Costly defense traits in structured populations.

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.