Evolution of individual variation in a competitive trait: a theoretical analysis

When competitive traits are costly, negative frequency-dependence can maintain genetic variance. Most theoretical studies examining this question assume binary polymorphisms, yet most trait variation in wild populations is continuous. We propose that continuous trait variation can result from continuous variation in resource quality and that the shape of the resource distribution determines trait maintenance. We used an individual-based model to test which conditions favour the stable maintenance of variation and which cause temporal fluctuations in trait values. This approach, inspired by contrasting outcomes of previous studies, clearly showed the decisive role played by the shape of resource distributions. Under extreme conditions, like the absence of resource variation or with very scarce resources for weak competitors, traits evolved to a single non-competitive or highly competitive strategy, respectively. Some distributions led to strong temporal fluctuations on trait values, whereas others led to the maintenance of large standing variation in competitive traits together with stable mean trait values. Our results thus explain the contradicting outcomes of previous theoretical studies and at the same time provide hypotheses to explain the maintenance of genetic variation and individual differences. We also suggest how the proposed effects of resource variation on trait maintenance can be tested empirically.