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I was thinking that geographical isolation has nothing to do with it. You got a bunch of deer over here in the Northwest and another bunch of the same deer in the Northeast, and they never get together. Nonetheless they are the same species. (later, of course, much later, this situation might change--they evolve differently).
Mangy Tiger stated about the same as well. You are both correct. They would be separate species if they were given the chance to breed in the wild but chose not too. This would rule out geographic isolation as a
de facto criteria for determining species. As you mentioned above, geographic isolation may lead to speciation but is not speciation in and of itself.
What would be interesting is if there was a small corridor that linked Eastern and Western deer populations. It is quite possible that these two areas used to meet in the Great Plains which has since been taken over by agriculture. There is still the possibility that there is a northern corridor through Canada. I'm not sure what the real situation is but it might be worth digging into.
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As regards physical compatibility, I suppose you mean something like big dog/little dog. But they are the same species.
Dogs are a ring species, at least in my opinion. A St. Bernard can not mate with a chihuahua, but there are intermediates through which they can be linked. For instance, a huge dog breed can mate with a large dog breed; a large dog breed can mate with a medium dog breed; medium to small; and then small to miniature. So we are able to move some St. Bernard genes into the chihuahua gene pool, but it is diluted by the time it gets there. Also, when the St. Bernard genes arrive in the chihuahua population it is not able to return directly back to the St. Bernard. There are examples of this type of speciation within nature as well that involves geographic rings, such as a series of subspecies that wrap around a mountain range. While dogs are not separate species it is accurate to state that they are in the process of speciation, even if it is through artificial means.
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I don't understand why DNA incompatibility would not be the deciding factor.
Because individuals do not pick mates through DNA incompatibility. Gene pools are not kept separate strictly through DNA incompatibility. Take the apple maggot fly (Rhagoletis pomonella). From
Observed Instances of Speciation :
Rhagoletis pomonella is a fly that is native to North America. Its normal host is the hawthorn tree. Sometime during the nineteenth century it began to infest apple trees. Since then it has begun to infest cherries, roses, pears and possibly other members of the rosaceae. Quite a bit of work has been done on the differences between flies infesting hawthorn and flies infesting apple. There appear to be differences in host preferences among populations. Offspring of females collected from on of these two hosts are more likely to select that host for oviposition (Prokopy et al. 1988). Genetic differences between flies on these two hosts have been found at 6 out of 13 allozyme loci (Feder et al. 1988, see also McPheron et al. 1988). Laboratory studies have shown an asynchrony in emergence time of adults between these two host races (Smith 1988). Flies from apple trees take about 40 days to mature, whereas flies from hawthorn trees take 54-60 days to mature. This makes sense when we consider that hawthorn fruit tends to mature later in the season that apples. Hybridization studies show that host preferences are inherited, but give no evidence of barriers to mating. This is a very exciting case. It may represent the early stages of a sympatric speciation event (considering the dispersal of R. pomonella to other plants it may even represent the beginning of an adaptive radiation). It is important to note that some of the leading researchers on this question are urging caution in interpreting it. Feder and Bush (1989) stated:
"Hawthorn and apple "host races" of R. pomonella may therefore represent incipient species. However, it remains to be seen whether host-associated traits can evolve into effective enough barriers to gene flow to result eventually in the complete reproductive isolation of R. pomonella populations."
In this case the flies mate at different times when they used to all mate at the same time. They are still able to hybridize in the lab, but in the wild this does not occur because of the timing. For this reason they are cautiously considered an example of speciation.
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