Evolution of Sexual Selection

What is sexual dimorphisms? How can it be shaped? Provide an example of sexual dimorphism.

Sexual dimorphisms are when males and females differ in aspects of phenotype like morphology or behavior. This can be shaped by both natural selection and sexual selection. An example is the difference in size, shape, and behavior of elephant seals

What is the assumption about sexual selection that we are making?

We assign “male” and “female” labels based on the size of the gametes reproduced by the individual. We are also assuming that females have some traits and males have other traits (sexual dimorphisms).

What is the primary driver of the evolution of sexual dimorphisms?

Sexual selection

What does the strength of sexual selection depend on?

The differences in amount of parental investment per offspring of each parent

What is parental investment?

The time and energy (and other resources) devoted to producing/rearing offspring

• Energy invested in producing each gamete

• Time/energy invested in caring for offspring (parental care)

What are the assumptions we make about the majority of females in the world? What about males?

Females produce larger gametes than males, and, thus, invest more energy and other resources per gamete. Males gametes are small, with little stored energy/other resources

What does “Total parental investment” equal? Is this greater for males or females?

The energy/time investment to produce each gamete + parental care. Females almost always have more total parental investment per offspring than males

Sexual selection is (stronger or weaker) for females? Describe this sex’s reproductive success.

Stronger

• invest so much time/energy in each offspring

Female reproductive success (= # of offspring) is limited by available time, energy, and access to other resources

• Doesn’t have time for more mates

Sexual selection is (stronger or weaker) for males? Describe this sex’s reproductive success.

Stronger

• Can sire many offspring with multiple females

Male reproductive success depends mainly on success in getting mates

What are the predictions that are made when sexual selection is stronger for on sex and weaker for the other?

1. The sex with stronger sexual selection (usually males) will compete more for mates

2. The sex with weaker sexual selection *usually females) will be more choosy about who they accept as mates

   1. Bc has a limited amount of time and energy to raise offspring

What are the different types of Male-male competition? Provide an example for each.

1. Outright combat - elephant seals

   1. Or male marine iguanas

2. “Alternative tactics” like sneaking

   1. Australian giant cuttlefish

3. Sperm or Pollen competition

   1. Northern giant mouse lemur

4. Competition in attracting pollinators

   1. Flowers

5. Infanticide

   1. Polar bears

Provide an example where female choice imposes sexual selection males.

In tree frogs, females prefer males who have longer calls versus shorter calls.

What are 4 things that may drive/influence the evolution of male displays & female preferences? Provide an example for each.

1. Preexisting sensory biases favor certain male traits

   1. Golden moles are unlikely to evolve visual visual displays bc both sexes are completely blind

2. Females choose males that provide the best direct benefits (like food, defensive chemicals, territory, nests, parental care, or protection)

   1. In some species, females choose mates based on the quality of a “nuptial gift” (dead fly, etc)

3. Females choose males with traits that indicated “good genes” that will increase the fitness of her offspring

   1. In tree frogs, females prefer males with long calls because offspring of long-calling males had higher fitness than offspring of short-calling males

      1. Consistent with good genes hypothesis

4. Arbitrary preferences/traits may evolve just because displays and preferences —> assortative mating —> genetic correlations

   1. Species where there are both red and white birds. Females choose who they prefer. (assortative mating)

Overall, in the special case of traits involved in mate choice, genetic correlations between display and preference may lead to…

Genetic variation and drift

• May be all you need to evolve costly male traits

Why is the idea that investment per offspring influences the strength of sexual selection supported by “sex-role reversals”? Provide an example.

Because when males invest more per offspring than females, sexual selection is stronger for females than males. Furthermore, females tend to compete for mates and males are more choosy.

• Ex: Male seahorses invest more time and energy into each offspring than females. Thus, males are choosier than females (reversal)