ABSTRACT
A population consists of interbreeding individuals: genes can be freely interchanged among them. Populations vary with time and from one site to another. Research may focus on variation in space, or follow a single population as it changes with time. The genetic composition of a population begins with a pool of genetic material carried by the founders, and keeps changing as new individuals are born, immigrate into it, emigrate from it, or die. The gene pool may change when external forces operate. It should be remembered that the phenotype, but not the genotype of individuals is affected directly by the environment. Genetic phenomena like dominance or incomplete penetrance reduce the observed phenotypic variation because not all individuals that carry a gene also express it in the phenotype. The simple models in population genetics and evolution enable the formulation of ‘working hypotheses’, which can be experimentally verified. The most important model in population genetics theory is the Hardy-Weinberg equilibrium law.
