Sex and Gender
By: Jesse Parrish
Tags: Biology, Sex, Trans, LGBT, Gender

Biological Sex

Biological sex as a concept is rooted in gene transfer and reproduction. Back in the dark ages, some 15 years ago, my high school biology textbook gave a definition of sexual reproduction: cellular meiosis — the production of haploid cells from a diploid cell — and recombination, usually with another haploid cell and potentially from another organism. This in contrast with asexual reproduction, in which organisms are reproduce by binary fission or mitosis, or genetic combination by other mechanisms, as found in retroviral insertions or horizontal gene transfer in bacteria. In multicellular organisms with well-defined cell types for specific functions, like animals, plants, and fungi, cells either play a role in reproduction or not, the latter being somatic cells. These cells have the DNA of an organism, but the DNA from somatic cells is not passed onto offspring. The evolutionary significance of this has been understand for over 100 years.

Given a modern understanding of genetics, sexual reproduction is contained within the biological concept of meiosis, in which cells with two copies of the genetic material (diploid) are divided into cells with a single copy (haploid). There is (probably) a single common ancestor of all organisms which utilize sexual reproduction, the descendants of which form a clade known as eukaryota. The eukaryotes have structures which are necessary for the process of meiosis. These definitions are applied to the biological domain in general, from fungi to rhododendrons to leopards. A key difference between kingdoms is in how meiosis and recombination occur in an organism’s lifecycle. In certain protozoa, plants, and fungi, the lifecycle of the organism can include haploid stages, these being capable of reproducing asexually or by recombination with another haploid cell. For example, plants and algae can form asexually reproducing spores, and in certain species, the lifecycle is haploid dominant, meaning that the somatic cells are haploid, with diploidy only occurring in zygotes. Closer to ourselves, the animal and fungal kingdoms comprise a defined lineage, opisthokonta, which is the clade of organisms having motile sperm with a single, posterior flagellum. In the animal kingdom, which is defined by having both differentiated tissue layers and heterotrophy, the lifecycle is simpler: haploid cells do not reproduce without first undergoing recombination, and the lifecycle of the organism is diploid dominant.

Animals reproduce sexually or asexually. Asexual reproduction can occur by regular cell division. A new, reproductively viable starfish can regrow from a severed limb. Otherwise, it can occur meiotically. In animals, meiotic asexual reproduction is known as parthenogenesis, and organism depending, it may be obligate or facultative. Aphids are facultatively parthenogenic, sometimes with alternating sexual and asexual generations within the same population living on the same host plant. Parthenogenesis may occur in one of two ways: apomictic or automictic. In the former, a new organism is derived from mitosis of a single cell — always the ovum, or egg — and in the latter, the new organism is derived from a meiotic process which results in a diploid cell without recombination involving another organism’s haploid cell, this being able to occur in a few different ways.

At this stage, male and female are already present. Parthenogenesis only occurs in female organisms. However it occurs, the heart of animal reproduction across the kingdom is the egg cell. There are no animals derived solely from sperm. This is absolute enough to function as a clean definition of the difference between sperm and egg, but the differences are physically obvious, and these are typically used. Eggs are always larger and non-motile. Sperm are smaller and usually motile, unless the flagellum has been lost. Organisms which have structures for the production and support of sperm or eggs are male and female, respectively.

With the introduction of sexual structures, definitions and concepts break down from the level of animal to that of particular families or organisms. We are moving from biological sex generally to how we talk about sex in animal species and organisms. However, the general concepts of male and female are already established.

Particular animal species instantiate these concepts. Whether an animal is sexed as male, female, neither, or both is defined on the species level, not the individual level. It makes sense to speak of the male reproductive structures of a specific species, these varying wildly, but they all instantiate the general concept of sperm production or facilitation. If a biologist discovers a new, sexually dimorphic species and claims that the males are larger, we can either confirm or falsify his hypothesis by confirming which types of sex cell the organism produces. That biologist is not entitled to redefine what constitutes “male” as it relates to that particular organism; such an effort would be treated as an absurdity.

This concept requires refinement, and as we are at the species-level, we can sensibly do so. Given a species and male reproductive structure, the presence or absence of that structure in an individual is a sufficient condition for that organism being male, but not a necessary one at any given moment. It’s the lifecycle, whether or not that structure is produced in the ordinary developmental pathway of the organism, that defines the particular organism as male. Necessarily this “ordinary lifecycle” is a species-level concept, undefinable by an isolated individual without phylogenetic inference. Zygotes which never survive to the point of sexual differentiation can be sexed if the genetic makeup and lifecycle of the organism are well understood, just as men do not cease to be biologically male if they are castrated.

Most importantly, there are two sexes in every species at this level of organization, defined strictly by the presence or absence of male or female reproductive structures in the lifecycle. In other words, there are two biological sexes, and species depending, an organism can be female, male, both, or neither. Further, to my knowledge, no animal species has “neither” as a part of its lifecycle. Species which produce sterile offspring as part of the lifecycle, such as ants, bees, and naked mole rats, generally produce sterile females. All parthenogenic organisms are female. Animals which can reproduce by fission nevertheless have a sexual lifecycle.

It would be non-arbitrary to classify a particular organism with certain mutations which precludes the possibility of sexual development entirely as “neither,” so long as we have a thorough understanding of the genetic components involved and the organism lifecycle. That said, it’s an evolutionary dead end, and being so, I doubt that animal species, even a eusocial species like bees, would naturally evolve a stable “neither.” Thus there is an argument that only three of these four combinations are applicable to the animal kingdom.