To explain explosive speciation and the production of species flocks, I have focused on how changes in the mating system can produce rapid reproductive isolation between diverging populations. By preventing interbreeding, reproductive isolation permits genetic divergence and, thereby, morphological, ecological and behavioral divergence. The formation of recognizable species follows.
Organisms with complex mating systems are predisposed to become reproductively isolated during allopatry or micro-allopatry. Sexual selection is a particularly potent force producing reproductive isolation. The coevolutionary nature of sexual selection leading to escalated contests, the potentially strong impact of successful variants, selection for novelty per se, and other features make divergence in mating characteristics likely to occur and rapid in isolated populations of sexually selected species. Social (West-Eberhard 1983a), rather than sexual competition, may be critically important in some cases, as in the monomorphically brilliant, biparental Lake Tanganyikan cichlids.
Three additional interactions between sexual selection and speciation are suggested. 1) Sexual selection can maintain variation which can later become the basis for "polymorphic speciation" following the fixation of one morph in a new environment. 2) Founder events may trigger genetic reorganizations in sexually selected traits. 3) Unstable equilibria or rapid drift in sexually selected traits may occur.
An examination of the two most impressive species flocks, the Hawaiian drosophilid flies and the African Great Lakes cichlids, supports the importance of sexual selection to species flock formation. The amphipods of Lake Baikal are proposed as a test case.
Other similarities between the cichlids and Drosophila which have contributed to
species flock formation are discussed. These include a weak intrinsic tendency to disperse and
presence in environments which promote isolation of local populations. The considerable
trophic diversity represented by each species flock is attributed to the relative absence of
potential competitors in the case of the Hawaiian Drosophila, and to the failure of
potential competitors to adapt to lacustrine conditions in the case of the African cichlids.
The failure of noncichlids to adapt to lacustrine conditions is attributed to continued gene
flow with fluvial-adapted populations.