there dog breeds that are so far apart genetically that they can't produce viable offspring?

Are there dog breeds that are so far apart genetically that they can't produce viable offspring?


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Obviously, a very large dog would have difficulties mating with a very small dog and vice versa.

But putting that problem aside (using, say, insemination), considering the large variation of dog breeds, are there any two breeds that are so different genetically that they can't produce viable offspring together?


I actually found some sort of reference for this. Apparently in the case of a beagle and irish setter pairing, they had a lot of difficulty in producing pups, but as breeds have a lot of genetic quirks, this might be due to a genetic accident; this is probably a case of mutual infertility rather than speciation. There are probably mutations in their genomes which are causing non viability in the pups the way that it does with some human parents who have difficulty having children - an immunological incompatibility or traits which cause most offspring non-viable.

In fact the discussion points out that the species designation does not always mean they cannot produce offspring, but simply do not. Coyotes and wolves will produce offspring too if they are encouraged to do so, but are competing and antagonistic in the wild and so never do mate in practice.

So the answer is probably no. (the beagle/setter pups did show up, but with a much smaller litter than usual with just 2 pups).


Breeding dogs has been a passion for people through many centuries. Part art, part science, and total devotion, breeding will show you all the best in the human-and-dog bond. It is exciting and challenging.

Breeding purebred dogs is also time consuming, expensive, and, occasionally, heartbreaking. If you go forward, your underlying purpose should be to improve the breed — not just increase its numbers.

Breeding a litter should begin with knowledge. Responsible breeders devote time to learning as much as they can about their breed, about canine health and training, and about AKC rules. How can you become an expert?

Study your breed standard. This is the official version of the “perfect” breed specimen and should be the starting place for any breeder. The AKC offers breed videos with real-life examples, and many parent clubs offer more detailed, illustrated versions of their standards for more in-depth research.

Attend dog events. Watch dogs in action and study the pedigrees of those you like. Ask questions of breeders involved in your breed. Research your breed by visiting the breed’s national parent club website. Find and attend a local club meeting to meet other breeders.

Read, read, read! Your library and bookstore are invaluable sources of information about canine health and breeding. Some books, including the Complete Dog Book and American Kennel Club Dog Care and Training, are available in the AKC Store. The AKC Gazette and other dog magazines have informative articles about breeding as well.


DNA testing looks into dog breeds and cat history

A chihuahua and a St. Bernard don’t look like they have much in common. But they’re both the same sub-species. What’s the difference? A few genes.

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October 24, 2019 at 5:35 am

St. Bernards are tall, hairy, muscular dogs, built for a life rescuing trapped travelers in the mountains of Europe. Chihuahuas are tiny, with shorter hair and rounder heads. They come from Mexico. Looking at them side by side, you might be tempted to question whether they’re the same species. Yet for all their dramatic differences, each can still mate with any other dog and produce pups. That’s because a big boy St. Bernard and an itsy bitsy chihuahua are the same subspecies — Canis lupus familiaris.

The differences in the appearances of these two dog breeds trace to tiny variations in their DNA. DNA is a long string of smaller molecules called nucleotides (NU-klee-oh-tydz). They come in four types — adenine (A), cytosine (C), thymine (T) and guanine (G). The order in which those four letters occur spells out the instructions that tell each cell what molecules to make. And those DNA strings are highly specific to each individual.

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One may have a string of letters that are almost identical to another’s. But the first may have a C at a site where the other has a T. That single difference might change what molecule is made from that long sequence of letters — giving one dog curly fur instead of straight, or short limbs instead of long.

Each parent passes along half of its DNA to its puppy. That DNA codes for traits that will collect over doggie generations. Eventually, if breeders select for certain traits (mating dogs with the same chosen traits over and over), they can create a new breed.

A couple of tweaks here might produce the long floppy ears characteristic of a basset hound. Another few tweaks over there might produce the short stumpy legs and elongated snout of a dachshund. Still more tweaks might make no changes at all.

Scientists refer to these small but important tweaks as SNPs (pronounced snips). That’s short for single nucleotide polymorphisms (Pah-lee-MOR-fizms). SNPs are places where one nucleotide has randomly substituted for another — where a G, for example, might have taken the place of a T. Millions of SNPs appear within the DNA of every dog (and cat, and human). Comparing patterns of SNPs in dogs that look alike or have other characteristic traits can help scientists find what subtly sets each breed apart.

By searching for those SNP patterns, scientists can later figure out from which breeds a dog or cat has descended.

Hunting down dog SNPs

To make that work, scientists first need to identify those patterns. Scientists like Angela Hughes. She’s an animal geneticist at Mars Petcare (yes, the Mars that makes M&Ms) in Vancouver, Wash. Hughes heads a team that makes Wisdom Panel. It’s a test to find out what breeds are in a dog’s ancestry.

Explainer: What are genes?

To figure out which SNP patterns define a breed, Hughes needs dogs. Her own mutt — a mix of an Australian cattle dog and a Jack Russell terrier — won’t cut it. She needs dogs that people have been specifically breeding for generations. “We go out and work with breeders and [dog] shows,” she says. “Sometimes we have to go out and find the breeders,” she says, because “they don’t always come to shows.”

Her team tries to test several hundred dogs of each breed. They also get different types of the same breed — such as retrievers that have been bred for hunting and retrievers that have been bred to be show dogs or pets.

Then, when someone sends in a sample from their pet to Wisdom Panel, scientists can look for distinctive SNPs in its DNA. To identify a dog’s lineage, they plug in 1,800 gene sequences, each with its own SNP. Then they’ll compare these to those in the pet.

A computer program then uses an algorithm (AL-go-RITH-um) to find the best match between this pet and the known SNPs of purebred breeds. “If the dog could be only one thing, which of these would it best match?” Hughes asks. “And if it were two things, what would be the best match?” The program does this all the way to figure out a dog’s great-grandparents.

