Genetic variation is produced when the genes of two parents are mixed during sexual reproduction by segregation and recombination. In asexual species, genes are mixed only by recombination, and less often than in sexual species.
The Hardy-Weinberg equilibrium occurs under idealized conditions in which no evolutionary forces are acting. At this equilibrium, the three genotypes at a locus with two alleles are in the ratios p2 :: 2p(1 – p) :: (1 – p)2. Deviations from those proportions can be used to detect selection and other evolutionary factors.
Linkage disequilibrium occurs when alleles at two loci occur together more often than expected by chance. It is eroded by recombination, and increased by selection and other evolutionary forces.
Horizontal gene transfer (HGT) is the movement of genes between organisms by mechanisms that do not involve meiosis, and is particularly important to prokaryotes. It can move genes between individuals of the same species and of different species. HGT is important to human health because it is the major pathway by which bacteria evolve resistance to antibiotics.
Mutation, which is an error that occurs when DNA or RNA is replicated, is critical to evolution because it is the ultimate source of genetic variation. Mutations can affect anywhere from a single base to a large piece of chromosome. Mutation rates vary greatly among species.
Mutations in coding regions are called synonymous if they do not change the protein, and are called nonsynonymous if they do. This distinction has important evolutionary implications.
Many mutations have no measurable effect on survival or reproduction. Those that do are typically deleterious. A small fraction are advantageous, and their spread leads to adaptive evolution.
Mutations that affect one trait virtually always have pleiotropic effects, meaning that they also affect other traits.
In species with separate somas and germ lines, a mutation can be inherited if it alters a gene in a cell in the germ line. Mutations to somatic cells leave no descendants to the next generation.
Experiments show that mutation is random with respect to what will increase fitness.
There are several mechanisms of nongenetic inheritance. One is cultural inheritance, an essential part of human civilization.