Lecture 24

Topics

• Speciation

• Character displacement

Study Sources

• Chapter 22 pp. 444-445; also Fig. 22-26 in Ricklefs.

• Study Guide: Competition

The Main Points

• The question of how new species come to be formed from existing species was brought to clear focus with Darwin's Origin of Species.

• The problem was that "selection" seemed to get rid of novel phenotypes and preserve others. As long as heterozygosity was creating new phenotypes there would be variants added at the extremes but these would be few and hardly seemed the way new species would be produced.

• The "mutationists" held that new species could be produced only by a large and rapid change in the phenotype such that backcrossing would be very difficult.

• Proponents of the two views held tenaciously to their views but by the end of the 1930's a new synthesis appeared with the work of Fisher, Huxley and others, that incorporated selection, mutation, population genetics, population ecology and statistics.

• Central to the modern synthesis was the idea that isolation of segments of a gene pool must occur before speciation may proceed. In isolation (let's say after a barrier forms across the range of a species), the smaller gene pools are subjected to the selection pressures of the local environments and since they are smaller, are more subject to rapid changes due to selection. Given enough time, these subunits of the original gene pool will become different. The only question is, How different? The test comes when contact between the subumits is again established.

• At this point, two things can happen
  1. Individuals of the subunits may resume mating and produce offspring which can backcross with the parental populations. Thus, gene flow is once again established and the merging produces a large gene pool with the sum of all the variation the two subunits have produced in isolation.

  2. Resumption of mating may or may not occur, but more importantly, individuals of the subunits are no longer interfertile. At best, they may mate and produce sterile hybrids and at worst, they may not even recognize each other as potential mates. In either case, no genes can flow between the two subunits. We look then for the isolating mechanisms, i.e. the specific behaviors, anatomical differences, chemical imcompatibilities, developmental discontinuities, etc. that now interrupt the gene flow.

• If there is no gene flow between the two subunits than the frequency distributions of the "new species" can be drawn separately. You should do that now. Draw a figure with an array of phenotypes on the X-axis and Frequency of occurrence on the Y-axis. You can assume slightly different ranges of phenotype and put both distributions on the same axis. Draw two of these, one for "species" populations (I'll just use Populations from now on) living together in the geographical region of contact (Sympatry) and one for the populations living apart (Allopatry).

• In allopatry, the populations will retain the differences that have developed in isolation. In sympatry, selection pressures may cause the two populations to become, on the average, different. The condition where two species are more different in sympatry than in allopatry is called character displacement. How can this happen?

• In sympatry, phenotypes that held in common by the two populations may experience competition for limited resources. When that happens, those phenotypes will experience the negative effect by having lowered reproductive potentials. Consequently, the numbers of those phenotypes in both populations will decline. (Draw some arrows on your figure showing declines in the phenotypes held in common by the two populations). The effect of these declines is that the modal phenotypes of both populations move in opposite directions. This is simply directional selection occurring in both populations. What we actually see, of course, is that, on the average, the two populations are more different than they were when they first came into contact.

• The same thing can occur if the phenotypes of the two populations that are most similar don't recognize the difference, successfully mate and produce sterile hybrids or hybrids that die in development. These matings have the effect of lowering the biotic potential for those phenotypes and their numbers decline. Again, as with competition, the observed effect is directional selection.