  We propose a model for the dynamics of frequencies of a costly defense trait.
More precisely, we consider Lotka-Volterra-type models involving a prey (or
host) population consisting of two types and a predator (or parasite)
population, where one type of prey individuals - modeling carriers of a defense
trait - is more effective in defending against the predators but has a weak
reproductive disadvantage. Under certain assumptions we prove that the relative
frequency of these defenders in the total prey population converges to
spatially structured Wright-Fisher diffusions with frequency-dependent
migration rates. For the many-demes limit (mean-field approximation) hereof, we
show that the defense trait goes to fixation/extinction if and only if the
selective disadvantage is smaller/larger than an explicit function of the
ecological model parameters.
