EVOLUTION (11:704-486)-SPECIATION SMOUSE –SPRING 2011 1SPECIATIONPreliminaries:One of the problems Darwin did not solve was that of speciation. He viewed species as particularly well-marked varieties, more differentiated than subspecies or varieties and less differentiated than genera. Darwin may have had it right; we may be making too big a fuss! Modes of Speciation– But for one interbreeding population to become two populations, each breeding freely within itself (but not with the other), would notseem to be a trivial process. Is that just a consequence of other changes or is reproductive isolation the point of the process? Futuyma describes four (4) modes of speciation, as in his Figure 16.1. I will spend the rest of our time talking about the four modes and various wrinkles on these themes, but this figure kind of says it all. Don’t lose this graphic, because it tells the story pretty well. Allopatricspeciation– The consensus is that speciation mostly occurs in allopatry, with subsequent populations unable to exchange genes. In two areas, genetic complexes emerge, gradually acquiring genetic isolation, while in the process of divergence. Peripatricspeciation– There is a small variant - peripatric speciation. A small population is isolated and diverges from the rest of the species, founder effect. Divergent popns later come back together, but they are found to be reproductively isolated, as an incidental side consequence of the separate adaptation to their differential ecological circumstances. Parapatricspeciation– For this version, the two forms are found “side by side” in adjacent habitat. The idea is that the process begins with divergence in allopatry, but then these diverging forms come back together, before they are completely isolated. Reproductive isolation then evolves under the influence of natural selection, across a narrow hybrid zone. Sympatricspeciation–The idea is that the selection for two very different adaptive optima is so strong that it can happen in the same location. While genetic exchange creates some problems, selection against hybrids is so strong that reproductive isolation develops in situ. People are still arguing about this one, and I’ll have more to say about it later.
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