Allele frequency is the number of times an allel occurs within the gene pool. THe Hardy-Weinburg Principle predicts the frequency of dominant and recessive alleles in a given percentage of the population with 2 equations given some conditions are met such as no mutations, random mating and a large population.
Individuals within a population of a species may show a wide range of variation in phenotype due to genetic and environmental factors.
There are 3 types of natural selection: Stabilising selection, disruptive selection and directional selection.
Allopatric speciation and sympatric speciation works under the principle of no gene flow between two groups of organisms under the same species. They show variation due to mutation. Different reproductive success causes different alleles to be passed down, leading to a change/increase in allele frequency. The two groups of organisms can no longer reproduce to produce fertile offspring - they are two different species.