ABSTRACT
Epigenetic interactions may affect the fate of individuals. The Russian geneticist Timofeev-Resovsky produced, by appropriate crosses, laboratory mutant strains of Drosophila of two kinds: one group carried each mutation singly on a different chromosome. In the other group, each chromosome carried two of the same mutations. All but one of the single mutations had deleterious effects on fitness. One kind of genotype-genotype interaction within populations – sometimes mediated by the environment – was termed genetic facilitation. R. Lewontin maintained Drosophila strains in single-strain populations, and in competition with a standard white-eyed strain. The expectation was that competition would have a negative effect because food or space could become limiting. Sudden, deleterious environmental changes may drastically reduce population size. A drastic reduction in population size, from which the population eventually recovers, is called a bottleneck.
