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3.22: Sympatric speciation

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  • 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, which was originally more controversial. It occurs when a single population of organisms splits into two reproductively isolated communities within the same physical region. How could this possibly occur? What stop (or inhibits) the distinct sub-populations from inbreeding and reversing the effects of selection and nascent speciation? Recently a number of plausible mechanisms have been identified. One involves host selection104. In host selection, animals (such as insects) that feed off specific hosts may find themselves reproducing in distinct zones associated with their hosts. For example, organisms that prefer blueberries will mate in a different place, time of day, or time of year than those that prefer raspberries. There are blueberry- and raspberry-specific niches. Through a process of disruptive selection (see above), organisms that live primarily on a particular plant (or part of a plant) can be subject to different selective pressures, and reproductive isolation will enable the populations to more rapidly adapt. Mutations that reinforce an initial, perhaps weak, mating preference can lead to what known as reproductive isolation - as we will see this is a simple form of sexual selection105. One population has become two distinct, reproductively independent populations, one species has become two.

    Questions to answer & ponder:

    •Make a model of interactions for how non-adaptive factors could influence species formation.

    •Describe the (Darwinian) cycle of selection associated with the development of the giraffe’s neck.

    •Provide a scenario that would explain why a small population associated with allopatric speciation would either speed evolutionary change or lead to extinction?

    •Which comes first, the behavior or the ability to carry out the behavior?

    •How would you model the process by which an asexual organism would be assigned to a specific species?

    •How would you go about determining whether an organism, identified through fossil evidence, was part of a new or a living species?

    •How would you determine whether two species are part of the same genus?


    104 Sympatric speciation by sexual selection: Sympatric speciation in phytophagous insects: moving beyond controversy?

    105 The sexual selection:


    • Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.