Three Reasons Why There are Two–and Only Two–Biological Sexes.
Zach Elliot is an author of two books on sex and gender and a producer of 20+ animated videos on sex differences. Here is the intro to his article: "What Are Sexes? There is much confusion in our current culture as to what sexes are and what they are not."
There is much confusion in our current culture as to what sexes are and what they are not. When biologists make a claim about the number of sexes in a species, they are not making a claim about chromosomes, body types, or personal identity; rather, they are making a claim about the number of distinct reproductive strategies in that species.
A reproductive strategy is an evolved system for propagating genes and forming a new individual. In sexually reproducing species, producing a new individual requires the combination of at least two distinct and complementary reproductive strategies. These strategies are fulfilled through the delivery of genetic material in sex cells called gametes, which have half the genetic material of the parent. When two gametes fuse, they form a genetically unique individual with a full set of chromosomes.
Some species reproduce through gametes of the same size (isogamy) and can have many unique reproductive strategies called mating types, which control what gametes can fuse with one another, but their differences do not go far beyond the molecular level. On the other hand, most species in the plant and animal kingdoms reproduce through gametes of differing size and form (anisogamy), where there is an asymmetry in size and behavior between the interacting gametes and often the individual organisms themselves.
When gamete sizes are differentiated (anisogamy), there are typically exactly two sexes, no more and no less. In such systems, the reproductive strategy that produces the smaller gametes is designated as male and the reproductive strategy that produces the larger gametes is designated as female. It is not the physical size of the gametes themselves that differentiates the male and female reproductive strategies, but rather what those size differences represent.