Wanted: Millions of mutts

There are several other tests for a dog’s DNA. Adam Boyko founded the company EmBark to make one of them. Boyko is a geneticist at Cornell University in Ithaca, N.Y. He developed the test to get more data for research. Most of his tests, though, start not with EmBark clients but with his dog, Penny — a mix of Jack Russell terrier, Pomeranian and miniature pinscher. “When she sees a swab, she jumps up. She knows there will be a treat,” he says. “We test a lot of prototypes on her.” But while Penny is a great experimental subject, she’s only one dog.

“There’s a billion dogs in the world, and most are not purebred,” he notes. “If you want to make discoveries about what makes unique behavior in dogs, what underlies cancer risk or risk of … allergies, you need way bigger sample sizes,” he says. He figured he could only get enough samples if he created a test that anyone might use on their pets.

Scientists Say: Chromosome

His computer program works a bit differently than a SNP test. It looks at more than 200,000 different genetic fingerprints. These are patterns of DNA changes that sit close to each other on chromosomes.

Chromosomes are long, tightly coiled pieces of DNA. When animals mate, their DNA mixes. In the process, chunks of their chromosomes tend to end up close to each other. Scientists can trace those chromosome chunks back to the parent who passed them on, Boyko explains. EmBark then compares the DNA in those chromosome chunks to the DNA in known dog breeds

Boyko can identify a dog’s ancestral breeds. He can even find that dog’s closest puppy cousins. That’s important, he notes, for people who breed purebred dogs. Many of these dogs have been inbred — bred with animals to which they are too closely related. That can be bad for the health of their puppies. By finding their close relatives, Boyko can help people breed healthier dogs.

Here kitty, kitty

“There’s more researchers in dogs than there are in cats,” observes Robert Grahn. Still, he points out, cats can get their DNA tested, too.

Grahn, a geneticist at the University of California, Davis, notes that there are fewer cat breeds. Moreover, he notes, most cats aren’t really one “breed” or another. They’re just, well, cats. Most cats meet on the street and mate randomly, he explains. Those are the “domestic short hair” or “domestic long hair” cats that come in black, white, tabby, calico and more. Persians, Siamese and other cat breeds are purebreds. They are often bred to compete in shows. But owners tend to keep these expensive cats to themselves, Grahn says. They “tend to stay inside. How many times have you seen a Persian wandering the street? You don’t let that out.” (Though he studies cats, Grahn himself is more of a dog person. “I had the best cat ever,” he explains. But once that cat died, “any other cat I would have wouldn’t measure up.” Now he has a Labrador named River.)

People who breed cats may still want to know about their pet’s family tree. A genetic test such as Basepaws can detect cat breeds. The test hunts for SNPs just as the dog tests do. But instead of trying to get a saliva sample from an indignant feline, a chunk of cat hair will do.

Such a test can tell you about possible coat colors and fur length. But with so many “domestic short hair cats” in the world — such as the many random tabbies and tuxedos — knowing their ancestral breed might not be as interesting as learning where in the world its ancestors came from. That’s why Leslie Lyons helped develop the Cat Ancestry test at UC Davis. It shows the part of the world where your cat’s ancestors might have developed.

Lyons is a geneticist. She works at the University of Missouri in Columbia where she and her laboratory have built a genetic library for cats. “I think they’re the perfect little species,” she says. She wouldn’t say she owns cats, herself. “Four cats share my home,” she explains. “I live in a rural area,” and so the cats come and go as they please.

Domestic cats, notes Lyons, are not native to North America. “They’re imports.” They came from Europe, southeast Asia — maybe even the Mediterranean, she says. The Cat Ancestry test identifies patterns of SNPs from these areas. Then it compares them to the SNPs in your cat. So sure, yours might be another domestic short hair. But it may turn out to have a Western European heritage, or an Asian one. Wouldn’t you like to know which?

Power Words

algorithm A group of rules or procedures for solving a problem in a series of steps. Algorithms are used in mathematics and in computer programs for figuring out solutions.

behavior The way something, often a person or other organism, acts towards others, or conducts itself.

breed (noun) Animals within the same species that are so genetically similar that they produce reliable and characteristic traits. German shepherds and dachshunds, for instance, are examples of dog breeds. (verb) To produce offspring through reproduction.

cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.

cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chromosome A single threadlike piece of coiled DNA found in a cell&rsquos nucleus. A chromosome is generally X-shaped in animals and plants. Some segments of DNA in a chromosome are genes. Other segments of DNA in a chromosome are landing pads for proteins. The function of other segments of DNA in chromosomes is still not fully understood by scientists.

code (in computing) To use special language to write or revise a program that makes a computer do something. (n.) Code also refers to each of the particular parts of that programming that instructs a computer's operations.

computer program A set of instructions that a computer uses to perform some analysis or computation. The writing of these instructions is known as computer programming.

data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

develop To emerge or come into being, either naturally or through human intervention, such as by manufacturing. (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.

DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell&rsquos production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

generation A group of individuals (in any species) born at about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans. The term also is sometimes extended to year classes of other animals or to types of inanimate objects (such as electronics or automobiles).

genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

irony A phrase, expression or action that seems to counter what had been stated or had been expected.

limb (in physiology) An arm or leg. (in botany) A large structural part of a tree that branches out from the trunk.

localized An adjective for something that has a very local impact. (antonym: broad or far-reaching)

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

native Associated with a particular location native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

nucleotides The four chemicals that, like rungs on a ladder, link up the two strands that make up DNA. They are: A (adenine), T (thymine), C (cytosine) and G (guanine). A links with T, and C links with G, to form DNA. In RNA, uracil takes the place of thymine.

pup A term given to the young of many animals, from dogs and mice to seals.

random Something that occurs haphazardly or without reason, based on no intention or purpose.

risk The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard &mdash or peril &mdash itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

sequence The precise order of related things within some series. (in genetics) n. The precise order of the nucleotides within a gene. (v.) To figure out the precise order of the nucleotides making up a gene.

single nucleotide polymorphism Abbreviated SNP (pronounced &ldquosnip&rdquo), this refers to DNA in which one of its original nucleotides has been naturally substituted for another. This variation may alter the function of DNA. SNPs are inherited.

species A group of similar organisms capable of producing offspring that can survive and reproduce.

subspecies A subdivision of a species, usually based on geographic separations. Over time, this separation may have allowed some of the genes in a population of a species to vary, creating differences in those organisms&rsquo appearance or adaptation to the local environment.

subtly An adverb to describe something that may be important, but can be hard to see or describe. For instance, the first cellular changes that signal the start of a cancer may be only subtly different &mdash as in small and hard to distinguish from nearby healthy tissues.

trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

unique Something that is unlike anything else the only one of its kind.

