Sex differences in lifespan have been labelled as one of the most robust features in biology. In human populations, women live consistently longer than men, a pattern that encompasses most mammalian species. However, when expanding both the taxonomic scope beyond mammals and the range of mortality metrics the female survival advantage over males is no longer the rule. Moreover, current evidence suggests that sex differences in actuarial ageing parameters (i.e. age at the onset of ageing and rate of ageing) are far from consistent across the tree of life. This chapter first reviews current knowledge of sex differences in mortality patterns across animals and appraises how these diverse patterns can be explained by the current evolutionary framework. It then emphasizes the relevance of going beyond the differences in mortality patterns by exploring how natural and sexual selection have shaped age- and sex-specific changes in reproductive performance and body mass across the tree of life, and by identifying some possible biological pathways modulating ageing in a sex-specific way. Finally, it highlights how evolutionary theories can be relevant to understand the widespread differences in causes of death between sexes, offering a complementary approach to gain a comprehensive understanding of the evolution of sex differences in health and ageing, with likely biomedical implications.

Threespine sticklebacks, numerous species of disease-causing bacteria, and Darwin’s finches have all shown rapid evolutionary change in response to changing environments. Evolutionary ecologists use a variety of genetic and molecular approaches to study evolutionary change in these and other species. Gene flow, genetic drift, mutation, and natural selection can cause evolutionary change within a population, but natural selection is the only evolutionary process that can lead to adaptation. The benefits and costs of adaptations are environment-dependent and reflect evolutionary tradeoffs, so a trait may be beneficial in one environmental context and costly in a second. Natural selection may lead to speciation when genetic divergence is maintained either by physical barriers to gene flow, or by assortative mating of similar genotypes within a population. Evolutionary ecologists compare morphological, behavioral, and, most commonly, molecular characters in related groups of organisms, and use similarities in these characters to create phylogenetic trees that reflect evolutionary relationships.

This chapter will focus on debunking the myth that sexual selection was Darwin’s afterthought to natural selection by more fully situating sexual reproduction and sex differentiation in Darwin’s intellectual trajectory, dating to the 1830s. This long-standing myth results, in part, from the scientific communities’ discomfort with sexual selection theory and, more to the point, the social radicals who embraced it in the late 19th and early 20th centuries. To unpack the gendered reception of the Descent, this chapter also analyzes the related myth that sexual selection theory naturalized female inferiority by highlighting the nuanced and varied reception of sexual selection theory within and, importantly, beyond the scientific community especially among socialists and feminists.

Darwin’s theory, in its uniformitarianism, its materialism, and its elimination of all metaphysical explanations and any element of intelligent agency from the world’s biological phenomena has been taken as an important influence in the growth of the idea that all living creatures are automata – more or less “conscious machines.” Darwin himself, in a least four different aspects of his writing, belies this inference from his theories: the metaphorical work done by his dominant idea – natural selection; his anthropomorphism; his views on instinct; and his theory of sexual selection.

Sexual selection theory has undergone both a theoretical and an empirical revolution over the last few decades. New understandings of female choice, partnership dynamics and the impact of sociodemographic factors on mating strategies have led to a flourishing of studies within human behavioral ecology and related evolutionary social sciences. Drawing from this literature, this chapter outlines how mating markets operate and the particular traits that are thought to be most important in human mate choice. Following this overview, the chapter examines how temporal and contextual facets of human partnerships affect mating decisions. Finally, the chapter shifts the focus to debates about the dual roles of fidelity and paternity and how these affect mating strategies. Throughout, the chapter focuses on data from non-WEIRD societies in order to highlight aspects of mating where universals are evident and where cross-cultural variation is important. In addition, the chapter highlights how studies of human mating by human behavioral ecologists offer a distinct and valuable perspective which cannot be found in other subfields of the evolutionary social sciences.

