Homework 7. Due April 2 in class. Please type your answers. Note: this homework isn't very long, but the study guide that's been online since last week has a lot of sexual selection quesitons. Get to work!

Sexual Selection. Three points each, a bonus point for the last question (so you can earn 16 out of 15!).

1. Under what conditions is sexual selection likely to be driven by female choice rather than male competition?When males are unable to monopolize access to females, either directly or indirectly through controlling resources required by females, then male-male competition is likely to be unimportant. When the number of offspring a female can successfully raise is more limited by resources than by access to mates, she should be choosy. [Obviously, if she choose a male that brings resources, this decreases resource limitation - but choosing a male with better genes, or even choosing multiple males to increase offspring fitness, may also be thought of as optimizing reproductive success under resource limitation].
   

1. The remaining questions are in reference to the following article:
   Reusch et al . 2001. Female sticklebacks count alleles in a strategy of sexual selection explaining MHC polymorphism. Nature 414: 300-303.
   Some terms you may need:
   -Disassortative mating: Mating between disimilar phenotypes or genotypes.
   -SSCP: A method of identifying matching and non-matching DNA sequences (can be used to determine # of alleles and which alleles are shared).
   - ANOVA: A method for determining whether a treatment effect is statistically significant. If p<0.05, then there is a significant effect of the treatment or factor.

2. The title suggest sexual selection is occuring.
a. What trait is experiencing sexual selection? Sexual selection is acting on number of MHC alleles.[However, the situation isn't entirely straightforward. Note, that the response to selection may be constrained by a) the maximum # of alleles. In addition, its seems that females seem to choose an optimal #, which is a form of stabilizing selection].
b. Why is it sexual selection? Because there is intersexual selection - females are choosing males based on male traits [again - its not entirely straightforward if they're actually choosing an optimal number of alleles, the number of which depends on a female's own number of alleles].

3. Which model of female choice is supported by this study, and is it related to the Hamilton-Zuk hypothesis (Explain)? The results support the good genes model of female choice - females gain a higher probability of defense against pathogens for their offspring. In addition, it supports the assertion of Hamilton and Zuk that disease resistance genes may often be the good genes that females gain. Because the identity of the most advantageous disease resistance alleles will change over time (as pathogens evolve in response to hosts), the good genes that females gain will not be driven to fixation by female choice. This explains how sexual selection can continue indefinitely.

4. In the Galapagos marine iguana case, large males gain more mates, but ability to meet metabolic demands sets an upper limit on male size. In the Stickleback case, do the authors think selection sets an upperbound on number of alleles? Yes, if an individual has too many different alleles it increases the risk that self-peptides will mount harmful autoimmune responses.

5. Summarize the main result(s) of this paper and their significance.Female sticklebacks tend to mate with males that have high numbers of MHC alleles, unless the female herself has a very high number of alleles. This phenomena does not seem to be driven by inbreeding avoidance, since female sticklebacks do not choose males that have disimilar MHC genotypes (unlike the results from some experiments in humans and mice). Nor does it seem to result in overall higher genome-wide heterozygosity. The authors conclude that this behavior increases individual heterozygosity and population-wide polymorephism, and suggest it is ubiquitous among vertebrate animals. [Key result: Females select males based on number of MHC alleles, and not based on MHC dissimilarity].

6. State your own opinion or thought about this study. Bishop says "What's not to like?". As we'll see in the coming weeks, these results dovetail beautifully with other aspects of sexual selection, play a role in speciation too. In addition, this system is beautiful because a) both the ecological and genetic factors involved in these evolutionary processes can be studied, and b) The worldwide distribution of sticklebacks makes them a very accessible study organism.