Genetics - A. V. Sivolob 2008

Population Genetics
Properties of a Population
Variability

The existence of variation among individuals of the same species is a prerequisite for evolutionary changes within a population. Phenotypic variation can be caused by environmental factors (non-heritable variation), genetic differences (heritable variation), or both, as is typically the case with Quantitative Traits.

In population studies, the variation of quantitative traits is characterized by the mean value of the trait, variance, and the coefficients of variation and heritability (see Chapter 3). The variation of alternative (qualitative) traits is determined by the proportion of a specific form (or morph) of that trait within the population, known as the phenotype frequency. The primary indicators of genetic variation in a population are Selection/33.html">Gene and genotype frequencies. The level of a population's genetic variation serves as the fundamental source for its potential adaptive change.

If a population possesses only one allele of a gene, it is termed monomorphic; if it has two or more, it is polymorphic. The presence of several allelic forms of a gene in a population is referred to as genetic polymorphism. The proportion of polymorphic genes among all analyzed ones—polymorphism ($P$)—is one of the key indicators of population genetic variation. The conventional criterion for gene polymorphism is its allele frequency: a gene is considered polymorphic if the frequencies of at least two alleles exceed 0.05 or 0.01 (two polymorphism thresholds). If this frequency does not exceed the threshold value, the allele is classified as a rare allele or a mutation. An example of calculating polymorphism $P$ is given in Table 8.2.

Class="center">Table 8.2. Calculation of average polymorphism (P) across four populations

Another indicator of population genetic variation is mean heterozygosity ($H$). In every individual within a population, a certain fraction of genes exists in a heterozygous state. The proportion of such genes (relative to those analyzed) characterizes the heterozygosity of that individual. The averaged value of individual heterozygosity scores across all surveyed individuals yields the population's mean heterozygosity, $H$.

Heterozygosity largely depends on allele frequencies. If the frequencies of two alleles of a single gene are 0.9 and 0.1, the proportion of heterozygotes is 2 x 0.9 x 0.1 = 0.18 (see Chapter 3 and the Hardy-Weinberg equation below). With allele frequencies of 0.4 and 0.6, the proportion of heterozygotes rises to 2 x 0.4 x 0.6 = 0.48. Heterozygosity reaches its maximum values when the allele frequencies for each gene are equal (1/2 and 1/2 for diallelic genes; 1/3, 1/3, and 1/3 for triallelic genes, etc.).