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Full question: How come some similar animals are different species, while with domestic dogs, wildly dissimilar types are considered different breeds?
-- Z. Kornberg, Jerusalem
Michael Bruford, a professor of biological sciences at Cardiff University in Wales, explains the thought process behind this seeming double standard.
Scientists have been distinguishing between species on the basis of how they look, behave or live since recorded history began. However, two famous scientists stand out in terms of how we perceive species differences today: Carl Linnaeus, an 18th-century Swedish naturalist, and Charles Darwin. Linnaeus was the first person to formulate a single approach for describing species in a hierarchical manner according to their similarity, using his binomial nomenclature of genus followed by species (Homo sapiens, for instance). Darwin was among the first people, and certainly the most celebrated among them, to develop a credible theory on how species evolve (via natural selection). Both of these scientists' insights underpin most of what modern science utilizes when studying species and speciation.
Currently species are still primarily distinguished by their appearance, but it is becoming very clear that looks don't always tell us all we need to know about whether two organisms are different. Many so-called cryptic species exist that, to the untrained eye, look very similar or even identical to another organism--commonly found, for example, in nocturnal mammals such as bats and bush babies. If the two mated, however, they may never be able to produce viable offspring; this, in fact, is the primary criterion for dividing similar organisms into different species. Because of these red herrings--and also because the process of describing species is very long and labor-intensive--scientists are increasingly turning to DNA to assist them in identifying and describing species.
Indeed, there is an endeavor under way at the moment called the Barcode of Life project, which aims to sequence all living organisms for a single gene that is common to them all, to produce a species "bar code." The key is that the sequence must vary greatly among species but not vary much within species. Such a bar code can then be used to identify organisms which may not be easily identified (such as tracing back what primate was the source of mysterious smoked meat in the rainforest) and even to distinguish organisms such as microbes that we cannot see or culture in the laboratory. There has been much debate among scientists about which DNA sequence is best for this purpose, and it is likely that a different sequence will work for each different kingdom of organisms. Currently a small gene found in the mitochondrial DNA of our cells--the cytochrome oxidase subunit 1 gene--is most commonly used in identifying members of the kingdom Animalia; a huge database is already in place for many of the world's animals using this gene.
Domestic animals fascinated Darwin and continue to enthrall those of us who own pets. It is certainly curious how domestic dogs, which we know--because DNA bar coding has told us!--were raised by man from a wild gray wolf (Canis lupus) ancestor, can take on such a dramatic variety of forms. But among dogs, which are well known for their hybrid (or mongrel) varieties, different breeds can mate and have viable offspring, so they are all found under the umbrella of a single species, Canis familiaris.
Dogs are highly unusual in their variation, from the Chihuahua to the Great Dane. (Recently, body size was found to be largely explained by differences in a single gene among dog breeds.) Darwin realized that man can force selection by picking particular individuals for breeding who show a particular characteristic that we want to see in our pets. So humans can accelerate the process of selection dramatically by exploiting the diversity naturally found in domestic forms and homing in on a form that is desirable. Natural selection usually acts more slowly, relying on what Darwin described as "descent with modification"--the chance arrival of new forms through DNA mutation.
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20 Comments
Add CommentAs a biologist I am a tiny bit disappointed in this response.
Reply | Report Abuse | Link to thisThe article is correct in stating that animals are generally considered different species if they cannot produce viable offspring. More precisely, animals are different species if there is no possibility of their genes being mixed in future generations.
A great dane cannot mate successfully with a chihuahua: it is simply not physically possible. However, Great Danes can interbreed with, say, Border Collies, who can interbreed with Shelties, who can interbreed with Chihuahuas. Consequently, their genes might still be combined in future generations of dogs. So Great Danes and Chihuahuas are not currently separate species. However, they would become separate species if all the medium-sized dogs died out.
This situation, where two groups cannot interbreed directly, but other groups can interbreed with both of them, also occurs in nature. The entire mess is then called a "ring species", and it is one of many natural examples of the speciation process occurring before our eyes. The fact that we have induced the same effect in a domestic animal is really rather thrilling, especially since we have good records of the history of many breeds.
