The Evolution of Sex

The unit of reproduction is the individual in the case of asexually reproducing organisms but the couple in sexually reproducing organisms.

Unless the sexual couple produces twice as many offspring as the asexual individual, there will be a cost of sex (fewer offspring per sexual parent than per asexual parent).

In the extreme case where each female produces a constant number of offspring whether she is sexual or not, then there is a two-fold cost of sex because asexual females produce twice as many female offspring.

In other words, the "sexual female propagates her genome, or any given element of her genome, only half as efficiently as the asexual female" (Bell 1982).

The Paradox of sexuality

Other costs of sex:

• Cost of finding a mate
• Risk of disease transmission in mating
• Risk of not finding a mate
• Loss of heterozygosity

The Evolution of Sex

What might offset the cost of sex?

• If males contribute to raising offspring such that couples produce more offspring than can a single female

What might pay the cost of sex?

In some fashion or other, all explanations of sex rely on the fact that sex can generate greater variability through recombination and segregation.

Fisher - Muller hypothesis

Related: Sex generates novel combinations of alleles - generating a wide variety of genotypes, some of which may have greater fitness.

Also known as the The Vicar of Bray Hypothesis after:

"an English cleric noted for an ability to change his religion whenever a new monarch ascended the throne [to emphasize] that there may be great advantages of easily and gracefully adapting to changed circumstances."

-- Bell 1986.

Red Queen Hypothesis

Since the world is constantly changing, organisms must, like the Red Queen in Alice in Wonderland, constantly adapt just to stay in place.

If sexual organisms adapt faster, they may be able to keep up with rapid changes in the environment, while asexual populations fall behind.

Asexuals may be more prone to extinction.

Parasites are an especially important part of the environment. Organisms must evolve defenses etc continuously and rapidly against viruses, bacteria, insect parasites, etc.

(When focused on the evolutionary arms race" between hosts and parasites, the hypothesis is known as the Host-Parasite Hypothesis.)

Problems with the Fisher-Muller and the Red Queen Hypotheses:

Sex and recombination can break apart new beneficial (e.g. resistant) genotypes once they arise, whereas asexuals can reproduce their genomes exactly.

Mutation and dispersal can sometimes generate and retain more beneficial genotypic combinations than sex.

Negative epistasis hypothesis

Loss of genetic variation

Sex and recombination can regenerate the missing extreme genotypes and increase the amount of genetic variance.

With this increased variability, selection is more efficient at increasing the frequency of the fittest alleles.

Muller's Ratchet

Sexual populations can regenerate these lost genotypes by recombination (as long as the alleles have not been lost).

Asexual populations cannot.

Once the least mutated class is lost in an asexual population, the population reequilibrates.

This reequilibration reduces the frequency of individuals in the next best class, which can then be lost, repeating the cycle [= the ratchet].

The ratchet will be fastest when mutation rates are high, selection is weak, and population sizes are small: Greatest advantage to sex under these circumstances.

SOURCES:

• Bell (1986) The Masterpiece of Nature. California Press.