Homework 8. Due April 12. Please type your answers.

Speciation Homework (Chapter 15)

1. What does it mean when the authors say there are "parallel speciation events"? (To help you think about this: How many biological species are in this system, vs. how many speciation events occurred? In what way are they parallel?). Parallel speciation means that the same speciation events occur in multiple independent lineages. [For example, in sticklebacks limnetic and benthic forms (ecomorphs) in the same lake are isolated by differences in spawning habitat and sexually selected characters. The exact same reproductive isolating mechanisms arise independently in pairs of species in 6 different lakes. However, there are only 2 biological species! So the same species is arising in "parallel"]

2. 2. State, in your own words, why parallel speciation is strong evidence for speciation caused by natural selection. (Note - this is an application of the comparative method, which saw as a way to provide evidence of adaptation). Parallel speciation means the occurrence of multiple, independent, but nearly identical (in terms of the reproductive isolating mechanisms that arise) speciation events. Just as with repeated independent evolution of the same trait in the same environment, this is considered strong evidence for the role of natural selection because repeated evolution of a trait given a similar set of environmental conditions in each instance suggests causality, and selection is the only force know that could cause this. [In this case, we also have detaied knowledge of the ecological, genetic, and historical circumstances associated with the scenario.] The other force that could cause divergence, genetic drift, would not be expected to cause the same pattern of divergence in each lake. [Note: all this assumes that the same forms in different lakes are not similar simply because of migration. This is thought to be the case in sticklebacks because phylogenies show that the limnetic & benthic ecomorphs from a lake are more closely related to each other than to fish in other lakes.]

3. Use about 3 sentences to summarize the main finding of the paper and its support.

McKinnon article
1. Using microsattelite data the authors the demonstrate that anadromaous and stream ecotypes found in the same area are much more closely related than they are to fish of either ecotype from different regions, strongly supporting the hypothesis that the stream ecotype evolved repeatedly (=parallel evolution).

2. In matings of fish between regions, despite independent origin and vast geographic barriers, females of both ecotypes preferred males of their own type.

3. Mating trials using female whose size was experimentally manipulated demonstrated that size is the main determinant of reproductive isolation, although there was also some additional preference for the same ecotype even after size was altered.

[Conclusion: "Species arise mainly as a by-product of ecological differences and divergent selection on a small number of traits [such as size]"]