  We study a birth and death model for the adapatation of a sexual population
to an environment. The population is structured by a phenotypical trait, and,
possibly, an age variable. Recombination is modeled by Fisher's infinitesimal
operator. We prove the existence of principal eigenelements for the
corresponding eigenproblem. As the infinitesimal operator is 1-homogeneous but
nor linear nor monotone, the general Krein-Rutman theory cannot be applied to
this problem.