What I am really curious about is, how do creationists deal with the existence of dogs? Do they think chihuahuas and great danes are different species, separately created? Or do they think chihuahuas and great danes can interbreed?
Or do they just... not think?
This may be a dumb question, but if the criteria of whether or not dogs are part of different "breeds" instead of "species" is based on if they can create viable offspring, then why are wolves considered a different species? Can't wolves and dogs interbred?
Reply | Report Abuse | Link to thisWell kids, this is a very wrong answer. Domestic dogs of many varieties interbreed well with wolves, and the results are fertile and breed true. Lots of the USMC dogs are wolf/shepherd hybrids. They're crossbred with other working dog varieties, too, including dobermans, rottweilers, and weimeraners. To be honest, most people can't tell the wolf from the shepherd by looking, and the hybrids are just big smart dogs that get sick a little less than the purebred "dog". Personality wise, some would rather bite you than smell you, and others are just big teddy bears. I think there's some kind of genetic behavior trait that's 50/50 in the crossing.
Reply | Report Abuse | Link to thisAs for a chihuahua not able to breed with a great dane, that's just wrong too. True, a dane sire with a chihuahua bitch is going to have problems as the pups will grow too big for natural delivery. But I've seen male chihuahuas mount some pretty big females (heck, I saw a little ratdog humping a camel once, although I don't think the camel noticed, and I'm pretty sure there were no offspring), and you could always fall back to AI. The thing is, the breeding would be likely to take, and you'd have some pretty strange puppies -- probably stupid and as mean as hell from what I know about chihuahuas, but you can never tell.
Dingoes are considered another species too, but they breed with dogs and wolves just fine, though the pups bark funny. And in the southwest US, coydogs (coyote/dog hybrids) show up from time to time, and they are fertile even though coyotes have major reproductive differences from dogs. And they're 50/50 on crazy, too.
Nope, the reason different breeds of dogs are considered the same species and wolves are something else is partly historical and partly political. After all, if we recognized the physiologically distinct chihuahua and great dane as different species, we'd have to recognize asian, negro, and caucasians as different species, and that's something that Political Correctness cannot countenance. Which is a shame, since we know the different human breeds have very different medical, dietary, and social issues to contend with, but science is "racist" if it tackles the problem that way.
And if we went the other way and claimed wolves and dogs to be the same species, then we'd have to deal with the issue that we smart humans didn't succeed in making the wolf into a new species after all.
We are not as smart as we think we are.
I think you're right in stating that mechanisms of defining "species" is a political arena. After reading Darwin's "Origin of the Species" (a rather tiresome read) I am forced to conclude two things. First we still don't have a proper method for determining what exactly constitutes a different species. Second, that with enough inbreeding or crossbreeding of animals it is possible to produce different variations which in the end look like different species. Until someone can properly define the word "species" from a scientific standpoint there isn't any answer to this question.
Reply | Report Abuse | Link to this> What I am really curious about is, how do
Reply | Report Abuse | Link to this> creationists deal with the existence of dogs? Do they
> think chihuahuas and great danes are different
> species, separately created? Or do they think
> chihuahuas and great danes can interbreed?
>
> Or do they just... not think?
This is an interesting question. My scientific training is a little off the subject of evolution (I'm a physiologist), and I'm really no expert in the area, but I have been pondering this idea for a long while. What I have come to believe is that different species do change over time and that they can change so much so that they can become different from their ancestors. Nevertheless, I have difficulty concluding that if you give infinite amounts of time and perpetual natural selection that you will be able to derive all current species. In my view there has to be a "starting block" on which speciation must occur. At this point I don't believe our current scientific understanding is convincing enough to rule out the supernatural. I'm interested to hear your response on this matter.
I believe the definition of a species should refer to a fertile, rather than a viable offspring. That many species have gone extinct and that many have evolved over time seems indisputable. The notion that God planted dinosaur bones to test the faith of fundamentalist believers seems whimsical at best!
