Let’s talk about (biological) sex—part II

When philosophy meets biology: should we stop talking about sex?

Philosophy and biology, by Midjourney.

Last time we began a discussion of a recent paper in the philosophy of biology, authored by Aja Watkins and Marina DiMarco, in which the authors suggested that it would be better for biologists to do away with the notion of sex.

Part of their argument was that there are exceptions in the biological world, species for which the concept of sex seems problematic. For instance: “In some shark species, many reptiles, and some birds, egg-producing individuals can reproduce asexually via parthenogenesis. New Mexico whiptail lizards (Aspidoscelis neomexicanus) now only reproduce this way; there are no remaining ‘males.’” (p. 5)

The implication seems to be that these cases are somehow problematic for the gametic view of sex. They aren’t. There are species of birds who have lost the ability to fly (e.g., penguins), which doesn’t negate the broad generalization that birds, usually, are flying vertebrates. Again, in biology exceptions are understood from an evolutionary perspective, within general frameworks provided by concepts like “sex,” “species,” “flying vertebrates,” and so forth.

New Mexico whiptail lizard (Aspidoscelis neomexicanus). Image from Wikimedia, CC license.

Watkins & DiMarco then go on to discuss more sophisticated philosophical accounts of sex, such as the “homeostatic property cluster” one. These are interesting, but get rather technical, and they don’t change the basic goal of their paper: the standard (i.e., gametic) definition of sex in evolutionary biology ought to be eliminated from scientific parlance because it is, in the authors’ view, “problematic” on multiple fronts.

Notwithstanding Watkins & DiMarco’s frequent “worries” (a standard locution in technical philosophy papers), biologists find the gametic definition of sex useful because it is causally and historically connected to other relevant biological properties, such as different morphologies, mating strategies, parenting behavior, and so on. As another philosopher, Paul Griffiths (quoted by the authors), puts it:

“The payoff for this way of thinking about sexes is that it helps to explain the evolution of reproductive systems and how they differ across the diversity of life.” (p. 8)

Addressing the issue of exceptions, Griffiths continues: “There are some species at the boundary between unicellular and multicellular life, such as some volvocine algae, which can be seen as representing transitional states in the evolution of distinct biological sexes and might be described as having more than two sexes. They produce slightly anisogamous gametes and in a range of sizes rather than two discrete types. But in complex multi-cellular organisms like plants and animals we find two very different kinds of gamete, each associated with a fundamentally different reproductive strategy, and so two biological sexes.” (p. 9)

This is an observation in search of an explanation, which is how biology (and science more broadly) works. Accordingly, evolutionary biologists have produced a large literature on the evolution of mating systems, a literature that includes theoretical models that are informed by, and attempt to explain, the available empirical evidence.

A bit later on, Watkins & DiMarco make a big deal (e.g., at p. 10) of the distinction between sexes and “mating types”—both terms in current biological usage—and rhetorically ask why not get rid of the concept of “sex” altogether and just use mating types instead.

Technically, mating types indicate distinct strains within species of microorganisms that are genetically compatible for sexual reproduction, essentially acting as the equivalent of “male” and “female” in more complex organisms. Mating types are found in fungi and algae, where individuals can be morphologically similar but have different functional types designated by symbols like “+” and “-“ to indicate compatibility for mating. These types are determined by specific genetic regions called mating-type loci on their chromosomes. The idea is that mating types evolved into usually morphologically and behaviorally distinct sexes. Hence the need to keep mating types and sexes conceptually distinct. Besides, I bet that if we simply replaced “sex” with “mating type” Watkins &DiMarco would still not be happy and would want us to be eliminativists about mating types as well. That’s because the motivation for their paper is, as we shall see, primarily political, not scientific.

Two mating types of the yeast Saccharomyces cerevisiae. Image from Wikimedia, CC license.

Again, Griffiths makes a lot of sense to me: “[The gametic] approach to the evolution of sexes is very widely accepted in contemporary biology. That does not mean that it is the final word, but it does mean that a philosopher who wants to dismiss it as a mass of error had better have some powerful arguments.” (p. 10)