Sex and Sex Ratios


I. Why sex?

A. Costs

1. Cost of making males - 2-fold penalty every generation
2. Cost of meiosis (outbreeding) - breaks up coadapted gene complexes

B. Benefits

1. Prevents mutation accumulation (Mullers ratchet) - impt if mutation rates high
2. Increases rate of evolution by creating novel genotypes

a. possible continual source of environmental variation is parasites (the Red Queen)

b. many asexual forms have sex when environment changes: i.e. aphids, daphnia when cold, dry or light changes

II. Consequences of sex

A. Evolution of two genders = anisogamy

1. Disruptive selection

If there is variation in gamete size, then small gametes will be more motile and compete better for access to other gametes while larger gametes will produces more viable zygotes. Expect disruptive selection on gamete size. Human egg = 200,000 times human sperm size! But, some Drosophila have sperm over 20 times their body length!!


2. Minimize genomic conflict

Small sperm are less likely to carry cytoplasmic parasites and organelles

B. Sex determination

1. Genetic, e.g. chromosomal or chromosomal region

a. heterogamety - male -> mammals, flies; female -> birds, butterflies
b. haplo-diploidy - hymenoptera (ants, bees & wasps), thrips, some beetles

2. Environmental, e.g. incubation temperature

3. Social, e.g. many fish undergo sex reversal depending on mating system


C. Sex ratio allocation

1. Fisherian sex ratios -- 1:1 is stable, noninvadable strategy if investment in each sex is equal
2. Unequal numbers of females and males when:

a. Local mate competition - avoidance of sibling competition for mates

b. Local resource competition - due to restricted dispersal
c. Maternal condition in polygynous species, or the Trivers/Willard effect