Citations

Journal:​ M.J. Montague et al. Comparative analysis of the domestic cat genome reveals genetic signatures underlying feline biology and domestication. Proceedings of National Academy of Sciences. Vol. 48, December 2, 2014, p. 17230. doi: 10.1073/pnas.1410083111.

About Bethany Brookshire

Bethany Brookshire was a longtime staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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How Will Dogs Reshape Nature Without Humans to Control Them?

How will dogs reshape and redecorate nature in a world without humans?

In a previous essay called "As Dogs Go Wild in a World Without Us, How Might They Cope?" I wrote about how dogs might adapt to a world in which we no longer control their lives. It will surely be a challenging time for our canine companions, and it seems that their losses would far outweigh their gains even if they are free of the numerous constraints we place on them. However, we need to be very careful about our predictions about what and how dogs will do in a world without us, as there's no straightforward shopping list of traits that would necessarily decrease their survival and those that would favor the ways in which they would adapt. It's also essential to consider how individuals would adapt, rather than to adopt species-wide predictions. In an essay published by Thomas Daniels and myself called "Feralization: The making of wild domestic animals," we focused on the ways in which individual domestic animals — in this case, dogs — either become desocialized from humans, or never become socialized, and thus come to behave as untamed, non-domestic animals. These and other topics are considered in my previous essay.

I received a number of very interesting comments about my essay, and some got me to muse on how artificial selection imposed by humans past and present would give way to different forms of natural selection. Additionally, I began to think of dogs as an invasive species as they become residents of a wide variety of ecosystems because it's clear that they would have to interact -- compete and cooperate -- with members of many different species who were not created by humans but whose lives also are greatly influenced by humans. Their lives, too, would change absent us.

Biologists generally categorize different forms of natural selection as stabilizing, directional, or disruptive selection. Stabilizing selection is "a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular trait value." For example, dog breeders generally practice artificial stabilizing selection when they try to produce dogs to satisfy breed standards. Directional selection "is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype." A simple example would be situations in which there would be selection for body size (large, medium, or small), running speed (slow or fast), or dull or bright coloration.

Finally, when disruptive selection occurs, extremes of a trait are favored over intermediate forms of that specific trait. An example of disruptive selection, also called "diversifying selection" would be the following: "if a population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks, the rabbits with black fur would be able to hide from predators amongst the black rocks, and the rabbits with white fur likewise amongst the white rocks. The rabbits with grey fur, however, would stand out in all areas of the habitat, and would thereby suffer greater predation." Another well-known example is Darwin's finches living on the Galápagos Islands who showed disruptive selection in beak size. Research showed that beak size "appeared to be adaptively related to the seed size available on the respective islands (big beaks for big seeds, small beaks for small seeds). Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive." Disruptive selection is the opposite of stabilizing selection.

Dogs going wild and reshaping nature by "reverse engineering"

If one thinks of the intensive artificial selection of dogs as a form of genetic engineering, humans surely engineered dogs for a wide variety of traits as they chose characteristics that satisfied human needs, some of which had negative effects on the dogs. As dogs go wild without us, one could view the ways in which individuals would change as a form of reverse engineering. Stabilizing selection would likely give way both to directional and disruptive selection, as for example, dogs of different sizes would likely show differential survival in different habitats, dogs of different sizes and different breeds or mixes would likely interbreed far more than they do absent human control of reproduction, and coat color, texture and other phenotypic traits would no longer be as tightly controlled without humans doing the work as dogs come to occupy vastly different ecosystems.

Will dogs become an invasive species and should they be labeled as such?

Dogs currently are creations of humans. While there are populations of free-ranging and feral dogs, individuals in these groups remain domesticated individuals. I mention the possibility of dogs becoming what some might call an invasive species because their presence, absent us, will markedly change the lives of many other animals as dogs become active members of a wide variety of populations and ecosystems. There will be changes in the behavior and geographic distribution of individuals of numerous other species, including those with whom they may form alliances and those with whom they might compete, and all of their homes will be reshaped and redecorated as dogs evolve absent humans. Dogs' genes also will enter into play, as they're able to interbreed with coyotes and wolves and produce viable offspring.

It's also interesting to note that just as humans, a pervasive -- some say the most -- invasive species, have had enormous global effects on all sorts of ecosystems, so too will dogs gone wild, and they exist because of us. Somehow, we can't get out of the picture even when we're not here, at least for the period during which dogs continue to reflect our widespread tinkering with their lives. Concerning labeling dogs as an "invasive species," when I was talking with someone, they noted with a chuckle that there won't be any humans to label dogs as "invasive," and members of other species won't be doing it either. Psychology Today writer and well-known author Mark Derr suggested to me that we might also refer to dogs gone wild after humans depart as being "abandoned" or "orphaned." Surely, both words could apply more appropriately thanks to their being "invasives." Regardless of what they're called, as time goes on, all the nonhumans who are left after humans leave need to figure out how to survive with new canine members of their communities.

Why care about how dogs would cope in a world without humans?

Many people who are interested in dogs will likely also be interested in how artificial selection will give way to different forms of natural selection when "reverse engineering" occurs without us. As we ponder what a world without humans will be like for dogs, not only do we need to focus on who dogs are, but we also need to consider the nature of dog-human relationships and the nature of dog-other animal relationships.

Please stand by for further discussions of how dogs would do in a world without humans. Each time I ponder how dogs will do without us more and more questions arise, and I'm constantly learning about all things dog including their relationships with other dogs, with other nonhumans, and with humans.

