In the summer of 1995, at least 15 iguanas survived Hurricane Marilyn on a raft of uprooted trees. They rode the high seas for a month before colonizing the Caribbean island, Anguilla. These few individuals were perhaps the first of their species, Iguana iguana, to reach the island. If there were other intrepid Iguana iguana colonizers of Anguilla, they died out before humans could record their presence.
|Evolutionary biologists would love to know what happens next: will the colonizing iguanas die out, will they survive and change only slightly, or will they become reproductively isolated from other Iguana iguana and become a new species? We could be watching the first steps of an allopatric speciation event, but in such a short time we can't be sure.|
A plausible model
We have several plausible models of how speciation occurs - but of course, it's hard for us to get an eye-witness account of a natural speciation event since most of these events happened in the distant past. We can figure out that speciation events happened and often when they happened, but it's more difficult to figure out how they happened. However, we can use our models of speciation to make predictions and then check these predictions against our observations of the natural world and the outcomes of experiments. As an example, we'll examine some evidence relevant to the allopatric speciation model.
Scientists have found a lot of evidence that is consistent with allopatric speciation being a common way that new species form:
|Spotted owl subspecies living in different geographic locations show some genetic and morphological differences. This observation is consistent with the idea that new species form through geographic isolation.|
|Diane Dodd's fruit fly experiment suggests that isolating populations in different environments (e.g., with different food sources) can lead to the beginning of reproductive isolation. These results are consistent with the idea that geographic isolation is an important step of some speciation events.|