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 chapter demonstrates ways in which Darwin challenged aspects of Enlightenment thought, including racial and sexual hierarchies, gendered stereotypes and androcentric perspectives. In doing so, he called into question Cartesian dualism—the separation of mind and body—and its colonial implications in configuring the body as unruly and in need of subjection to a scientific control that was masculine and European. Situating Darwin’s work in relation to contemporary political debates over race, slavery, and sex, it explores the forceful argument against innatism presented by Darwinian evolution, which undid biologistic arguments for biologically determined roles or behaviors, and shows that while he is often assumed to have occupied a separate and opposing camp to John Stuart Mill, which foregrounded biology rather than ethics, Darwin and Mill in fact shared notable common ground. It argues that, in a climate emergency and at a time of devastating and rising global poverty, Darwin’s strong sense of interdependence and interrelations counters authoritarian disregard for the vulnerable and disadvantaged.

Chapter 1 demonstrates that the nineteenth-century medical record undermines the idea that each person could have only one sex. Throughout the period, several doctors made a stand for “true hermaphrodism,” many more could not identify the sex of their living patients, and “experts” constantly disagreed not only about findings, but also about how best to establish sex in unclear cases. Precisely because no one method for determining sex proved entirely foolproof, doctors and medical forensics experts often relied on narrative to support their claims – a narrative that closely mirrors the one being developed simultaneously in contemporary fiction, and especially, but not exclusively, by realist fiction. Herculine Barbin’s memoirs (the only extant memoirs of a nineteenth-century intersex person in Europe) find their literary corollary in popular fiction that shares much with the creativity and exploitation of narrative techniques in the medical record. Newly uncovered case studies challenge the longstanding representation of the rigidly polarized binary in nineteenth-century France as well as the Foucauldian thesis of “true sex.” Patients sometimes made their own sex determinations, or sought out multiple doctors in order to meet their needs.

Chapter 3 covers the natural history of reproduction. A first section on asexual reproduction deals with the different forms of cell division in unicellular prokaryotes and eukaryotes. A short interlude introduces the notion of sex and describes sexuality phenomena uncoupled from reproductive processes, both in prokaryotes and in unicellular eukaryotes. The main types of sexual reproduction (gametogamy, gamontogamy, autogamy) are described, as are the distinction between sexes and mating types and the different ways in which sexual roles can be distributed within the population. Short paragraphs are devoted to secondary sex characters and to conditions such as aneuploidy, gynandromorphism and intersexuality. Attention is then shifted to the reproductive organs of the metazoans and the morphology of eggs and spermatozoa, and also to the reproductive organs of the plants and the morphology of their gametes. Finally, the fate of gametes is described, both in typical biparental reproduction, with particular regard to its ecological context, and in uniparental sexual reproduction (self-fertilization, parthenogenesis, gynogenesis, androgenesis and hybridogenesis).