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All conclusions of this section apply to selection for habitat preference due to balanced polymorphism, which also shows positive frequency dependence.
Therefore, balanced polymorphism at loci affecting fitness in the two habitats is not necessary for natural selection to initiate sympatric speciation via habitat or resource specialization, contrary to the opinion expressed by other authors (Maynard Smith 1966; Dickinson and Antonovics 1973; Rosenzweig 1978; Pimm 1979; Udovic 1980; Felsenstein 1981; Kondrashov and Mina 1986; Butlin 1987; Diehl and Bush 1989; Johnson et al.
It should be noted that antagonistic pleiotropy is a necessary but not sufficient condition for maintenance of balanced polymorphism in traditional models of sympatric speciation.
s = t) then it will be true that p = q and hence rs+/- will be present in 50 per cent of the population, there is no sense in which the balanced polymorphism occurs or is necessary because of the particular numerical value taken by the frequency of the rs+/- genotype; the precise numerical value of the frequency of heterozygotes is simply irrelevant to the question of whether a balanced polymorphism is present, and the polymorphism could still have been balanced if either of the homozygotes was the most frequent in the population.
It should also be noted that the presence of a balanced polymorphism for the rs system as a whole does not mean, as Annett (1985) puts it, 'that there must be advantages associated with sinistrality'.
iii) To show that a balanced polymorphism exists one needs to specify the costs incurred for both homozygotes.
It was precisely because a genetic balanced polymorphism does not require equal gene frequencies that the estimate of 49 per cent for the rs+ locus was particularly striking.
MSB's third point about the BP + HA hypothesis is that 'To show that a balanced polymorphism exists one needs to specify the costs incurred for both homozygotes'.
Handedness and educational success: The hypothesis of a genetic balanced polymorphism with heterozygote advantage for laterality and ability.

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