Evolutionary wanderlust: Sexual selection with directional mate preferences

In the pantheon of evolutionary forces, the optimizing Apollonian powers of natural selection are generally assumed to dominate the dark Dionysian dynamics of sexual selection. But this need not be the case, particularly with a class of selective mating mechanisms called ‘directional mate preferences’ (Kirkpatrick, 1987). In previous simulation research, we showed that nondirectional assortative mating preferences could cause populations to spontaneously split apart into separate species (Todd & Miller, 1991). In this paper, we show that directional mate preferences can cause populations to wander capriciously through phenotype space, under a strange form of runaway sexual selection, with or without the influence of natural selection pressures. When directional mate preferences are free to evolve, they do not always evolve to point in the direction of natural-selective peaks. Sexual selection can thus take on a life of its own, such that mate preferences within a species become a distinct and important part of the environment to which the species’ phenotypes adapt. These results suggest a broader conception of ‘adaptive behavior’, in which attracting potential mates becomes as important as finding food and avoiding predators. We present a framework for simulating a wide range of directional and non-directional mate preferences, and discuss some practical and scientific applications of simulating sexual selection.