Terms to know: sexual reproduction, asexual reproduction, recombination, the two-fold cost to sex, group selection, species selection, individual level natural selection

Questions:

1. Sexual reproduction results in much more genetic variation, because of recombination, than does asexual reproduction.  As a result, a species with sexual reproduction is likely to have a higher ability to adapt to changing conditions than is a species with only asexual reproduction.  Why do these facts NOT explain the evolution of sexual reproduction?
2. What are the costs to sexual reproduction?  Which of these costs is likely to be the main cost, in that it is necessarily true in all sexually reproducing organisms?
3. Explain why it is true that an allele for asexual reproduction is expected to be reproduced twice as much, over some set time period, as an allele for sexual reproduction.
4. A variety of species, including planarians, several aphids and the microscopic crustaceans Daphnia, reproduce through parthenogenesis (asexual production of female offspring by females) during the spring and early summer when conditions are relatively constant, but reproduce sexually at the end of summer when producing offspring that will be dormant over winter and emerge the next spring.  Give a hypothesis for the evolution of sexual reproduction that is consistent with these data.
5.  
1. Explain why species that have parasites are predicted to be more likely to evolve sexual reproduction than are species with few or no parasites.


The following situations are real examples that have been used to support the hypothesis that parasites favor the evolution of sex in their hosts.  Explain how each supports the hypothesis, and also think of an alternative hypothesis that could explain the same situation:
 

2. A species of snail can reproduce sexually or asexually.  When populations of a species of snail are compared, in populations with high levels of parasites most individuals reproduce sexually, while in populations with few parasites most individuals reproduce asexually.
3. Several species of lizard have originated as hybrids between two other species.  The parental species reproduce sexually, the hybrid-origin species have problems with meiosis because of their hybrid origin and can only reproduce asexually.  In the asexual species, there are more parasites than in the sexual species.
6. How might sexual and asexual species be expected to differ in speciation rate? Why? How could this difference help to explain why so many species have sexual reproduction?
7. The fact that many species reproduce sexually has been explained through selection at the individual level, group level, and species level. Give all of these explanations. Which one is unlikely to be true?. Could either of the more likely explanations be the only explanation for the evolution of sex? Which? Are these explanations mutually exclusive (i.e. if one is true, does it mean the other is not)? Why/ why not?