Those who survive to produce offspring, which then survive to reproduce, are more fit than those who do not meet both requirements. Natural selection acts on individuals, but individuals do not evolve. Phenotype is the description of visible traits that characterize an individual or members of a population. Phenotype can reflect both an individual s genetic makeup and the effects of the environment on an individual. Genotype is the hereditary makeup of an individual.
Mutations are changes in the genetic material.
Random genetic drift refers to a change in gene frequencies that is completely random, i.e., there is no necessary selection for or against a particular allele. The allele simply occurs, by chance alone, more or less often in a generation than in the previous generation. In order for this drifting to influence the entire population s genotype frequency, it is necessary to have a small population. Migration refers to the flow of genes into or out of an isolated population through inbreeding. A small amount of gene flow often occurs between populations that normally do not interbreed.