3.21: Sympatric speciation

While the logic and mechanisms of allopatric speciation are relatively easy to grasp (we hope), there is a second type of speciation, known as sympatric speciation. Its mechanism(s) were originally more controversial. Sympatric speciation occurs when a single population of organisms splits into two reproductively isolated communities within the same physical region. How could this occur? What stops (or inhibits) the emerging sub-populations from inbreeding? How might these subpopulations become reproductively isolated? A number of plausible mechanisms have been identified. One involves host selection.^[161] In host selection, animals (such as insects) that feed off a specific host may find themselves reproducing in distinct zones associated with their hosts. For example, organisms that prefer blueberries may mate in a different place, time of day, or time of year than those that prefer raspberries. There are blueberry- and raspberry-specific niches, and organisms that specialize to one or the other may have a reproductive advantage when they restrict themselves to that food source. Through a process of disruptive selection (see above), organisms that live primarily on one particular plant (or part of a plant) can be subject to different selective pressures. Reproductive isolation will enable the populations to "stay evolutionarily focussed" and so adapt more rapidly. Mutations that reinforce an initial, perhaps weak, mating preference can lead to reproductive isolation. One population becomes two distinct, reproductively isolated populations, one species has become two.

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