Various drivers of female-limited diversity

Abstract

Many ecologists and evolutionary biologists are motivated by a fascination for the immense diversity of the natural world. Here, I focus on one aspect of that diversity: individuals of the same species often differ from one another in important ways. In species with separate sexes, it is common for males and females to have very different phenotypes, but there are also cases in which distinct phenotypes exist among individuals of the same sex. Here, I discuss various systems in which males all look alike, while females come in two or more distinct morphs. In the general introduction (chapter 1), I first introduce some important concepts and highlight common pitfalls in research on distinct morphs. In particular, I highlight the importance of negative frequency dependent selection for understanding genetic polymorphism, and explain why trade-offs alone are insufficient explanations for polymorphism maintenance. I then introduce three examples of female-limited color polymorphism that occur in some butterflies, some damselflies, and some hummingbirds, respectively. In each case, I discuss the drivers behind negative frequency dependent selection, and highlight important similarities and differences between the systems. Chapter 2, titled ”Fast or Fancy: Modeling a trade-off between female attractiveness and development time”, aims to discover the mechanism by which the Alba polymorphism in Colias butterflies is maintained. In these butterflies, females come in two distinct color morphs, the white Alba morph and a colorful morph (orange or yellow), while males are all colorful. Female morph is determined genetically, and is characterized by several trade-offs. We propose a possible evolutionary driver that takes into account that Alba females have faster development but are less attractive to males compared to colorful females. Because males emerge earlier than females do, the faster development of Alba females allows them to emerge at a time when many males are still unmated and courtship is intense. The more attractive colorful females emerge later when a larger proportion of males have already mated and intense courtship is less prevalent. Chapter 3, titled ”Ecological Sexual Dimorphism and Population Consequences when one Sex Uses a Superior Resource”, is concerned with the evolution of ecological sexual dimorphism. While differences between sexes usually result from sexual selection or other intrinsic differences between the sexes, it is also possible for sexual dimorphism to be driven by competition for some resource or niche. Males and females sometimes specialize on different resources, resulting in reduced competition. We identify a constraint that limits the effectiveness of ecological sexual dimorphism: if one resource is more abundant than the other (which is very often the case), then resource use would be most efficient if more than half the population specialized on the more abundant resource. However, sex ratios are strongly constrained by the rarer-sex effect, and therefore each sex tends to make up half of the population. If morph is determined by a single locus, a sex-limited polymorphism can solve this problem, with one sex specializing on the more abundant resource, while the other sex is polymorphic for niche use. However, the efficiency of polymorphism is reduced when resource use is affected by multiple loci. In that case, we show that sexual dimorphism can evolve even if it results in suboptimal resource use. Chapter 4, titled ”Sex ratio theory for facultative parthenogens: from fortuitously optimal stick insects to the origin of haplodiploidy in Hymenoptera”, shifts the topic away from female-limited diversity and instead investigates how asexual reproduction (parthenogenesis) affects whether it is better to produce male or female offspring. We first show that, if females can reproduce both sexually and parthenogenetically, this favors the evolution of female-biased sex ratios. We then investigate how the evolution of parthenogenesis is affected by constraints on the sex of parthenogenetically produced offspring. In many animals, details of the sex determination system (e.g. sex chromosomes) constrain the sex of parthenogenetically produced offspring, such that parthenogenesis produces either only daughters or only sons. We show that under stable rates of parthenogenesis, the female-producing parthenogenesis of stick insects and mayflies can achieve optimal sex allocation as if it were not affected by a constraint. Furthermore, we investigate scenarios in which parthenogenetically produced offspring are all male. This includes birds and also haplodiploids (a system where parthenogenesis produces only sons, and sexual reproduction produces only daughters). Finally, we highlight an old paper by J. J. Bull from 1981 on the origins of haplodiploidy in Hymenoptera (bees, wasps, and ants). The general discussion (chapter 5) contains three short essays that are loosely connected to chapters 2-4. First, I focus on the Alba polymorphism and review hypotheses that have been suggested (and often rejected) concerning the maintenance of this polymorphism. Second, I discuss what our model on ecological sexual dimorphism can teach us about the female-limited color polymorphisms introduced in the general introduction. Third, I discuss why in some eusocial animals, such as Hymenoptera, workers are always female. In the general discussion, I also write about the role of theory in biology. I end on some thoughts on how theory can help bridge gaps between fields and discover general principles in evolution, while also highlighting the value of more taxon-specific research.