Biology 106 Lecture Topic #11: SEXUAL SELECTION

I. History

A. Almost as old as theory of Nat. Sel.

B. Proposed by Darwin

1. "Descent of man and selection in relation to sex" (1871)

C. Problem

1. How can conspicuous characters evolve??

D. Common theme in animal mating systems

1. Polygyny- 1 male, many females (common)

2. Polyandry- 1 female, many males

3. Sexual Interactions

1. within sexes

2. between sexes

II. Sexual Selection = selection on traits that confer higher reproductive success on individuals over other such individuals of same sex and species

III. Components of Sexual Selection

A. Intrasexual

1. contests among 1 sex (males) for access of other (female) sex.

B. Intersexual

1. Preference of 1 sex (often females) for members of other sex.

IV. Consistency of Sexes: Why do males often compete for females & not vice versa?

A. Robert Bateman (1948)

1. Experiment: # of offspring vs. # of mates

2. Biological basis?

a. female reprod. success limited by egg prod. & resources

b. males rs limited by no. of mating events or access to mates

B. Strategy

1. Intrasexual competition

a. males should fertilize as many eggs as possible

i. mate competition

b. females should choose to mate with males that can best help rear their offspring

ii. mate choice

V. Genetic basis

A. Good genes e.g.. attractive male has low parasite loads

1. Handicap principle = If you can survive to reproduce with large expensive secondary male characteristics then you must be really fit

B. Fisher's Runaway trait model = genes cause greater exaggeration of trait in males causes greater discrimination among females

Order of events

1. conspicuous characters

2. LEADS TO RUNAWAY EVOLUTION

3. EVENTUALLY OPPOSED BY NATURAL SELECTION

VI. Take home message

A. Darwin's theory

B. Sex. Sel. explains why males often have conspicuous traits

C. Two components: 1. Mate competition, 2. Mate choice

D. Good genes vs. runaway sex. sel..

VII. Terms to Know: sex sel., intra vs. inter sex sel., mate competition and choice, Good genes vs. runaway sex sel., polygyny and polyandry