From Biol301

Contents 1 Sexual Selection 1.1 Darwin's observations 1.2 Revisiting Fitness 1.3 Assymetric Costs 1.4 Male-Male Competition 1.5 Female Choice 1.5.1 Three (3) Theories of How Female Preferences are Established 1.5.1.1 Fisherian (runaway) sexual selection 1.5.1.2 Sensory Bias 1.5.1.3 Indicator

Sexual Selection

• occurs due to competition over mates
• Darwin's natural selection doesn't explain beauty and other traits that limit ability to survive (big feathers make one easier to see / capture, etc.)

Darwin's observations

1. There are many traits that seem to reduce fitness.
2. Sexual dimorphism: make and female differ in phenotypes

• Primary Sexual Characteristics: organs or structures directly involved in reproduction.
• Secondary Sexual Characteristics: traits not directly involved in reproduction; everything else.
  • Theory of sexual selection attempts to explain the secondary sexual characteristics

Revisiting Fitness

• Fitness defined by two measures

1. Ability to reproduce
2. Ability to survive

• The ability to reproduce involves a mate, therefore it is not random.

Assymetric Costs

• Sperm are cheap and eggs are costly; therefore, there are limitations on the female.
• Bateman (1948) showed the reproductive success of Drosophila
  • Males reproductive success increased over a series of mates
  • Female reproductive success plateaued after one mating.
• There is, therefore, a beneficial strategy for each sex:
  • Males should attempt to mate as many times as possible with as many mates as possible. Males will have to compete with other males to do this.
  • Females should be choosy and mate once with the best partner.

Male-Male Competition

• Also Known As (AKA) direct or intra-sexual competition.
• Four traits develop in competing males:

1. Size
2. Weapons: antlers, etc.
3. Sperm competition: remove another males sperm from female; plug the female; apply male hormones to mitigate chances of another male coming along and removing one's own sperm; etc.
4. Behaviors, such as infanticide: kill off weening offspring to bring females back into heat.

Female Choice

• AKA indirect or inter-sexual competition
• Leads to ornaments (beauty); elaborate coloration; displays; etc.

Three (3) Theories of How Female Preferences are Established

Fisherian (runaway) sexual selection

• This is a coupling of a male display gene with a female preference gene.
  • Example: Lions with long tails give birth to males with long tails and females who like long tails.
  • It is called runaway because it forms a positive feedback loop and can quickly lead to really long tails.
• The trait is arbitrary so you cannot know before it happens which trait will be pronounced.

Sensory Bias

• Requires the female preference to occur before the male trait.
• Example: Female swordtails choose mates with longer tails. A closely related species called Plattyfish do not have tails. However, when male plattyfish have tail added (artificially), female plattyfish show a preference for the tailed male plattyfish. Therefore we can see that the preference for long tails came before long tails (from an evolutionary / phylogenetic tree view).
  • We drew a phylogram of this in class.

Indicator

• The idea that males have traits that indicate what quality they are as a mate.
• There must be an honesty mechanism here somewhere.
• Hot topic in biology.
• Indicators could represent: ability to find food; carrier of good or bad genes; resistance to parasites; etc.
• Examples of indicators: vigor (can call longer, etc.); complexity or song; size; frequency of call (larger birds usually call at a lower frequency); etc.
• Dr. Freeland gave us this link [1] to a recent article on indicator theory.