Mate Choice and Genetic Quality: A Review of the Heterozygosity Theory

Publisher Summary

This chapter presents a review of mate choice and genetic quality, defines mate choice, and discusses some of the key issues about how individuals can benefit from being choosy, including the distinction between two main types of genetic benefits. It also focuses on heterozygosity and fitness and presents an overview of the methods that have been used to estimate individual heterozygosity or relatedness among individuals. It also presents a review of the studies that have found correlations between individual heterozygosity and a variety of fitness‐related traits. The chapter consists of two parts—mate choice based on relatedness with the partner, which is choice to optimize offspring heterozygosity and the link between the evolution of promiscuity and inbreeding avoidance.

Introduction

Mate choice has been and still is a very popular research topic in behavioral and evolutionary ecology. This is not surprising, given the role it plays in our own lives, and given its importance for understanding phenomena ranging from sperm form and function (Holman and Snook, 2006) to speciation (McPeek and Gavrilets, 2006). Research on mate choice beautifully illustrates how theoretical work in the form of verbal and mathematical models develops alongside simple and more sophisticated empirical studies (e.g., Gustafsson 2006, Kokko 2006, Qvarnström 2006). The study of mate choice has also created its share of scientific debate and controversy, as can be seen in the literature, and observed at scientific conferences. As a graduate student, I witnessed (from a safe distance) some surprisingly emotional exchanges about why females paired with an already mated male, and later I myself got caught up in discussions about whether and how females benefit from engaging in extra‐pair copulations. A lively research field it is indeed.

Arguably the most controversial issue of all is the importance of indirect genetic benefits for the evolution of mate choice. The key questions are these. Are genetic benefits important enough to explain the evolution of more or less sophisticated (and costly) choice mechanisms? What is the nature of these genetic benefits? The main aim of this review is to consider a specific type of genetic benefit, namely the fitness gains resulting from the production of offspring with higher individual genetic diversity (heterozygosity). Research on inbreeding and on the relationship between individual heterozygosity and fitness‐related traits in free‐living populations has blossomed, particularly after molecular tools such as microsatellite markers became widely available. However, we are only beginning to understand why individual heterozygosity is related to fitness and how this affects the evolution of mate choice.

To set the stage for this review, I briefly define mate choice, and discuss some of the key issues about how individuals can benefit from being choosy, including the distinction between two main types of genetic benefits (Section II). The next section focuses on heterozygosity and fitness. Here, you will find an overview of the methods that have been used to estimate individual heterozygosity or relatedness between individuals, and a summary of an ongoing debate about the importance and interpretation of heterozygosity–fitness correlations. Section III also contains a review of studies that have found correlations between individual heterozygosity and a variety of fitness‐related traits. Section IV is about mate choice and heterozygosity and consists of two parts. In the first part, I discuss mate choice based on relatedness with the partner, that is, choice to optimize offspring heterozygosity. I consider when it will pay females to mate with a more or less‐related male, and how females should trade off choice for different types of genetic benefits. Then, I emphasize the link between the evolution of promiscuity and inbreeding avoidance, and I discuss why it is important to consider the costs and mechanisms of choice. Finally, I review the evidence that social mate choice, choice of a copulation partner, and choice of sperm under multiple mating depends on relatedness. The second part of Section IV is about mate choice favoring heterozygous partners. First, I discuss why one should expect such choice and why the heterozygosity of the choosing female may affect her choice. Then, I review the evidence that individuals prefer heterozygous mates, in different contexts.

This review is probably biased toward studies on birds, because those are the studies I am most familiar with. However, I attempted to review the entire recent literature, and the fact that many examples are from birds also reflects the relative amount of research on avian mate choice.