Sexual Selection

Chapter 9

I) Sexual Dimorphism – males and females are different.

A) Sexual Dimorphisms are difficult to explain

1) If characteristic is good for the male why not everybody (i.e., female)

2) Characters often appear to be maladaptive

a) bright red coloration and long tail feathers in birds

II) Sexual Selection

A) Definition – differences among individuals in attracting mates

B) Sexual reproduction is asymmetrical between males and females

1) Females generally make larger parental investments

a) larger cost in producing gametes

b) increased energy in producing offspring

c) increased energy in caring for offspring

d) reduced ability to produce other offspring

2) Lifetime reproductive potential is lower in females

a) because of the above cost differences, females are limited in offspring production by the number of eggs (or pregnancies) she can have

b) males lifetime reproductive success is high, but limited by the number of females he can mate with.

3) Access to males is not a limited resource for females but access to females is a limited resource for males (i.e., females run the show).

4) consequences of asymmetrical fitness limits

a) males will compete for females

b) females will chose mates carefully (i.e., choosy)

C) Male – male competition or Intrasexual competition – males compete with each other for access to mates.

1) Combat – males physically interact with each other

a) gain physical access to females

b) hold limited resource (e.g., food) necessary for female

c) generally results in selection for larger body size, aggressive behavior, weaponry, armor, etc.

2) Sperm competition – competition can be among the sperm from different males after copulation

a) when females are inseminated by more than one male, the male leaving the most sperm has higher reproductive success

b) can result in strategies like males may remove other males sperm, block future copulations, prolong mating, etc.

3) Infanticide – when social structure is effected by outcome of male-male competition, new males kill any young they did not father.

a) reduces females reproductive success, but may not be avoidable.

4) Selection for alternate strategies

a) sneaker males – small, female looking reproductively viable males.

D) Female choice – females select characteristics in males that increase their own reproductive success.

1) Good Genes hypothesis – females may chose males that have the best, most fit genes so that the offspring will have good genes.

a) Direct display – vigorous, good looking males are that way because they are genetically superior (e.g., feather color intensity and mite density in cliff swallows)

b) Handicap – any male that can maintain an energetically expensive or risky character is more fit.

2) Good Father hypothesis – females chose males that can best provide for their offspring

a) large territories with lots of food, best nest, most devoted, etc.

3) Preexisting Sensory bias – females preference for a particular characteristic evolved first for different reasons then male mating display followed (frog calls and species identity).

4) Sexy Son hypothesis – female choice can be arbitrary relative to current reproductive success (i.e., fitness).

a) once a particular male characteristic is favored by females then selection on females will increase the frequency of the favorable trait (i.e., females will produce sons that are more attractive to females)

b) Runaway Sexual Selection – expensive and/or elaborate male characteristics may be preferred by females but result in a lower mean population fitness than if not.

E) Exceptions prove the rules – in cases where the initial assumption of access to males not being a limited resources for females but access to females is for males is known to be false, predictable outcomes are observed

1) Emperor penguins – males and females share rearing and care and there is no sexual dimorphism

2) Pipefish – males incubate and rear young and females compete for access to males.