# Genetic structures of Populations

The genetic structure refers to the frequencies of the alleles of a certain population.

If the phenotype is observed, only the genotype of the homozygous recessive alleles can be known; the calculations provide an estimate of the remaining genotypes. Since each individual carries two alleles per gene, if the allele frequencies (p and q) are known, predicting the frequencies of these genotypes is a simple mathematical calculation to determine the probability of getting these genotypeswhen two alleles are drawn at random from the gene pool. So in the above scenario, an individual pea plant could be pp (YY), and thus produce yellow peas; pq (Yy), also yellow; or qq (yy), and thus producing green peas (Figure below). In other words, the frequency of pp individuals is simply p_{2}; the frequency of pq individuals is 2pq; and the frequency of qq individuals is q_{2}. And, again, if p and q are the only two possible alleles for a given trait in the population, these genotype frequencies will sum to one: p^{2} + 2pq + q^{2} = 1.

When populations are in the Hardy-Weinberg equilibrium, the allelic frequency is stable from generation to generation, and the distribution of alleles can be determined from the Hardy-Weinberg equation. If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play.

Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction.

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. When two species evolve in diverse directions from a common point, it is called divergent evolution.