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.
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.
The balanced polymorphism hypothesis does not make a clear prediction as to whether the rs+/- or rs-/- genotype should be better at mathematical ability, although it is stated clearly that the rs+/+ should be at a disadvantage.
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.
MSB say that 'Balanced polymorphisms, it is claimed, require equal frequencies of the two alleles'.