Allopatric reproduction

Anisogamy is a reproductive system defined by unequal gametes: the female gamete is large and carries food reserves, while the male gamete is small, numerous, and principally transports genes. In The Selfish Gene, this difference is more than a way of classifying sex. It is the proposed evolutionary starting point for unequal parental investment, divergent reproductive strategies, and conflicts between mates, though the argument also stresses that genes act through complex interactions rather than as isolated causes.

Definition and Biological Basis

The book distinguishes the sexes by the gametes they produce, not by a list of bodily features that varies among species. Mammals may commonly be classified by traits such as pregnancy, milk production, or particular chromosomes, but those criteria do not apply universally: frogs, for instance, do not have a penis. The more general distinction is that one sex produces large gametes and the other small ones. The terms female and male are applied respectively to these large- and small-gamete producers across animals and plants. Even in humans, where the egg is microscopic, it is many times larger than sperm; in birds and reptiles, an egg may contain enough nourishment to sustain a developing young animal for weeks.

This definition also clarifies what is distinctive about anisogamy by contrasting it with isogamy, found in some primitive organisms such as certain fungi. In an isogamous system, individuals are not divided into two sexes: they produce equivalent gametes, and any compatible gamete can fuse with another. If three gametes, A, B, and C, are available, A can fuse with either B or C, and B can fuse with either A or C. In a conventional sexual system, by contrast, a sperm can fuse with eggs, but the eggs do not fuse with one another. The division into male and female therefore rests on gamete difference, not simply on the existence of sexual reproduction.

From Equal Gametes to Divergent Strategies

The evolutionary account begins with an imagined isogamous population whose gametes are interchangeable and roughly equal in size. Some gametes might happen to be slightly larger than average. Their additional food reserves could give the resulting embryo a better start, creating selection favoring larger gametes. Yet larger investment also creates an opportunity for exploitation: a producer of smaller gametes could benefit if those gametes were more mobile and more likely to find and fuse with the larger ones.

Small, fast gametes could be produced in greater numbers, potentially allowing their producer to have offspring with more partners. Selection would thus favor two diverging strategies. The large-gamete strategy invests more resources in each gamete; the small-gamete strategy invests less in each but produces more and enables them to seek larger gametes. Once divergence began, intermediate sizes would be disadvantaged because they would gain neither the advantages of abundant, mobile small gametes nor the substantial initial resources supplied by large ones.

The proposed divergence then becomes a runaway process. Small gametes become smaller and more mobile, while large gametes become larger to compensate for the declining contribution of their partners and eventually remain stationary as the smaller gametes actively seek them out. The large-investment strategy is described as the honest strategy and the small-investment one as exploitative. Although a large-gamete producer would benefit from fusing with another large one, the account argues that selection to bypass that barrier would be stronger among small-gamete producers, who had more to gain. In this model, eggs and sperm are the outcomes of the two strategies.

Investment and Conflict Between Mates

The unequal gametes carry unequal resources into reproduction. When two isogametes fuse, each contributes equal numbers of genes and equal amounts of food reserve. Sperm and eggs contribute equal numbers of genes, but eggs contribute far more food; sperm contribute none and serve to transport their genes to an egg. At conception, therefore, the father has invested less than half of the resources in the offspring. Since a male can produce many millions of sperm a day, he can potentially sire many offspring in a short time with different females, whereas the female's supply of eggs limits how many she can produce.

The book links that initial imbalance to a continuing conflict over care. Mates share half of each other's genes through their children, which can give both an interest in cooperating to rear them. But each parent may also benefit by investing less in a given child and directing resources toward other offspring with other partners. In this gene-centered framing, each partner can be understood as having an interest in the other contributing more. The potential for cooperation therefore coexists with mutual exploitation; sexual partnership is not assumed to be an uncomplicated joint enterprise.

The general prediction is that males will tend toward promiscuity and less paternal care, while females have two broad counter-strategies, called the he-man and domestic-bliss strategies. Which tendency prevails depends on ecological circumstances, and real breeding systems range from monogamy to promiscuity and harems. Some cases even have fathers providing more childcare than mothers. The account presents these patterns as outcomes of conflicting reproductive interests shaped by circumstances, not as a single behavior that applies uniformly to every species.

From Individual Reproduction to Population Arguments

Anisogamy also figures in the book's challenge to explanations based on benefit to the species as a whole. Wynne-Edwards proposed that individual animals deliberately reduce birth rates altruistically to regulate population size. The hypothesis is attractive because human populations can grow rapidly when the average couple has more than two children who survive to reproduce. At the scale of the world, immigration and emigration do not change the total population, leaving births and deaths as the relevant factors; population growth also depends on generation spacing, not only on the number of children.

The book's contrasting explanation asks how reproductive decisions affect the transmission of genes, rather than whether they benefit a population collectively. The difference between egg and sperm production supplies part of its account of why individual reproductive interests can diverge: a male's potential offspring number is much less constrained by gamete production than a female's. That distinction does not itself settle every question about care or population regulation, but it helps explain why a proposal that individuals limit reproduction for the group's good requires scrutiny at the level of individual and genetic advantage.

Scope and Limits of the Argument

The treatment of anisogamy belongs to a wider argument about what a gene can and cannot explain. A modern survival machine contains thousands of genes, and building a body is a cooperative process in which each gene's effects depend on many others. Sexual reproduction shuffles genes between generations, so an individual is a temporary vehicle for a combination that may not persist, while particular genes can continue through many bodies. The book's gene-centered perspective depends on this distinction between short-lived combinations and longer-lived replicators.

It also cautions against treating expressions such as a gene for a behavior as literal descriptions of a single cause. No gene independently builds a body part; development involves interacting genes and the external environment. Likewise, the claim that genes influence behavior is not a claim that they make moment-to-moment decisions: the nervous system directs immediate action, while genes influence the development of the survival machine and its nervous system. Anisogamy is therefore presented as an evolutionary basis for differences in reproductive investment, not as a complete explanation of every sex difference or a direct prescription for how individuals behave.

The 1976 first edition retained a youthful, revolutionary quality, and its author chose to leave the original text standing in the second edition rather than revise it page by page. The account of anisogamy remains part of that larger gene-centered project: a theory that uses differences in replication and investment to explain conflict, while acknowledging that development and behavior emerge through intricate relationships rather than single, self-sufficient causes.