Sexual Selection

What is sexual reproduction?

The production of new living organisms by combining genetic information from 2 individuals of different types (sexes)

How are sexual reproduction and parasitism related?

Sexual reproduction subtly alters cell structure

bacteria and viruses must evolve new attack mechanisms each generation

asexual organism are quickly exploited by bacteria and viruses

How is sex defined?

Sex is defined by gamete size (sex cells)

For females the gamete is bigger

How does disruptive selection on gamete size work?

small gametes do well if it bumps into a large one

favours the extremes (large and small) over the medium

genotype starts producing large gametes which gives it a headstart but it is at danger of being exploited by the far more common smaller gametes

The small gametes need ti find the large gametes to get a larger zygote, so they developed a tail to seek them out (sperm)

What is Bateman’s principle?

the sex which invests more in the young becomes a resource for which the less parental sex competes

Why is there sex differences between males and females?

asymmetry in male/female investment → different selection pressures on males and females → sexual dimorphism (physically and behavioural)

What is the difference between monogamy and polygamy?

monogamy = one mate

polygamy = more than one mate

what is the difference between polygyny and polyandry?

polygyny = one male, several females

polyandry = one female, several males

What are the 3 different types of polygyny?

1. female defence polygyny (harem)

2. resource defence polygyny (territory)

3. lek polygyny

what is female defence polygyny?

aggressively preventing other men getting to camels

What is resource defence polygyny?

females come to a territory and male defends the border of the territory (polygyny threshold)

what is lek polygyny?

males congregate and are chosen by females

what is promiscuity?

many males, many females

What is the polygyny threshold?

The point where it is better for a female to be the second mate of a well-resourced male than the only mate of a poorly-resourced one

3 examples for human polygyny?

1. Size - larger differences in size between males and females is more common

2. Sexual maturation - females tend to arrive at puberty earlier than males

3. Mortality - male mortality exceeds female mortality

What is intrasexual selection?

one sex imposes selection pressure on other members of that sex

What is intersexual selection?

One sex imposes selection pressure on members of the other sex

What are examples of precopulatory intersexual selection?

1. dispersion (move across larger distances to find mates)

2. aggression

What are examples of post copulatory intersexual selection?

1. Sperm competition

2. Infanticide

3. Induced abortion (the Bruce affect)

What is female sexual selection based on?

Based on male phenotype because it gives clues to the genotype

Why do females choose phenotype?

1. One-off investment

2. Future investment

What are the 2 different hypothesis for male ornamentation?

1. Runaway sexual selection

2. The handicap principle

What is the runaway sexual selection hypothesis?

All features start as something useful e.g. long tail indicates a better flier, short tail may indicate malnourishment

As females decide to mate with the longest tail, the tail gets longer

Sexual selection is stronger than natural selection

What is the handicap principle?

Animals with handicaps (peacock’s tails) demonstrate their good genes by being able to survive despite the handicap