The sexual selection continuum

The evolution of mate choice for genetic benefits has become the tale of two hypotheses: Fisher's 'runaway' and 'good genes', or viability indicators. These hypotheses are often pitted against each other as alternatives, with evidence that attractive males sire more viable offspring interpreted as support for good genes and with a negative or null relationship between mating success of sons and other components of fitness interpreted as favouring the Fisher process. Here, we build a general model of female choice for indirect benefits that captures the essence of both the 'Fisherian' and 'good-genes' models. All versions of our model point to a single process that favours female preference for males siring offspring of high reproductive value. Enhanced mating success and survival are therefore equally valid genetic benefits of mate choice, but their relative importance varies depending on female choice costs. The relationship between male attractiveness and survival may be positive or negative, depending on life-history trade-offs and mating skew. This relationship can change sign in response to increased costliness of choice or environmental change. Any form of female preference is subject to self-reinforcing evolution, and any relationship (or lack thereof) between male display and offspring survival is inevitably an indicator of offspring reproductive values. Costly female choice can be maintained with or without higher offspring survival.