I hope that musing about what and how dogs will do in a world without humans will benefit them now as they currently try to adapt to an increasingly human-dominated world that many dogs find highly stressful. This surely would be a win-win for all, even if we'll never know what happens to them in our absence.

1 Once again I thank Jessica Pierce for her help with this essay and for continuing to talk with me about what a world without humans would be like for dogs and other animals.


How big do silver Labs get?

The recommended breed standard height for a Labrador is up to 24 and a half inches for a male. And an inch shorter for a female.

However, individuals can vary a couple of inches or more either side of that.

When it comes to body weight, the variations can be even greater and will depend on which of two groups (American or English) a silver Lab falls into.

Male Labs often reach about 70lbs in weight. Females about 10lb lighter.

But there can be as much as 20lbs difference either side of that average.

American Labs bred for hunting and retrieving are slimmer, taller and often lighter than the chunkier English type that you see in the show ring


How Will Dogs Reshape Nature Without Humans to Control Them?

How will dogs reshape and redecorate nature in a world without humans?

In a previous essay called "As Dogs Go Wild in a World Without Us, How Might They Cope?" I wrote about how dogs might adapt to a world in which we no longer control their lives. It will surely be a challenging time for our canine companions, and it seems that their losses would far outweigh their gains even if they are free of the numerous constraints we place on them. However, we need to be very careful about our predictions about what and how dogs will do in a world without us, as there's no straightforward shopping list of traits that would necessarily decrease their survival and those that would favor the ways in which they would adapt. It's also essential to consider how individuals would adapt, rather than to adopt species-wide predictions. In an essay published by Thomas Daniels and myself called "Feralization: The making of wild domestic animals," we focused on the ways in which individual domestic animals — in this case, dogs — either become desocialized from humans, or never become socialized, and thus come to behave as untamed, non-domestic animals. These and other topics are considered in my previous essay.

I received a number of very interesting comments about my essay, and some got me to muse on how artificial selection imposed by humans past and present would give way to different forms of natural selection. Additionally, I began to think of dogs as an invasive species as they become residents of a wide variety of ecosystems because it's clear that they would have to interact -- compete and cooperate -- with members of many different species who were not created by humans but whose lives also are greatly influenced by humans. Their lives, too, would change absent us.

Biologists generally categorize different forms of natural selection as stabilizing, directional, or disruptive selection. Stabilizing selection is "a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular trait value." For example, dog breeders generally practice artificial stabilizing selection when they try to produce dogs to satisfy breed standards. Directional selection "is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype." A simple example would be situations in which there would be selection for body size (large, medium, or small), running speed (slow or fast), or dull or bright coloration.

Finally, when disruptive selection occurs, extremes of a trait are favored over intermediate forms of that specific trait. An example of disruptive selection, also called "diversifying selection" would be the following: "if a population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks, the rabbits with black fur would be able to hide from predators amongst the black rocks, and the rabbits with white fur likewise amongst the white rocks. The rabbits with grey fur, however, would stand out in all areas of the habitat, and would thereby suffer greater predation." Another well-known example is Darwin's finches living on the Galápagos Islands who showed disruptive selection in beak size. Research showed that beak size "appeared to be adaptively related to the seed size available on the respective islands (big beaks for big seeds, small beaks for small seeds). Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive." Disruptive selection is the opposite of stabilizing selection.

Dogs going wild and reshaping nature by "reverse engineering"

If one thinks of the intensive artificial selection of dogs as a form of genetic engineering, humans surely engineered dogs for a wide variety of traits as they chose characteristics that satisfied human needs, some of which had negative effects on the dogs. As dogs go wild without us, one could view the ways in which individuals would change as a form of reverse engineering. Stabilizing selection would likely give way both to directional and disruptive selection, as for example, dogs of different sizes would likely show differential survival in different habitats, dogs of different sizes and different breeds or mixes would likely interbreed far more than they do absent human control of reproduction, and coat color, texture and other phenotypic traits would no longer be as tightly controlled without humans doing the work as dogs come to occupy vastly different ecosystems.

Will dogs become an invasive species and should they be labeled as such?

Dogs currently are creations of humans. While there are populations of free-ranging and feral dogs, individuals in these groups remain domesticated individuals. I mention the possibility of dogs becoming what some might call an invasive species because their presence, absent us, will markedly change the lives of many other animals as dogs become active members of a wide variety of populations and ecosystems. There will be changes in the behavior and geographic distribution of individuals of numerous other species, including those with whom they may form alliances and those with whom they might compete, and all of their homes will be reshaped and redecorated as dogs evolve absent humans. Dogs' genes also will enter into play, as they're able to interbreed with coyotes and wolves and produce viable offspring.

It's also interesting to note that just as humans, a pervasive -- some say the most -- invasive species, have had enormous global effects on all sorts of ecosystems, so too will dogs gone wild, and they exist because of us. Somehow, we can't get out of the picture even when we're not here, at least for the period during which dogs continue to reflect our widespread tinkering with their lives. Concerning labeling dogs as an "invasive species," when I was talking with someone, they noted with a chuckle that there won't be any humans to label dogs as "invasive," and members of other species won't be doing it either. Psychology Today writer and well-known author Mark Derr suggested to me that we might also refer to dogs gone wild after humans depart as being "abandoned" or "orphaned." Surely, both words could apply more appropriately thanks to their being "invasives." Regardless of what they're called, as time goes on, all the nonhumans who are left after humans leave need to figure out how to survive with new canine members of their communities.

Why care about how dogs would cope in a world without humans?

Many people who are interested in dogs will likely also be interested in how artificial selection will give way to different forms of natural selection when "reverse engineering" occurs without us. As we ponder what a world without humans will be like for dogs, not only do we need to focus on who dogs are, but we also need to consider the nature of dog-human relationships and the nature of dog-other animal relationships.

Please stand by for further discussions of how dogs would do in a world without humans. Each time I ponder how dogs will do without us more and more questions arise, and I'm constantly learning about all things dog including their relationships with other dogs, with other nonhumans, and with humans.

I hope that musing about what and how dogs will do in a world without humans will benefit them now as they currently try to adapt to an increasingly human-dominated world that many dogs find highly stressful. This surely would be a win-win for all, even if we'll never know what happens to them in our absence.