Reply | Report Abuse | Link to thisYes, the entire argument about what constitutes a species is very vague, but I'm surprised no one has spouted out some very straight forward criteria that I remember from class a couple years ago, and also surprised the article doesn't mention it:
Reply | Report Abuse | Link to thisthere's the Genetic, but also Behavioral and Physical aspects, and debatable geographical aspects for what constitutes a species:
Temporal: if they mate at different times, are active at different times, this can affect if they ever interbreed.
Geographical: are they physically able to even meet and find out of they are compatible.
Behavioral: Are they aggressive against each other, and thus would never mate. Also, maybe they have different mating rituals which prevent them from attracting one another.
Physical attraction: do they find the other group physically attractive to them to mate with.
Physical mechanisms: do they mate in the same fashion, can they even copulate in nature.
Embryonic Viability: Does the sperm and egg combine at all, or combine to form a teratoma.
Can the embryo physically leave the mother without dying or killing the mother?
Is the result fertile, able to survive in the wild, or have reduced genetic survival, best way I can describe is if that generation produces a slightly worst generation upon itself, and each generation gets less viable until they die out, more from a genetic than any other affect.
And, obviously, there's the pure genetic analysis.
Now, my point is simply that there are many, MANY ways to determine if something is a species or not. All breeds of dogs pass this criteria, and thus could be claimed as the same species. While wolves and dogs are fairly different, but still be genetically viable, but may be behaviorally so different in the wild that they would never mate.
> At this point I don't believe our current scientific
Reply | Report Abuse | Link to this> understanding is convincing enough to rule out the
> supernatural. I'm interested to hear your response on
> this matter.
First of all, I'm not a scientist, just a guy with an interest in scientific matters and hence a long time reader of this magazine and website, so do not take anything I say for the absolute truth (heck, don't take anything anybody says for the absolute truth, BTW). That being said, to the best of my knowledge science has never claimed to give all the answers all the time, merely to provide models of the way nature works. And models are by their very nature simplifications of reality - what we call a species is a handy label to apply to a group of individuals with enough traits in common, and in some cases (like the different breeds of dogs, the dingo and the wolf) it's not entirely clear (disregarding historical reasons) why the boundaries are as they are, when indeed some dogs can interbreed with wolves or dingos while other dogs can't breed with each other. The complete "book of life" would have to include every individual organism that ever lived, but that is simply not feasible, nor desirable, so science applies labels, observe groups of individuals, makes hypothesis and theories and tries to formulate, sometimes more successfully than others, general tendencies and more universal laws (we have a good grasp, but not a complete understanding, of how speciation works, some vague ideas about how life may have started, but we're pretty damn sure apples fall down, not up. "We" in this context means the scientific community at large, I'm merely an interested reader so only a marginal part of that "we").
Now, about speciation and the starting point of it, like I said, we have some pretty good theories about how speciation works (evolution through natural selection), but not a good understanding about how life began. Several competing hypothesis have been put forth, some of them more promising than others, but until somebody builds a time machine and goes back to see the first self-replicating chain of carbon or whatever we can never really be sure. My main objection against including a supernatural being in the hypothesis is that it complicates matters further (we would then have to account for the existence of this supernatural being) without really providing a satisfying answer (unless you want to believe in a supernatural being - personally I'm indifferent).
In short, science can never rule out the supernatural, but by it's very nature science can not acknowledge the existence of a supernatural being as long as a simpler explanation exists (Occam's razor). And most scientists would agree that chances are better that some organic molecules attained a sufficient level of complexity to start replicating themselves than they are that a complete supernatural being suddenly sprang (or otherwise came) into existence and started putting together those self-replicating molecules.
Hope you found this useful.
Once again, this author makes the statement that all dogs are descended from the wolf. Don't wild dogs count? Most breeds look like them-- and most mongrels revert to the type. Only a few northern breeds look like wolves.
Reply | Report Abuse | Link to this"In short, science can never rule out the supernatural, but by it's very nature science can not acknowledge the existence of a supernatural being as long as a simpler explanation exists (Occam's razor). And most scientists would agree that chances are better that some organic molecules attained a sufficient level of complexity to start replicating themselves than they are that a complete supernatural being suddenly sprang (or otherwise came) into existence and started putting together those self-replicating molecules."
