Paul Griffiths in Aeon:
It’s uncontroversial among biologists that many species have two, distinct biological sexes. They’re distinguished by the way that they package their DNA into ‘gametes’, the sex cells that merge to make a new organism. Males produce small gametes, and females produce large gametes. Male and female gametes are very different in structure, as well as in size. This is familiar from human sperm and eggs, and the same is true in worms, flies, fish, molluscs, trees, grasses and so forth.
Different species, though, manifest the two sexes in different ways. The nematode worm Caenorhabditis elegans, a common laboratory organism, has two forms – not male and female, but male and hermaphrodite. Hermaphroditic individuals are male as larvae, when they make and store sperm. Later they become female, losing the ability to make sperm but acquiring the ability to make eggs, which they can fertilise with the stored sperm.
This biological definition of sex has been swept up into debates over the status of transgender people in society.