1 Once again I thank Jessica Pierce for her help with this essay and for continuing to talk with me about what a world without humans would be like for dogs and other animals.


Answers and Replies

Does anyone who is an expert know the answer to the questions I asked?

Does anyone who is an expert know the answer to the questions I asked?

I would have though an expert on evoltion woul dbe quite hard to come across. I don't think of it as any natural form of evolution, as there is no selection between mates, and they do not evolve to better thei chances of survival.

I think the fact that they are domestic pets would suggest that the evolution you are talking about isn't the same as natural evolution. I myself am not 100% so yes, if there is anyone clued up on this please try an explain :shy:

The reason I excluded mutations, is because my specific question is how far could an animal be changed with pure breeding. If your goal was to continue to increase a certain trait in an animal how far could that be increased without mutations, and whether their is some limit, and to increase that trait any further you would need mutations. I guess that isnt such an important question anymore, since I would assume that it would be possible, if the original population had the genes somewhere, but it would be extremely unlikely that those genes would all end up in one animal at one point.

By the way you say that mutations happen all the time, are you talking about mutations that get passed down to the child? I always assumed that they were very rare, because otherwise most people would probably have all kinds of genetic problems, since if you pick a random gene which has been chosen by billions of years of trial and error, and change it then you are almost guaranteed to have made a negative change, and only maybe 1 in 10 million DNA mutations would be positive.

And I agree, I think that breeding and natural selection are basically the same, you could see the environment of the domesticated dog as being such that it favored small size. Its still selection for a certain trait, only that trait is chosen by someone.

The biological definition of evolution is "the change in alelle frequency with time." This is certainly happening in selectively-bred dogs, and thus it certainly counts as evolution. In this case, the "natural" selection pressures are being overwhelmed by human decision-making, but the biological result is evolution all the same.

The biological definition of evolution is "the change in alelle frequency with time." This is certainly happening in selectively-bred dogs, and thus it certainly counts as evolution. In this case, the "natural" selection pressures are being overwhelmed by human decision-making, but the biological result is evolution all the same.

There's no sense in trying to isolate "natural" evolution from "artificial" evolution, in the same way that there's no sense in trying to isolate "natural" chemicals from "artificial" chemicals.

Dog breeding is performed by allowing only the best animals to breed. This is still "survival of the fittest," only the assessment of fitness is done by a human being rather than by an ecosystem.

It's still evolution, either way.

There's no sense in trying to isolate "natural" evolution from "artificial" evolution, in the same way that there's no sense in trying to isolate "natural" chemicals from "artificial" chemicals.

Dog breeding is performed by allowing only the best animals to breed. This is still "survival of the fittest," only the assessment of fitness is done by a human being rather than by an ecosystem.

It's still evolution, either way.

This is where I dissagree, dog breeding is performed by allowing only the best animals to breed, that tick the boxes of the dog breeder. An example would be image, or bone structure. These characteristics are not necessarily key in their survival naturally, as their life does not depend on it. I think it is important to isolate both natural evolution and selective evolution, as they are not the same thing. Your example of chemicals does not really have much similarity to the one of dog breeding I don't think.

These characteristics are critical to their ability to reproduce, at least in the false "ecosystem" created by the human breeders. If they escape their pens, all bets are off.

You're free to have whatever opinion you like, but the biological definition of evolution is clearly met in both "natural" evolution and in selective breeding. Your personal biases aren't relevant to that determination.

You're free to have whatever opinion you like, but the biological definition of evolution is clearly met in both "natural" evolution and in selective breeding. Your personal biases aren't relevant to that determination.

What I am trying to put forward is that there are two types of selection in evolution that we are talking about. One involves natural selection and the other involves artificial selection. I can accept that the biological definition is that, but I don't see what is wrong with dividing them into atleast these two groups. I am not dissagreeing with the definition, but saying that natural selection is the reproduction of a species with certain traits which is attributed towards its ability to survive and reproduce. Artificial selection is for the good of the human, who intentionally breeds (dogs) seeking particular traits.

I think you have shown that I was initially incorrect in thinking that evolution fitted into more than one catagory, but I still think that it is important to note that specifically in selection that natural and artificial both result in something different, an adaptation to either their environment or to what their owners seeks to achieve,

Again, you're welcome to your opinion -- just don't expect anyone else to have the same opinion.

Again, you're welcome to your opinion -- just don't expect anyone else to have the same opinion.

I don't think you could everyone to have the same opinion on anything, so I wouldn't expect the near on impossible. Thanks for the insight I think I will look at this all in a different light, and maybe I should actually read a bit more into the topic. Thanks.

So I guess getting back to the thread starters question. Yes it is evolution. :tongue:

Yes, mutations happen all the time. If you were to compare any 2 chromosomes, about 1 in every 1,000 bases would be a SNP (Single Nucleotide Polymorphism) on average. Keep in mind though, that the frequency of SNPs are less within genes than they are in between genes due to gene conservation.

Oh and BTW, people DO have all sorts of genetic problems. Many may be obvious, and much more probably not so obvious.

On the subject of dog breeding though, I think dogs have a somewhat unique genome that allows then to vary in size fairly drastically. I would say that the variation we see in dog breeding is more a result of epigenetics, than it is what many of you would think of as "evolution". But then again, IMHO epigenetics is a form of evolution as well.

My 2 cents if you will:
The original question regarding a hypothetical scenario in which you wished to obtain a distinct breed of dog that has a significantly lower average mass than its intial ancestors, without new mutations occuring, is an interesting question.

Firstly though, you simply could not prevent mutations from occuring. Most mutations are the result of infidelities of the DNA copying process. These could not be prevented in large and only enviromental mutation inducing factors could be eliminated, which as we have established play a small part in mutation occurence. Nevertheless it is a hypothetical situation.

