Selections

Cited from "Human Evolutionary Genetics: Origins, Peoples&Disease"

Selections can occur at
a) survival into reproductive age - viability and mortaility;
b) success in attracting a mate - sexual selection;
c) ability to fertilize - fertility and gamete selection;
d) number of progeny - fecundity.

The sum of these is the ability of an individual genotype to survive and reproduce, its fitness, which which partly dependent on the environment.

The relative fitness of a genotype compared to other genotypes competing for the same resources is measured by a selection coefficient (s), which compares a genotype to the fittest genotype in the population. A selection coefficient of 0.1 represents a 10% decrease in fitness compared to the fittest genotype.

Mutation that reduce the fitness of the carrier are subject to negative selection, also known as purifying selection, whereas mutations that increase fitness undergo positive selection.

To understand the dynamics of selection at diploid loci, we must consider the impact of mutants on the fitness of the genotypes, and not on the individual alleles.

Codominant selection: a novel deleterious allele will be eliminated more rapidly from the population if it reduces the fitness of a heterozygote as well as the homozygote.

Overdominant selection: a new allele may increase the fitness of a heterozygote relative to that of both homozygotes. The two homozygous genotypes may exhibit different reduction in fitness (s1 and s2). This creates a balanced polymorphism.

Underdominant selection: new alleles reduce the fitness of the heterozygote alone.

Overdominant selection is no the only mechanism by which balanced polymorphisms can be generated, but is one of a number of processes described collectively as balancing selection. One example of an alternative mechanism for balancing selection is that of frequency-dependent selection., whereby the the frequency of a genotype determines its fitness. If a genotype has a higher fitness at low frequencies but lower fitness at higher frequencies, an intermediate equilibrium value will be reached over time.

Neutral theory of molecular evolution: The theory that the majority of mutations do not influence the fitness of their carriers.

Neutrality test: A statistical method for exploring whether observed genetic diversity is compatible with neutral evolutionary processes.

The power of neutrality tests to detect selection depends on, amongst other factors, the characteristic of the selective regime:
a) the type of selection operating;
b) the strength of selection;
c) the period during which selection occurred or is occurring.
The results of a neutrality test will often depend on whether the region being analyzed is itself under selection or is linked to a region under selection.