Because of the long generation times of most organisms, however, researchers must often assume (rather than prove) that a stable polymorphism exists.
Thus, although there was a trade-off between growth rates at high and low glucose concentrations, it could not have been primarily responsible for the stable polymorphism that we observed.
The range of parameters compatible with stable polymorphism proved to be much narrower in cases of temporal fluctuations compared with spatial ones.
This analysis showed that stable polymorphism in this system with rather high volume of the attraction domain (say, v [is greater than] 80% of the total volume of the simplex) is possible with sufficiently large r (say, r [is greater than] 0.34).
Sympatric speciation, which has been the subject of many theoretical studies based on principally genetic models, requires at least two fundamental conditions; (1) the establishment, through disruptive selection, of a stable polymorphism in a heterogeneous environment; and (2) the evolution of reproductive isolation between the morphs (Maynard Smith 1966; Rice 1984, 1987).
mansoni populations (shift to a new host, stable polymorphism, and reduction of gene flow between host-adapted populations) strengthen the probability that speciation will occur [ILLUSTRATION FOR FIGURE 2 OMITTED].
Such a stable polymorphism
appears when [W.sub.ND] < [W.sub.D] as p [arrow right] 0 and [W.sub.ND] as p [arrow right] 1, or
It might seem logical that temporal fluctuations in the direction of selection could help maintain adaptive variation, but the conditions necessary for a stable polymorphism
are very restrictive.