1 Week 9 - Speciation Define: gene flow, reproductive isolation, assortative mating Define the Biological Species Concept. Why is reproductive compatibility a logical criteria to use when naming species? Describe a situation where the Biological Species Concept is difficult to apply. Explain the difference between allopatric vs. sympatric speciation How do (a) genetic drift, and (b) differential selection, contribute to allopatric speciation? What kind of selection did Thoday and Gibson use on fruit flies in their experiment on sympatric speciation? What was surprising about their results? Why have most studies not been able to repeat their results? Under what conditions is sympatric speciation theoretically possible? Why? Why does size-assortative mating evolve between benthic and limnetic morphs of lake fish? What is selection acting against when it drives the evolution of strong assortative mating? List 4 traits that may contribute to pre-zygotic reproductive isolation. Be able to explain graphs showing greater pre-zygotic isolation among presently co-occurring (sympatric) pairs of species than among recently diverged allopatric pairs of species. Why does pre-zygotic isolation rapidly evolve when species co-occur in sympatry? Why don’t allopatric species pairs show pre-zygotic isolation when you put them together? Which do you expect to show more post-zygotic isolation - pairs of allopatric or sympatric species, or neither? Why? What causes post-zygotic isolation, anyway? You compare the DNA sequences of a protein-coding gene from two related species, and note that most of the nucleotide differences will cause amino acid changes between the species, whereas relatively few are silent substitutions. How can amino acid changes accumulate faster than silent substitutions? Wouldn’t you expect most amino acid changes to be eliminated be selection, compared to silent substitutions? What are gamete recognition proteins? Why does positive selection often act them?