Genetic Associations and the Evolution of Sex

Sex can only mix genes (alleles) at different loci. It does not create new genes, just new combinations of existing genes. The affects of sex can be understood by considering just two gene loci A and B with two alleles at each gene locus: A₁, A₂ and B₁ and B₂. There are 4 combinations of alleles at these two loci A₁B₁, A₁B₂, A₂B₁, A₂B₂. If all these combinations are present in their expected frequencies, then sex has no effect. In this situation, at best sex is neutral, at worst, because of the costs of sex, sex is disadvantageous. Sex can only recombine the 1 and 2 alleles at the two loci. It can produced mixed associations (A₁B₂ or A₂B₁) from pure associations (A₁B₁, or A₂B₂) or it can produce pure associations from mixed associations. That is all sex can do at the genetic level as far as its affects on genetic variation is concerned. So for sex to be interesting the population must be unmixed. In other words, there must be more, or less, mixed associations (A₁B₁, or A₂B₂) than expected. The different theories in Chapter 5 are just different ways of creating mixed associations (A₁B₁, or A₂B₂), so that sex can generate the pure associations  (A₁B₁, or A₂B₂) and have an effect.

Muller’s Ratchet

Let us assume the 1 alleles are non-mutant and the 2 alleles are the deleterious mutants. We start out with non-mutants A₁B₁. Mutation occurs and so we have A₁B₁, A­₁B₂, and A₂B₁. The double mutants A₂B₂ are so rare they can be ignored. The least loaded class A₁B₁ is lost by chance and so we are left with A₁B₂ and A₂B₁. Now sex can produce the non-mutant A₁B₁ class.

Fisher-Muller (Vicar of Bray)

Lets us assume the 2 alleles are the advantageous mutants. We start out with non-mutants A₁B₁. Mutation occurs and so we have A₁B₁, A₁B₂, and A₂B₁. The double mutants A₂B₂ are so rare they can be ignored. A₁B₁ individuals are less fit and so their frequency declines and again we are left with A₁B₂ and A₂B₁. Now sex can produce the super-fit double mutant A₂B₂ class.

Flip-flopping Environments

There are two features of the environment, say temperature and moisture, each with two states: hot and cold for temperature and wet and dry for moisture. Let us assume the A locus adapts organisms to temperature with the 1 allele working best in hot environments and the 2 allele working best in cold environments. Let us assume the B locus adapts organisms to the moisture regime with the 1 allele working best in wet environments and the 2 allele working best in dry environments.  Finally, assume that we start with two kinds of environment: environments that are hot and dry or cold and wet. The hot-dry environments will select for A₁B₂ and the cold-wet environment will select for A₂B₁. Now if the environments “flip-flop” they will be hot-wet and cold-dry. Sex can help the populations adapt to these new states by creating the needed genotypes A₁B₁ and A₂B₁.

Intermediate Optimum

In this theory the 1 alleles subtract some value from the trait and the 2 alleles add some value to the trait. Since an intermediate value of the trait is assumed to be most fit, most individuals will have mixed combinations A₁B₂ and A₂B₁. Now if the intermediate optimum starts changing (say it starts increasing) sex can help create the needed A₂B₂ combinations.

Interacting mutations

In this case, the effect of each additional mutation is greater and greater up to some point, the threshold, at which the organism dies (as shown in the figure). Most individuals will have mixed combinations A₁B₂ and A₂B₁ and sex can create the most fit A₁B₁ class. Sex will also create the least fit class A₂B₂. As a result of sex creating the two extreme classes,A₁B₁ and  A₂B₂, fewer deaths are required to purge the population of the bad mutations.