The evidence of strong disruptive selection
for host use is consistent with the possibility that the populations could have diverged in sympatry.
This is the only study to date to consider lifetime fitness across hosts in a population which has not been subject to disruptive selection in the field.
An initial trade-off may be sufficient to generate disruptive selection between utilization of alternate hosts, but the intense selection pressure of insecticides and the lack of opportunity to choose insecticide-free habitats selects strongly for resistant generalists in this context.
These intrinsic behavioral mechanisms may have evolved as a consequence of the reduced fitness of hybrid production--that is, through reinforcement--or they may be pleiotropic effects of disruptive selection
on other characters.
The cubic spline analysis did not detect any local stabilizing or disruptive selection
within the fitness surface (fig.
Directional selection for fecundity was highly significant and continuous with size [no significant quadratic terms to suggest stabilizing or disruptive selection
(P [is greater than] 0.
The one disruptive selection
study that would appear to lend support to Thoday and Gibson's experimental result turns out to be a "destroy-the-hybrids" experiment by virtue of the extremely strong level of disruptive isolation that was applied.
Each entry in the table shows an ordered pair of letters for hosts and parasites, respectively, that denote stabilizing selection (S) to a monomorphic state or disruptive selection
(D) to numerous widely separated phenotypes.
By contrast, disruptive selection
on axis 2, which represents proportional allocation to the two resistance traits, suggests that for a given total allocation to resistance, resistance to either parasite alone was favored, but intermediate levels of resistance to both were disfavored.
We therefore constructed an alternative selection function for Y (but not for X) that captures the relevant features of the disruptive selection
fasciculata, I used a regression technique (Pearson, 1903; Lande and Arnold, 1983) which estimates parameters describing both directional and stabilizing/ disruptive selection
on phenotypic traits.
In these models, changes in the sign of directional selection over space or time can impose a net disruptive selection
regime and hence maintain genetic polymorphism, given certain population structures.