When individuals from two different species mate, they frequently produce hybrids that are less fit than the parents. To prevent wasting parental resources on hybrids, choosiness for conspecific mates may be selected in hybridizing species. Such evolution of choosiness for mates (i.e. reinforcement) is considered to be an important driver of speciation. When the hybridizing species have unequal population sizes, reinforcement theory posits that the rarer species will evolve to be more choosy than the common species because costly hybridization will be more common in rarer species (“Howard pattern”). An alternate hypothesis is that scarcity of conspecific mates will lead to less choosiness in rarer species (“Hubbs pattern”); producing hybrid offspring is better than producing none. Empirical studies have found both patterns. Yet, these two patterns have not been evaluated systematically, so the mechanisms underlying them are poorly resolved. We hypothesize that opportunity cost in mating (e.g. finite time available to find a partner) limit the evolution of choosiness in the rarer species. We use Integral Projection Model, a simple continuous trait model, to simulate the evolution of mating preferences when a choosing individual can mate once over each time step, but only has two chances to find a partner.
Results/Conclusions
Our model shows that, under unequal population sizes, the Howard pattern never evolves; the common species becomes more choosy than the rarer species (Hubbs pattern). Other factors, such as unequal cost of hybridization, must be present in order to generate the Howard pattern. Furthermore, we find that to override a large asymmetry in population size requires an equally large, or larger, asymmetry in the cost of hybridization. We therefore hypothesize that such additional asymmetries can be found in cases where the Howard pattern is observed. Our study highlights the importance of the trade-off between the opportunity cost of choosiness (missing a mating opportunity) and the cost of hybridization (lower reproductive success from a mating). Previous works on reinforcement largely ignored this trade-off, so the rarer species was expected to become more choosy under reinforcement. However, our study shows that the reverse is true when mating opportunities are limited. Therefore, a test for reinforcement where the rarer species becomes more choosy should not be viewed as providing unambiguous support for reinforcement theory. Population sizes, encounter rates, opportunity costs of choosiness, and the costs of hybridization should all be evaluated to understand the adaptive strategies used by individuals in hybridizing species.