Reply | Report Abuse | Link to thisThis is absurd nonsense. Scientists have nothing at all to say about the "chances" of a supernatural being existing. In order to scientifically evaluate such a question, scientists would need to be able to examine many, many universes, and test how many of them were created by supernatural beings.
As things stand in reality, scientists have as much authority in judging this question as plumbers have for judging whether Shakespeare really wrote Hamlet.
"A great dane cannot mate successfully with a chihuahua: it is simply not physically possible."
Reply | Report Abuse | Link to thisArtificial insemination? Doh!
Hello: Can I have a Gray Wolf as a Pet ! Joe
Reply | Report Abuse | Link to thisyaga bomb OMG
Reply | Report Abuse | Link to thisspartans are going south OMG ROFL
science is the BEST i love it
Reply | Report Abuse | Link to thisI am a chemical engineer, but hopefully an informed layperson in other areas. I think of "species" as a useful way to categorize the living world (so your definition might vary depending on what you are trying to accomplish). Reality is not so neat and tidy - the comments I read discuss species that exist at the same point in time - today. How about across time, do parents of one species give birth to offspring of a different species? Species blend together at the edges (in space and time) and reality refuses to fit into the neat little boxes we humans like to think within. Don't let the fact that the world is bigger than whatever box or model you have created upset you.
Reply | Report Abuse | Link to thisNew Mexican
I'm not a scientist, let alone a biologist, but I have heard this explained using the concept of "ring species"
Reply | Report Abuse | Link to thisAccording to this definition, in a group of animals such as that commonly classified as dogs where no specific genetic (that is DNA coded) barrier to interbreeding exists, if specimen 1 can interbreed naturally with specimen 2 and specimen 2 can interbreed with 3, then 1 and 3 can be considered members of the same species irrelevant of the fact that 1 and 3 cannot or will not interbreed without artificial interference.
Therefore, as long as there are intermediate types of dogs between the Great Dane and Chihuahua, they are members of the same ring species. If however all other types of dog were to go extinct, we would classify them as distinct species--at least eventually. This of course does nothing to explain the whole wolf/dog thing.
EDIT: I just realized I've repeated something already posted here, sorry about that.
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Edited by Art for Science at 05/10/2008 2:51 PM
A bit of a side topic here, but it seems to me that the general confusion over what makes a species, who defines species, the overall appearance of inconsistency in the application of species definition, etc, is in and of itself a refutation of the standard ID line of thinking. After all, if all hereditary lines poof into being as a fully developed species and can never change, then why aren't the divisions--especially over multiple generations--extremely clear?
Reply | Report Abuse | Link to thisI'm just sayin'...
Another thought: James Mallet, a London geneticist, has stated that as much as 10% of animals we consider distinct species are capable of naturally interbreeding in the wild, although very few (though not zero) in any given population actually do so.
Reply | Report Abuse | Link to thissee [url http://www.washingtonpost.com/wp-dyn/content/article/2006/05/17/AR2006051702158.html][/url]
(it's on the 2nd page)
Additionally, often the definition of a species (specifically among vertebrates) is derived from individuals selected as "type specimens" which are used as the reference for what is considered a species. This is generally accepted, since the normal processes of natural selection, sexual selection, genetic drift, etc., do not generally create large differences in a population over merely a few generations.
However, when we begin to introduce artificial selection to a species as heavily as this has been applied to the domestic dog, it seems logical that we would engender much greater individual differentiation over much shorter time than the use of these type specimens was originally intended to measure against, adding to the overall confusion particularly within this given species.
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Edited by Art for Science at 05/10/2008 4:09 PM
As a thinking person, I am a "tiny bit disappointed in this response."
Reply | Report Abuse | Link to thisDanes (and other other dogs) are a bane to Darwin and his coreligionists for, after thousands of years of intentionally selecting for diversity of form and purity of breed, you've still got the same species!
What makes you think that random, minor mutations are going to result in that first, miraculous single cell's evolving eventually into the most complex machine known to Man, himself?
Talk about "not thinking"!
"and other dogs," obviously.
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