Wednesday, March 17, 2010
Dog Diversity
On pages 125 to 126, Jerry Coyne discusses the large variety of breeds found in dogs today. Having stated that the dog species as a whole "comes in all shapes, sizes, colors, and temperaments," it makes sense that these numerous breeds would be considered separate species if found in fossil form at a later date, as the wide variation of traits would lend itself to that conclusion. However, it is quite clear that "dog" is still a singular species today, as shown when one breeds a purebred toy poodle with a purebred pug. Both very different in terms of traits, yet they still can make "puggles" no problem at all. Yet, it wouldn't be such a good idea to breed a toy poodle with, say, a great dane as the fetus of a great dane - poodle cross would certainly not be able to develop in such a small dog. They are still breeds of the same species, so how could they not reproduce? How could this be? The implication behind this is that not all breeds of dogs can mate with one another successfully, so some separation exists within the species. When does that line between "breed" and "species" get crossed? It would be wise to take your response beyond a simple "when they can't reproduce with each other anymore" as the line is obviously blurred in the case of dog breeds.
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The line between "breed" and "species" gets crossed when one breed is unable to reproduce with any other dog belonging to the current species of Canis lupus familiaris. Of course, there are going to be instances when the size of the dog influences who it can and cannot mate with. All dogs are genetically different, but they still remain the under the same species because although there are selective advantages, these advantages have there own categories. For instance, breeders have be able to create the 150 different breeds we see today. Why so many one may ask? Since humans are in control of the development of new breeds, they can create whatever they desire. A great example is the grey hound, which was bred for racing by selecting its streamlined shape and long legs.
ReplyDeleteJerry Coyne explains that "If evolution meant only gradual genetic change within a species, we'd have only one species today- a single highly evolved descendant of the first species. Yet we have many: well over ten million species inhabit our planet today, and we know of a further quarter million as fossils" (5). This is obviously not the case because of speciation, one of the six components of evolution. This component involves the splitting of one common ancestor into multiple descendant species over millions of years that could no longer reproduce with one another. The way this begins is with a variation (mutation or sexual reproduction) in a population in which one species is given a selective advantage over another species in the environment. The organisms possessing this selective advantage are able to survive and reproduce passing along the advantageous trait, while the other organism slowly dies off and becomes extinct. Then over time, some form of the advantageous trait appears in the entire population, with variation. After an immense period of time and many generations with events of natural selection for numerous advantageous traits, the population becomes so genetically different that it could no longer breed with the original, which is most likely extinct. Dog breeding most likely began with the Eurasian gray wolf and has been going on ever since for almost ten thousand years. The artificial selection of domestic dogs, took "Only 0.1 percent of the time that it took wild dog species to diversify from their common ancestor in nature" (126). Artificial selection is very similar to natural selection except for the fact of "who's in charge" of the evolution of the species. In artificial selection, it is human desire that drives the gradual change while nature drives natural selection (hence its name).
All in all, there are some separations that exist within the species of dogs and due to the amount of time we have been breeding them (<10,000 years) we still have a long way to go until speciation will occur.
As stated in the first post, the line between species and breed is crossed when one individual of one type is unable to produce fertile offspring with an individual of another type. However, the reason why dog types are considered breeds even if they are unable physically to reproduce together is because this rule applies to the genetic ability of organisms to reproduce. Consider the opposite scenario of a poodle impregnating a great dane; the resultant offspring obviously shows that the two breeds are able to reproduce to produce a fertile individual. Just because physical attributes may constrain the ability of two individuals to reproduce does not mean they are of different species.
ReplyDeleteAs Coyne mentioned on page 126, turkeys have become so genetically altered to have large breasts that males are unable to even mount females and thus females must be artificially inseminated to reproduce. Such physical barriers do not undermine their common status as species, however; genetically, they are still able to produce a fertile offspring.
However, there is some controversy over what constitutes a 'species' in the scientific community. Some argue for the biological specification, in which individuals must be able to produce a fertile offspring, although some discrepancies do arise among asexual species and fossils as well; as Coyne explained, if scientists millions of years later were to discover the fossils of our own diverse dog breeds, they would likely categorize them as different species. Others argue for a phylogenetic specification, in which the two types must have distinctly different characteristics and traits that can not be subdivided into smaller categories. However, once again, there are problems with this categorization as some species can have wildly different physical attributes but are still able to reproduce to form fertile offspring.
In the end, I suppose it depends on what your view of what a 'species' really is, though I believe that the former description is more accurate and thus dog breeds would not fall into seperate species categories.
Sources:
http://www.utm.edu/departments/cens/biology/rirwin/391/391Species.htm
http://www.sciencedaily.com/releases/2003/12/031231082553.htm
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