For most people, the most obvious thought or image that sexual reproduction brings to mind is that of sexual intercourse, a mating between two individuals of opposite sexes, which will result in the birth of their common offspring. While biparental reproduction is certainly the most common mode of sexual reproduction among all eukaryotes, it is not the only one, and the way it is carried out can depart substantially, in many different ways, from the ‘canonical’ description above. What is common to all these modes is that two distinct sexually compatible individuals (parents) undertake a sexual exchange that leads to the generation of new individuals with a genetic constitution obtained from the association and/or the reassortment of those parents’ genomes. The key event in this mode of reproduction, technically called amphigony, is the fusion of two gametes or two nuclei functioning as gametes (syngamy), each produced by one parent, to form a zygote. While in species with anisogamy (i.e. with distinct male and female gametes; Chapter 4), only gametes of opposite sex are compatible, the two individuals that produce them are not necessarily a male and a female.

We are all familiar with the changes in an organism during development, followed by its reproduction, which are repeated generation after generation. Biologists describe this development–reproduction sequence as the life cycle: the series of transformations and reproductive events that, from a given stage of life of an organism, leads to the corresponding stage in a subsequent generation. We can describe a biological cycle as going from zygote to zygote, but also from adult to adult, or from embryo to embryo: in a cyclical process, the choice of the ‘initial phase’ is arbitrary or conventional, as the notorious ‘the chicken or the egg’ dilemma beautifully illustrates.

In the course of their lives, organisms spend time and energy on a number of activities and functions, of which reproduction is only one – think of growth, defence against predators and pests, and others. How many resources are used for reproduction, how much time is devoted to it and how this time is distributed over the course of life are all elements that characterize the different reproductive strategies. From an even wider perspective, in those organisms that at certain times in their lives can opt for one or another reproductive mode (e.g. sexual or asexual reproduction, as in many plants and many marine invertebrates), a reproductive strategy includes also this reproductive policy.

On February 1997 the birth was announced of a sheep named Dolly, the first mammal to be cloned from an adult cell of a mother individual. The event attracted enormous media attention. Dolly, born on 5 July 1996, actually had three ‘mothers’: one provided the egg (whose nucleus was removed), another the nucleus with the DNA picked out from a somatic cell (i.e. a cell of the body not specialized for reproduction), while the third mother carried the cloned embryo in her womb until parturition.

Ever since living beings arose from non-living organic compounds on a primordial planet, more than 3.5 billion years ago, a multitude of organisms has unceasingly flourished by means of the reproduction of pre-existing organisms. Through reproduction, living beings generate other material systems that to some extent are of the same kind as themselves. The succession of generations through reproduction is an essential element of the continuity of life. Not surprisingly, the ability to reproduce is acknowledged as one of the most important properties to characterize living systems. But let’s step back and put reproduction in a wider context, the endurance of material systems.

Acquiring the traits specific to a given sex, during early development or at another point during the life of an organism, is usually a complex process. Although the sex condition of an individual is conventionally defined based on the type of gametes it is able to produce (Chapter 4), the sex-specific phenotype is generally not limited to the organs of reproduction. Each of these characters can maintain a certain degree of independence from other sexual traits in the same organism, be subject to different developmental control, and show different degrees of sensitivity to the environment. Therefore, sexual differentiation extends to the development of the secondary sexual characters, which can be morphological, physiological, behavioural, or combinations of these. An exploration of this fascinating subject requires some preliminary clarification about systems and mechanisms of sex determination and sex differentiation.

In Chapter 1 we defined sexual reproduction as a form of reproduction that generates new individuals carrying a genome obtained by the association and/or the reassortment of genetic material from more than one source. In the most familiar form of sexual reproduction, the new genome is formed by the union of (partial) copies of the genomes of two parents through the fusion of two special cells produced for that purpose, the gametes, into a single cell, the zygote. This is the way most multicellular eukaryotes, ourselves included, reproduce sexually.

