either of the two main categories (male and female) into which humans and many other living things are divided on the basis of their reproductive functions (OED).
It is a biological category that divides people according to their genetics as well as sex characteristics such as reproductive organs. Nobody has any choice in what sex they are born as. They pop out into the world as either one or the other [1], and must deal with that as they will.
There are processes that exist in the material world, and then there are the concepts (or universals) that we humans abstract from the material particulars in order to think and talk about them: ‘sex’, ‘male’, ‘female’, etc. These concepts are culturally mediated, but changing the words, or changing the way we think about and categorise sex, will make no difference whatever to the material facts. Those exist regardless, independently of human thought.
As a concept sex seems simple enough, until you look at it closely. Some of what I’ll cover below is a bit technical, but as with genetics, it’s important to ground ourselves in facts before we get onto the social and philosophical questions.
Methods of reproduction
No living organism is immortal, so for a species to keep existing, it has to reproduce, i.e. to create new generations that will live on when the older generations die. Natural selection has produced, over billions of years, a rich diversity of methods, but all can be categorised as either asexual or sexual. All bacteria reproduce asexually, most animals reproduce sexually, and some species can adopt either method, depending on the circumstances (e.g. wasps, starfish, aphids, komodo dragons).
In essence, sex is an evolved mechanism for the reproduction of species.
Asexual reproduction
In asexual reproduction, one organism makes a clone of itself. Bacteria, some plants and even some animals (e.g. some sponges and sea anenomes) reproduce this way.
There are several methods of asexual reproduction. We needn’t look at them in detail, as my concern here is with human beings, but very briefly, the four main types are:
- Binary fission: meaning ‘division in half’, this usually occurs in bacteria. A single parent cell splits into two, copying its DNA into the new cell.
- Budding: the parent organism grows a bud or outgrowth which develops into a new organism, which may or may not separate. Used by some plants and some animals.
- Parthenogenesis: meaning ‘virgin birth’, this is reproduction from an egg without fertilisation by sperm. Used by some plants and some animals.
- Fragmentation: part of the parent organism breaks off and then develops into a new individual. This is different to budding because the new organism begins as a separate piece to the parent, not just an outgrowth.
What the methods have in common is that they allow one individual to churn out offspring all by itself, with relatively little expenditure of energy.
Sexual reproduction
In sexual reproduction, two parents of the same species – in anisogamous species (see below), one male, one female – combine their genetic material to create offspring that inherits a mixture of its parents’ genes and will therefore be slightly different to both.
Sexual reproduction requires the fusion of the two parents’ genetic material in the form of gametes, or sex cells. In animals, the male gamete is the sperm and the female is the egg, and fertilisation happens when the two come together. Fertilisation doesn’t necessarily involve sexual intercourse – and thanks to the technology of artificial insemination, this can now be true even in human beings.
The two parents don’t always have to be separate individuals: hermaphroditic organisms (e.g. earthworms) can supply both male and female gametes, and self-fertilise. Since two gametes are required, however, this is still considered sexual reproduction.
In summary, there are two models for organisms to reproduce themselves:
| Asexual | Sexual |
|---|---|
| No sexes, one parent | Two sexes, two parents |
| Individual makes a copy or clone of itself | Two gametes whose fusion leads to fertilisation |
| Offspring genetically identical to parents | Offspring genetically variant to parents |
| Quick and easy, but no genetic variation | More resource-costly, but gives genetic variation |
Of course there are exceptions. Some plants and fungi can do either asexual or sexual reproduction depending on the conditions. Even some animals can do this, via parthenogenesis. There are plants and a nematode worm that are trioecious, i.e. the organism has three forms – male, female, and bisexual or hermaphrodite. However the latter possesses both sex parts at once, so even this ‘third sex’ is built upon the male-female binary.
Evolution is conservative. Sexual reproduction involves two parents, and two gametes.
Isogamy and anisogamy
The vast majority of sexually reproducing species are divided into ‘male’ and ‘female’, defined by the size of the gametes they produce:
Male: small gametes
Female: large gametes
This is the textbook, scientific definition of male and female.
This dichotomy in gamete size, which is found in almost all complex multicellular eukaryotes [2] (a group which humans belong to), is known as anisogamy. When the differential in size is accompanied by a differential in motion where the small gamete is mobile and the large gamete is immobile, this more extreme form of anisogamy is known as oogamy, which is found in almost all animals. This dichotomy leads to varying degrees of sexual dimorphism, i.e. when males and females have different body types, as in our own species.
In many single-celled organisms, sexual reproduction occurs without a dichotomy in gamete size (and thus also no dichotomy of sexual dimorphism). This is known as isogamy, and the outcome is the same as in anisogamy, i.e. the fusion of the two gametes leads to fertilisation. However, because the gametes in such species are the same size, we don’t classify them as male and female. Instead, isogamous organisms use a system of mating types. Most commonly there are two, called + and - (plus and minus), though there can be more.
Isogamy is thought to be the ancestral form from which anisogamy evolved – the origin of male and female lies in this transition. In isogamy the parental investment is equal, whereas in anisogamy the female invests more resources in each gamete than the male, and the potential appears for natural and sexual selection to produce morphological differences between the sexes.
Overwhelmingly, organisms have either no sex or two sexes.
Why have two sexes?
Sexual reproduction is more costly in time and energy. It requires organisms to find and court a mate, involves lengthy gestation, and demands parental care, and therefore constrains reproduction. Asexual reproduction is quicker and easier: one aphid for example can have 600 billion offspring in one season.
So why have two sexes? The most likely explanation relates to genetic diversity.
In asexual reproduction, because there is no other parent, i.e. no other source of genetic material, every new generation will be genetically identical to the previous one. (Genetic variation does arise, but through genetic mutation.) This works fine when conditions are stable, but it can potentially put the species at risk.
- Each generation has the same vulnerabilities. If a disease appears that can kill one individual, it can kill the entire population.
- Each generation depends upon the same habitat, or at least there must be no change in habitat that will harm the species’ survival. If the selection pressures change, the population will adapt much more slowly.
Sexual reproduction by contrast relies upon the combination of genetic material from different parents, which over several generations can result in a lot of variation.
- Each generation has genetic variation within the population. If a disease appears, there is more chance that some individuals will have resistance and survive.
- If habitat changes, there is more chance that some individuals will have traits that allow them to adapt.
These are two strategies developed by evolution to ensure the survival of organisms. The fact that organisms of both kinds still exist tells us that both strategies have (so far) been successful in perpetuating species. No one method of reproduction is ‘superior’ to another: there are simply different strategies that have proved successful for certain species under certain conditions.
The answer, then, to the question ‘what is sex?’ is – from a narrow biological perspective – that sex is a means for certain species to reproduce themselves by fusing male and female gametes to ensure genetic diversity. In the next article I’ll look at how this works in our species, Homo sapiens.
Footnotes
[1] Even people with DSDs are basically either male or female, unless they’re one of the tiny percentage whose sex is ambiguous. I discuss intersex/DSDs separately here.
[2] Jussi Lehtonen, Hanna Kokko, and Geoff A. Parker, ‘What do isogamous organisms teach us about sex and the two sexes?’ (2016).
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