Isolation---by distance as well as barriers?

Hi, InGodITrust.Welcome to EvC! There is an ecological phenomenon referred to as the Allee effect, which is that reproduction decreases as a species's density decreases.So, if you have a population that is spread out over a very wide area, and has different densities in different parts of its range, you will find little interbreeding in low-density areas and more breeding in high density areas.Thus, you can have new mutations build up quickly in the densely populated areas, and diffuse very slowly through the sparsely populated areas. So, two densely populated areas separated by a wide stretch of sparsely populated area can accumulate mutations that don't cross between populations very readily.-----Similar to what Dr Adequate is saying, many species of fly and non-stinging wasp lay their eggs inside a host plant, and the larvae develop inside the host (these insects are called "parasitoids"). Some of these parasitoids may use multiple different host species. But, each plant will exert different selection pressures on the insect, so it's possible that those laying eggs in different plants will start to diverge.I supplied a link to a discussion about the topic of parasitoid flies a few months ago: Speciation in Fruit Flies.-----You can also have speciation by sexual selection.If females of a species prefer either of two mutually-exclusive male phenotypes (external traits), you can easily restrict gene flow between male types, provided the female preference is genetic (and there are many cases when it is).-----There are probably many other examples people could give, too, but these are sufficient for now.

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