1. In the runaway process of sexual selection, what trait is under selection in the males and what trait is under selection in the females. Which male traits are favored, and which female traits are favored? Why is runaway selection an example of maladaptive evolution?

Points were awarded for each of the three parts of this question.

1 pt for understanding that ornamentation was under selection in the males, and that preference for males was under selection in females.

2pts More elaborate/showy traits in males are favored by selection over less elaborate (not just larger traits…the trait just has to catch the female’s eye in some way). More choosy females are favored over those that choose mates randomly.

2pts It is maladaptive in the eyes of natural selection…ie selection for adaptations to the physical/biotic environment. You needed to point out some cost of having an elaborate trait…survivorship is a great example. However, it is not overall maladaptive for the individual (or else it would not evolve). The drawbacks of the trait are balanced by increased mating success. However, a population in which runaway occurs can have reduced average fitness over time, when averaged over all the individuals in the population.

Below is a great answer I got from one student. Note how examples are used to help the answer.

During runaway sexual selection choosy females choose the most highly adorned males, the males with the most highly developed secondary sexual characteristics (eg peacocks’ plumage). This means that 75% of the time, highly adorned males are chosen as mates. Therefore males are selected for secondary sexual characteristic traits. Traits for choosy females are correlated to select ornate males. Therefore, if ornate males have a selective advantage, females who are choosy will have a selective advantage. Therefore the female trait under selection is their behavior of choosiness.
This is an example of maladaptive evolution because the traits selected by sexual selection to increase in the population are not those selected for by natural selection. Natural selection would not select for males which are ornate as they would be more prone to predation as they may be caught more easily (eg a peacock’s tail hinders mobility and flight). They may also be more conspicuous (peacock coloration is not cryptic) or they may invest a lot of energy in ornate features which might otherwise have been invested in size. In this way natural selection and sexual selection select for opposing traits which could increase fitness. Features selected by sexual selection to increase fecundity may be maladaptive as they may decrease survivorship.


2. How is the t-allele in mice a selfish gene? How is the force of genic selection balanced by the process of individual selection in this example? How does the process of group selection favor non-t-allele mice?


1pt The t-allele is a selfish gene because the gene spreads despite the fact it has deleterious effects on the individual.

2pts Individuals that are tt are sterile. This is how individual selection is balances genic selection...the t allele can never fix in the population because all tt individuals die. (This is a case of overdominance where heterozygotes have the fittest genotype for the t allele gene).
Credit was also given to those who pointed out that female mice often prefer mice that lack the t allele (as this is a case of selection against those individuals with a t allele). However this is more of an evolutionary mechanism that stops the spread of the allele. Pointing out that the tt individuals are sterile better answers the question.

2pts This is a case of group selection because different groups (of mice) have differential fitness. Since groups of mice are typically small, random chance (ie genetic drift) can lead to some groups having only tt males. Since these mice are all sterile the group goes extinct. Groups without the t allele do not have this disadvantage, and thus groups of non-t allele mice are favored (which increases the frequency of the non-t allele in the overall, metapopulation).

Below is a really good answer that I got. Note how it directly and succinctly answers the questions.

The t-allele in mice is a selfish gene in that it works to spread through a population despite the fact that homozygous males are sterile, thus severely curtailing their fitness. In this way individual selection helps curtail genic selection because of the decreased fitness of individuals with the two t alleles.
Group selection also favors non-t allele mice. Mice live in small groups, generally 2 males and several females. If the t-allele spreads too much, eventually both resident males will be homozygous for the t-allele, and thus be sterile. That group will die out. Group selection would favor groups that have contained the spread of the t-allele, because these groups would have fertile males.