Obtaining this distinct breed of lower mass dog without mutations probably wouldn't be possible. Such a drastic difference in average mass would most likely be the result of a number of mutations. But a less drastic result could be attained. Unique combinations of alleles that already exist in the population you see can have massively varying effects depending on what other alleles they find themselves with. Genes do not act singularily and in isolation. The effect of an allele in the phenotype is as much a property of its genetic enviroment (and for that matter the 'traditional' enviroment) as the protein it codes for (look into additive gene effect). So it is possible that alleles already existing in the population could be combined so a much lighter dog is produced over many generations scrutinised by the human breeder. For as I have alluded to: traits of the phenotype, mass in this instance, are polygenic (determined by a number of genes). Hence there are a suprising number of combinations of genes for a given trait, each producing suprisingly drastic effects.

Hopefully that did not come across as an uninformed rant. As for this debate over what evolution is and whether we can distinguish between natural and artificial selection. I understand evolution as the increasing adaptedness of a population over generations. This may appear to defy the idea that the artificially selected dogs are evolving, but as just stated there is a degree of plasticity in biological 'definitions' which may not be familiar to physical scientists. It could be said the dogs are becoming more adapted, as they are becoming more like what the human breeder wants, and that is their main selection pressure. So I would show little caution in calling both types of selection causing evolution.

The OP asked a second question, regarding the likelyhood of genetically idnetical individuals arising. Possible - yes. Likelyhood - ridiculously unlikely. Lets just consider in short what would have to happen and the approximate probabilities associated with these events. Two identical gametes would have to be produced by two different people: one male one female conventionally. Without going into details this is close to impossible (but technically not). Given this highly unlikely event occurs, the gametes would have to fuse on two occasions. The chance of a particular pair of gametes fusing is another stupidly improbably event. The offspring would also have to be viable (another probability assigned here). These events may as well be assumed independent, so the probability of two identicals being born on different occasions is calculated by mutlipying all these very, very small numbers together obviously we obtain an even smaller number. Given sufficient time though, even the most improabable event, if it occurs continuously, should occur. But the time required for something like this to happen probably couldn't be counted on two hands. Considerations also have to be given to the different genotypes of individuals and the limited number of copulations in an organisms lifetime. All in all this ain't ever gonna happen.


18 Hybrid Animals That Are Hard To Believe Actually Exist

You might have probably heard about the most common animal hybrid between a female horse and a male donkey, called a mule, but did you know there are more of these mixed animals? Though this kind of species and breeds crossing does not usually appear in nature, with the intervention of humans, we now have zonkeys, ligers, and Savannah cats. These offsprings are typically infertile, with some exceptions such as the coywolf (not to be confused with a wolf that is coy), that is a mix of a coyote and a wolf and can further reproduce.

Though the internet is full of photoshopped images of strange creatures, this list is full of absolutely real and amazing animals. What does the future hold, with advances in genetic engineering and cloning? Only time will tell! Keep reading to find out more about these weird animals.

Liger ( Male Lion + Female Tiger)

Although there are rumors of wild Ligers, as far as we know, they exist only in captivity where they are deliberately bred. They grow to be very large very quickly, and are the biggest cats in the world. Hercules, the largest non-obese liger, is the largest living cat on Earth, weighing over 410 kg (904 lb). (source: wikipedia.org/wiki/Liger)

Tigon (Male Tiger + Female Lion)

How far can you go? Did you know that Ligers and Tigons also reproduce? We&rsquoll leave it up to you to figure out what their offspring are called! (source: wikipedia.org/wiki/Tigon)

Zonkey (Zebra + Donkey)

A variation of the aforementioned zebroid. (source: wikipedia.org/wiki/Zebroid)

Jaglion (Male Jaguar + Female Lion)

A rare combination. These photos are of Jahzara and Tsunami, born at Ontario, Canada&rsquos, Bear Creek Wildlife Sanctuary. (source: wikipedia.org/wiki/Panthera_hybrid)

Geep (Goat + Sheep)

Another rare animal, as the offspring of goat and sheep pairings are usually stillborn. (source: wikipedia.org/wiki/Geep)

Grolar Bear (Polar Bear + Brown Bear)

Also called &ldquopizzly bears,&rdquo most grolar bears live in zoos, although there have been a few confirmed sightings in the wild. (source: wikipedia.org/wiki/Grizzly polar_bear_hybrid)

Coywolf (Coyote +Wolf)

Coyotes and eastern wolves only diverged some 150-300,000 years ago, and the two are able to produce offspring. The resulting Coywolves share many behavioral characteristics, and are between the coyote and wolf in size. (source: wikipedia.org/wiki/Coywolf)

Zebroid (Zebra + Any Other Equine)

Darwin was one of the first to mention the Zebroid, an unruly animal that is hard to tame, and is more aggressive than a horse. (source: wikipedia.org/wiki/Zebroid)

Savannah Cat (Domestic Cat + Serval)

These beautiful creatures have been described as dog-like, enjoying games of fetch, wagging their tails, and having no fear of water. They are extremely expensive. (source: wikipedia.org/wiki/Savannah_cat)

Wholphin (Male False Killer Whale + Female Bottlenose Dolphin)

False killer whales actually come from the same family as dolphins, but despite this, they are extremely rare. Only one wholphin exists in captivity. (source: wikipedia.org/wiki/Wholphin)

Beefalo (Buffalo + Cow)

Also called &ldquocattalo,&rdquo they&rsquove been around since 1800, and are heartier than cattle and do less ecological damage when grazing. Unfortunately, as a result of the breeding, it&rsquos believed that only four wild buffalo herds exist that aren&rsquot contaminated by cow genes. (source: wikipedia.org/wiki/Beefalo)

Hinny (Female Donkey + Male Horse)

Slightly smaller than mules, they&rsquore also much less common. (source: wikipedia.org/wiki/Hinny)

Narluga (Narwhal + Beluga)

Extremely rare, although there has recently been an increase in sighting in the North Atlantic.

