Question

Explain the condition and consequences of directional selection and balancing selection.

*This is a Population Genetics topic.

Answer 1

Directional Selection

- It occurs when individuals homozygous for one allele have a fitness grater than that of individuals with other genotypes and individuals homozygous for the other allele have a fitness less than that of individuals with other genotypes. at equilibrium the population will be composed entirely of individuals that are homozygous for the alleles associated with the highest probability of survival. The rate at which the population approaches this equilibrium depends on whether the favored alleles is dominant, partially dominant, or recessive with respect to survival probability. An allele is dominant with respect to survival probability as homozygotes for the favored allele, and it is recessive if heterozygotes have the same survival probability as homozygotes for the disfavored allele. An allele is partially dominant with respect to survival probability if heterozygotes are intermediate between the two homozygotes in survival probability. This pattern of selection because one of the two alleles is always increasing in frequency and the other is alwayes decreasing in frequency.

- Directionlal selection usually seen in environments that have changed over time. Change in weather, climate, or food availability can lead to directional selection

- The result of this type of selection is a shift in the population's genetic variance toward the new ,fit phenotype

Balancing Selection

- It refers to a number of selective processes by which mulitiple alleles are actively maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone.This can happen by various mechanisms, in particular, when the heterozygotes for the alleles under consideration have a higher fitness than the homozygote. In this way genetic polymorphism is conserved.

-The effect of balancinng selection is to shift the site frequency spectrum (SFS) of selected loci toward an excess of intermediate frequency varients.

- There are several ways balancing selection works to maintain polymorphism.the two major and most studided are heterozygote advantage and frequency dependent selection. Heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either homozygous dominant or homozygous recessive genotype. Frequency dependent selection includes when biological interactions make an individual's fitness depend on the frequencies of other phenotype or genotypes in the population.