A zygote does not necessarily derive from the fusion of gametes or gametic nuclei produced by different individuals. Both egg and sperm may instead be produced by the same individual, a sufficient simultaneous hermaphrodite (Chapter 4). In this case, the offspring has only one parent. However, the gametes that merge are the products of independent processes of meiosis undergone by different germ cells, although in the same individual: this distinguishes self-fertilization (or selfing) from some forms of parthenogenesis where there is the fusion of two of the four nuclei deriving from the same meiosis, as we will see in the next sections (Figure 6.1).

This introduction asks what Darwin’s writings still offer to contemporary criticism and humanist thought, highlighting the implications of his work on ecologies, human difference, and aesthetics.

Darwin’s theory of sexual selection has presented queer and trans theorists with a number of stumbling blocks, in that it centers heterosexual reproduction orchestrated between aggressive males and coy females. Some critics have suggested rectifying this problem by imagining sexual selection only with the pursuit of pleasure and aesthetics. But because it severs sexual selection from the “economy of nature,” this split is not able to offer a robust theory of how sexual object choices come into being or circulate. This chapter suggests that Darwin’s thinking about domestication may prove more useful for queer theory because it encompasses criteria pertinent to both sexual and natural selection and entails theorizing how it is that aesthetic criteria matter within the economy of a world in which an organism finds itself.

Tumultuous nineteenth-century political debates, fears of violent revolutions, and the rise of women’s rights campaigns in Britain, the United States, and France provide a context for considering Darwin’s theory of sexual selection and its engagement with feminism. The Descent of Man, and Selection in Relation to Sex (1871) identified sexual differences, male–male combat, and female choice in courtship as key elements of animal copulation, while insisting that male choice controls human sexual relations, ideas that inspired radically different reactions from feminists, who objected to what they regarded as Darwin’s sexism, and fiction writers, who highlighted women characters resisting patriarchal expectations and making independent decisions. The long history and profound consequences of the concepts of sexual difference and sexual selection call for careful consideration of the intertwining of Darwin’s scientific theories about sexual difference and choice with divergent cultural formations, ranging from social Darwinism to feminist theory, and propose a more fluid understanding of sex and gender that supersedes the earlier two-sex model.

Although largely accepted in animal biology, sociobiology has proven to be a controversial model for explaining human behavior. Nowadays sometimes termed ‘behavioral ecology’, the application of biological models to human behavior has the potential to explain a wide array of human instincts and actions. This chapter reviews the models for such putative human universals as violence, sexual reproduction strategies, coalition-building, and altruism, and compares them with similar models applied to animals. It also emphasizes that environment and culture provides critical influences on the ultimate expression of behavior: for example, across societies, mate preferences are partially mediated by society’s economic opportunities, so that culture can act as a buffer for underlying biological instincts. Since this topic is controversial, the chapter review the historical antecedents, starting with Plato’s theory of universals, through John Locke’s Enlightenment ideal, the ‘tabula rasa’, to Margaret Mead and Napoleon Chagnon in modern anthropology. Much of this debate has an underlying moral element, so the chapter discusses the naturalistic fallacy and point out the fact that our morality need not be determined in any way by potential evolutionary influences on instincts or behaviors. However, it also notes the potential logical pitfalls of treating humans as ‘special case’ animals.

This chapter provides an outline of Mendelian and molecular genetics, with a particular emphasis on how DNA provides evidence of a single origin for all living things. It discusses the role of genetics in serial homology, and how that demonstrates genetic relatedness among vertebrates especially, and it shows how homologous HOX genes provide the head–tail orientation and body plan segmentation for animals as diverse as the house fly and human, and that we see this same pattern of segmentation as far back as the Cambrian. It also explains how the independent DNA of mitochondria and chloroplasts provide evidence for a deep history of symbiosis between the host and parasite for both plants and animals. This chapter provides a support for later chapters reviewing principles of evolution and the evolutionary history of complex organisms.