Evolution in closely adjacent plant populations IV. Barriers to gene flow

TLDR: Since no evidence has been presented for the origin of breeding barriers under disruptive selection in natural populations, the occurrence of mechanisms reducing gene flow were investigated in closely adjacent plant populations at metal mine boundaries.

Abstract: THE importance of isolation in promoting population divergence and speciation has long been recognised (e.g. Mayr, 1942; Dobzhansky, 1941; Baker, 1959). Isolation was considered a prerequisite for population divergence until Thoday (1958) showed that disruptive selection could effect such divergence in the absence of isolation. Recently the occurrence of divergence in nature in the face of gene flow has been shown in Papilio dardanus (Clarke and Sheppard, 1962), Maniola jurtina (Creed et al., 1959) and various grasses (Jam and Bradshaw, 1966; Aston and Bradshaw, 1966; McNeilly, 1967). However, gene flow is not without effect. Generally it slows down population divergence (but see Millicent and Thoday, 1961, and Streams and Pimentel, 1961) and produces ill-adapted genotypes from the crossing of two adapted types. In such situations we might expect the evolution of mechanisms to restrict gene flow. Evidence for the development of breeding barriers between adjacent (parapatric), or sympatric populations, and their absence between allopatric populations of the same species or group of species, has been presented in Drosophila (Dobzhansky and Koller, 1938; King, 1947; Ehrman, 1965), cotton (Stephens, 1946), Streptanthus (Kruckeberg, 1957), Solanum (Grun and Radlow, 1961) and Gilia (Grant, 1966). The process has also been demonstrated experimentally (Knight et al., 1956) and theoretically (Crosby, 1964). In all these instances there is evidence that breeding barriers have been developed between populations that have undergone prior allopatric divergence and which have subsequently met. However, Thoday and Gibson (1962) have shown that in Drosophila divergence and the evolution of breeding barriers can occur without isolation under disruptive selection. Since no evidence has been presented for the origin of breeding barriers under disruptive selection in natural populations, the occurrence of mechanisms reducing gene flow were investigated in closely adjacent plant populations at metal mine boundaries. The evidence suggests that considerable gene flow occurs in such situations (McNeilly, 1967; McNeilly and Bradshaw, 1967) and that mine populations are the product of recent evolution by disruptive selection (McNeilly, 1967; Antonovics, 1966).