• Population genetics for two or more loci is concerned with changes in the frequencies of haplotypes, which are the multi-locus equivalent of alleles.

• Recombination tends, in the absence of other factors, to make the alleles of different loci appear in random (or independent) proportions in haplotypes. This condition is called linkage equilibrium.

• A deviation from the random combinatorial proportions of haplotypes is called linkage disequilibrium. It can arise because of non-random mating, random sampling, and natural selection.

• For selection to generate linkage disequilibrium, the fitness interactions must be epistatic.

• Pairs of alleles at different loci that co-operate in their effects on fitness are called coadapted. Selection acts to reduce the amount of recombination between coadapted genotypes.

• When selection works on one locus, it will influence gene frequencies at linked loci. The effect is called hitch-hiking.

• Recombination is selectively disadvantageous in so far as it breaks down favorable gene combinations.

• The mean fitness of a population can be drawn graphically for two loci; the graph is called a fitness surface or an adaptive topography.