Cama (Camel + Llama)

First produced at the Camel Reproduction Centre in Dubai in 1998 via artificial insemination, they were created for their fur and use of pack animals. Only 5 were ever made. (source: wikipedia.org/wiki/Cama)

Dzo (Cow + Wild Yak)

Prized in Tibet and Mongolia for their meat and quantity of milk they produce, they are larger and stronger than both cows and yaks. As with the beefalo, however, it&rsquos believed that both animal breeds in the region now have contaminated genes. (source: wikipedia.org/wiki/Dzo)

Leopon (Male Leopard + Female Lion)

These beautiful animals have only ever produced in captivity. (source: wikipedia.org/wiki/Leopon)

Mulard (Mallard + Muscovy Duck)

Bred for food, the mulard is unable to produce offspring. (source: wikipedia.org/wiki/Mulard)

Żubroń (Cow + European Bison)

Stronger and more resistant to disease, they were initially thought to be a possible replacement for cattle. Now, only a small herd exists in Bialowieski National Park in Poland. (source: wikipedia.org/wiki/Żubron)

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What I think is people should stop breeding animals that normally would not breed in nature. The babies that turn out well may be sold or exhibited, and those that don't wind up at sanctuaries if there is room, or dig neglected, or dying. There's a reason most of these are "found" only in a captivity.

I know for a fact that some times these couples do happen in nature although rare

@katieparker not sure why i cant directly reply to your comment but here is a great article. http://www.discoverwildlife.com/animals/why-do-animals-interbreed where ligers apparently did actually occur naturally in asia originally. this isnt the only article i have read on this matter but it is a great one non the less. I dont agree with cross breeding species because of all the genetic defects that occur, it really is quite sad. But some species definitely are blind when it comes to mating in the wild and produce beautiful hybrid offspring.

Can you tell me where you got this "fact?" I'd love to read about it. Having worked in the veterinary field, as well as volunteered at a sanctuary for big cats, I can assure you that the interbreeding of lions and tigers, for example, does not happen in the wild. And, what's more, that the offspring of such a match which cannot be sold live horrible lives, if they live at all. Even white tigers are being inbred because of the high demand for them as pets. Several of the tigers where I worked were "rejects" from the trade, with major medical issues. Not white, so. not wanted.


How Will Dogs Reshape Nature Without Humans to Control Them?

How will dogs reshape and redecorate nature in a world without humans?

In a previous essay called "As Dogs Go Wild in a World Without Us, How Might They Cope?" I wrote about how dogs might adapt to a world in which we no longer control their lives. It will surely be a challenging time for our canine companions, and it seems that their losses would far outweigh their gains even if they are free of the numerous constraints we place on them. However, we need to be very careful about our predictions about what and how dogs will do in a world without us, as there's no straightforward shopping list of traits that would necessarily decrease their survival and those that would favor the ways in which they would adapt. It's also essential to consider how individuals would adapt, rather than to adopt species-wide predictions. In an essay published by Thomas Daniels and myself called "Feralization: The making of wild domestic animals," we focused on the ways in which individual domestic animals — in this case, dogs — either become desocialized from humans, or never become socialized, and thus come to behave as untamed, non-domestic animals. These and other topics are considered in my previous essay.

I received a number of very interesting comments about my essay, and some got me to muse on how artificial selection imposed by humans past and present would give way to different forms of natural selection. Additionally, I began to think of dogs as an invasive species as they become residents of a wide variety of ecosystems because it's clear that they would have to interact -- compete and cooperate -- with members of many different species who were not created by humans but whose lives also are greatly influenced by humans. Their lives, too, would change absent us.

Biologists generally categorize different forms of natural selection as stabilizing, directional, or disruptive selection. Stabilizing selection is "a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular trait value." For example, dog breeders generally practice artificial stabilizing selection when they try to produce dogs to satisfy breed standards. Directional selection "is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype." A simple example would be situations in which there would be selection for body size (large, medium, or small), running speed (slow or fast), or dull or bright coloration.

Finally, when disruptive selection occurs, extremes of a trait are favored over intermediate forms of that specific trait. An example of disruptive selection, also called "diversifying selection" would be the following: "if a population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks, the rabbits with black fur would be able to hide from predators amongst the black rocks, and the rabbits with white fur likewise amongst the white rocks. The rabbits with grey fur, however, would stand out in all areas of the habitat, and would thereby suffer greater predation." Another well-known example is Darwin's finches living on the Galápagos Islands who showed disruptive selection in beak size. Research showed that beak size "appeared to be adaptively related to the seed size available on the respective islands (big beaks for big seeds, small beaks for small seeds). Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive." Disruptive selection is the opposite of stabilizing selection.

Dogs going wild and reshaping nature by "reverse engineering"

If one thinks of the intensive artificial selection of dogs as a form of genetic engineering, humans surely engineered dogs for a wide variety of traits as they chose characteristics that satisfied human needs, some of which had negative effects on the dogs. As dogs go wild without us, one could view the ways in which individuals would change as a form of reverse engineering. Stabilizing selection would likely give way both to directional and disruptive selection, as for example, dogs of different sizes would likely show differential survival in different habitats, dogs of different sizes and different breeds or mixes would likely interbreed far more than they do absent human control of reproduction, and coat color, texture and other phenotypic traits would no longer be as tightly controlled without humans doing the work as dogs come to occupy vastly different ecosystems.

Will dogs become an invasive species and should they be labeled as such?

Dogs currently are creations of humans. While there are populations of free-ranging and feral dogs, individuals in these groups remain domesticated individuals. I mention the possibility of dogs becoming what some might call an invasive species because their presence, absent us, will markedly change the lives of many other animals as dogs become active members of a wide variety of populations and ecosystems. There will be changes in the behavior and geographic distribution of individuals of numerous other species, including those with whom they may form alliances and those with whom they might compete, and all of their homes will be reshaped and redecorated as dogs evolve absent humans. Dogs' genes also will enter into play, as they're able to interbreed with coyotes and wolves and produce viable offspring.

