Abstract
Divergence in the genic and chromosomal architecture of local populations from the population system norm has been documented in numerous plant and animal species (cf. Grant, 1963, 1964; Mayr, 1963; Ford, 1965). The stimulus for such may be provided by one of an array of environmental challenges. The immediate presence of a potentially interbreeding congener, or one which competes for biotic or physical resources of the environment may constitute such a challenge. A shift in the adaptive mode of one of the congeners could intensify existing external and internal barriers to gene exchange, or could permit the two species more efficiently to exploit their habitats. Population in areas of congener sympatry, which may involve morphological, physiological, ecological, or ethological traits, has been termed character divergence (Darwin, 1859) and character displacement (Brown and Wilson, 1956; Wilson, 1965). The term Wallace effect has been applied to shifts in population structure whose primary adaptations are to strengthen reproductive isolating barriers (Grant, 1966).
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