Lecture 9: Sexual Selection

Question 1

Observations regarding sexual selection:

Answer 1

1. Many species exhibit sexual dimorphism
2. In the great majority of these species, males are the more elaborate
3. The elaborate trait often reduces survival
4. The elaborate trait is often not expressed until after sexual maturity
5. The elaborate trait is usually only used during the breeding season

Question 2

What is Sexual Selection?

Answer 2

Sexual Selection is differential reproduction due to variation in the ability to obtain mates (or mates of a certain kind/quality)

Question 3

What is the significance of Sexual Selection?

Answer 3

Produces traits disfavored by survivorship selection, but the critical role of sexually-selected traits in mating success offsets this reduction

Question 4

What are the types of sexual selection?

Answer 4

1. Male-male competition (i.e. intra-sexual selection)

2. Female choice (i.e. inter-sexual selection)

Question 5

What are examples of intra-sexual selection?

Answer 5

1. Direct combat
2. Sperm competition (i.e. removal of sperm from previous males)
3. Infanticide

Question 6

What are examples of intra-sexual selection?

Answer 6

1. Direct combat
2. Sperm competition
3. Infanticide

Question 7

What is the advantage to inter-sexual selection?

Answer 7

These sexually-selected traits often reduce the survivorship of the individuals carrying those traits; while there is no personal gain for the female, there is potential for the transfer of good genes from father to son

Question 8

What are the 2 components of sexual selection under female choice?

Answer 8

1. Selection on female preference for male trait
2. Selection on male trait for female preference

Female preference and male traits evolve together

Question 9

What is definition of Direct Selection for female preference?

Answer 9

Direct selection for female preference affects female survivorship and fecundity

Question 10

What are examples of Direct Selection for female preference?

Answer 10

A female choses a mate that provides her with critical resources:

1. Good territory
2. Protection
3. Prenuptial gifts
4. Help with care for offspring
5. Disease-free mate

Question 11

What is the significance of Direct Selection for female preference?

Answer 11

Direct selection on female preference dictates the male trait in the population –> population stable

It is important when heritability of the sexually selected trait is low, and when variance in offspring fitness is mediated by paternal effects

Question 12

What is definition of Indirect Selection for female preference?

Answer 12

Indirect Selection for female preference does not affect female survivorship and fecundity, but is genetically correlated with the male trait under sexual selection

Question 13

What is the significance of Indirect Selection for female preference?

Answer 13

Under indirect selection on female preference, the population can move along the line –> population unstable

Question 14

How are female preference “genes” and male trait “genes” correlated?

Answer 14

Direct selection on male trait selects indirectly for female preference genes

Question 15

Describe the Fisherian runaway model.

Answer 15

For direct selection:

1. Female evolves preference for a type of male that betters her fitness
2. Female preference will then directly dictate what male traits are present in a population
3. Population is stable

For indirect selection:

1. Population and preference can arbitrarily move along the line without affecting survivorship and fecundity
2. Population is unstable

Female preference genes and male trait genes are genetically correlated; females expressing preference have sons with the preferred trait (“sexy sons”), but also genetic material for female preference (not expressed in sons)

Question 16

Discuss the good genes model.

Answer 16

Female preference genes are genetically correlated to both male trait genes and with good genes