It's also interesting to note that just as humans, a pervasive -- some say the most -- invasive species, have had enormous global effects on all sorts of ecosystems, so too will dogs gone wild, and they exist because of us. Somehow, we can't get out of the picture even when we're not here, at least for the period during which dogs continue to reflect our widespread tinkering with their lives. Concerning labeling dogs as an "invasive species," when I was talking with someone, they noted with a chuckle that there won't be any humans to label dogs as "invasive," and members of other species won't be doing it either. Psychology Today writer and well-known author Mark Derr suggested to me that we might also refer to dogs gone wild after humans depart as being "abandoned" or "orphaned." Surely, both words could apply more appropriately thanks to their being "invasives." Regardless of what they're called, as time goes on, all the nonhumans who are left after humans leave need to figure out how to survive with new canine members of their communities.

Why care about how dogs would cope in a world without humans?

Many people who are interested in dogs will likely also be interested in how artificial selection will give way to different forms of natural selection when "reverse engineering" occurs without us. As we ponder what a world without humans will be like for dogs, not only do we need to focus on who dogs are, but we also need to consider the nature of dog-human relationships and the nature of dog-other animal relationships.

Please stand by for further discussions of how dogs would do in a world without humans. Each time I ponder how dogs will do without us more and more questions arise, and I'm constantly learning about all things dog including their relationships with other dogs, with other nonhumans, and with humans.

I hope that musing about what and how dogs will do in a world without humans will benefit them now as they currently try to adapt to an increasingly human-dominated world that many dogs find highly stressful. This surely would be a win-win for all, even if we'll never know what happens to them in our absence.

1 Once again I thank Jessica Pierce for her help with this essay and for continuing to talk with me about what a world without humans would be like for dogs and other animals.


18.2 Formation of New Species

Although all life on earth shares various genetic similarities, only certain organisms combine genetic information by sexual reproduction and have offspring that can then successfully reproduce. Scientists call such organisms members of the same biological species.

Species and the Ability to Reproduce

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction.

Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce (Figure 18.9).

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group (Figure 18.10). If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.

Speciation

The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially interbreeding individuals. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. In fact, the presence in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be completed.

Given the extraordinary diversity of life on the planet there must be mechanisms for speciation : the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration found in On the Origin of Species (Figure 18.11a). Compare this illustration to the diagram of elephant evolution (Figure 18.11b), which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.

For speciation to occur, two new populations must be formed from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation (allo- = "other" -patric = "homeland") involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym- = "same" -patric = "homeland") involves speciation occurring within a parent species remaining in one location.

Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why there might not be more than two species formed at one time except that it is less likely and multiple events can be conceptualized as single splits occurring close in time.

Allopatric Speciation

A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across the range of the species, is relatively free because individuals can move and then mate with individuals in their new location. Thus, the frequency of an allele at one end of a distribution will be similar to the frequency of the allele at the other end. When populations become geographically discontinuous, that free-flow of alleles is prevented. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion forming a new valley, a group of organisms traveling to a new location without the ability to return, or seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the biology of the organism and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are, individuals from each population would fly back and forth continuing gene flow. However, if two rodent populations became divided by the formation of a new lake, continued gene flow would be unlikely therefore, speciation would be more likely.

Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms.

Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate sub-species of spotted owls exist. The northern spotted owl has genetic and phenotypic differences from its close relative: the Mexican spotted owl, which lives in the south (Figure 18.12).

Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely it is that speciation will occur. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls: in the north, the climate is cooler than in the south the types of organisms in each ecosystem differ, as do their behaviors and habits also, the hunting habits and prey choices of the southern owls vary from the northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur.

Adaptive Radiation

In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species. This is called adaptive radiation because many adaptations evolve from a single point of origin thus, causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island which leads to geographical isolation for many organisms. The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved, including the six shown in Figure 18.13.

Notice the differences in the species’ beaks in Figure 18.13. Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another example of adaptive radiation in an archipelago.

Link to Learning

Click through this interactive site to see how island birds evolved in evolutionary increments from 5 million years ago to today.

Sympatric Speciation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. The process of speciation within the same space is called sympatric speciation the prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” A number of mechanisms for sympatric speciation have been proposed and studied.

One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition called aneuploidy (Figure 18.14).

Visual Connection

Which is most likely to survive, offspring with 2n+1 chromosomes or offspring with 2n-1 chromosomes?

n +1 chromosomes are more likely to survive.

Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy (Figure 18.15). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes produced by this plant species. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.

The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid . The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. Figure 18.16 illustrates one possible way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations described here are unlikely to survive and produce normal offspring.) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.

Reproductive Isolation

Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction: if individuals of the two populations were to be brought together, mating would be less likely, but if mating occurred, offspring would be non-viable or infertile. Many types of diverging characters may affect the reproductive isolation , the ability to interbreed, of the two populations.

Reproductive isolation can take place in a variety of ways. Scientists organize them into two groups: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of the development of an organism that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place this includes barriers that prevent fertilization when organisms attempt reproduction. A postzygotic barrier occurs after zygote formation this includes organisms that don’t survive the embryonic stage and those that are born sterile.

Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, called temporal isolation , can act as a form of reproductive isolation. For example, two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May (Figure 18.17).

In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the other populations of the same species. This situation is called habitat isolation . Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. For example, a cricket population that was divided after a flood could no longer interact with each other. Over time, the forces of natural selection, mutation, and genetic drift will likely result in the divergence of the two groups (Figure 18.18).

Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction from taking place. For example, male fireflies use specific light patterns to attract females. Various species of fireflies display their lights differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern and would not mate with the male.

Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place this is called a gametic barrier . Similarly, in some cases closely related organisms try to mate, but their reproductive structures simply do not fit together. For example, damselfly males of different species have differently shaped reproductive organs. If one species tries to mate with the female of another, their body parts simply do not fit together. (Figure 18.19).

In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and diameter, which prevents the plant from being cross-pollinated with a different species (Figure 18.20).

When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages. This is called hybrid inviability because the hybrid organisms simply are not viable. In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own this is called hybrid sterility.

Habitat Influence on Speciation

Sympatric speciation may also take place in ways other than polyploidy. For example, consider a species of fish that lives in a lake. As the population grows, competition for food also grows. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new food source was found at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish therefore, they would breed together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area and keeping separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them.

This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 18.21 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location but have come to have different morphologies that allow them to eat various food sources.