Apparent independent and multiple evolution of binary sex

February 14, 2025 • 9:35 am

My friend Phil Ward at UC Davis found this reference and called it to my attention. It’s from the Philosophical Transactions of the Royal Society (B), and access is free (click on title below).  The pdf with the numbered references is here.

The paper is about how the evolution of two different types of gametes (“anisogamy”: a requirement for the origin of biological sexes) can originate from isogamy (same-sized gametes) under certain conditions. It is a theoretical paper, and I haven’t read it closely as I’m math-averse. However, what’s of interest is the first paragraph of the paper, which reviews the literature on anisogamy.  That paragraph states that anisogamy (ergo biological sex) has originated independently in many groups of eukaryotes (organisms with true cells).  I’ve put that first paragraph below and have bolded the relevant part. I’ve also linked to each group so you can see what they are.  The numbers lead to the references, which I have not checked.

Multicellular organisms typically produce gametes of two distinct size classes: larger eggs and smaller sperm. This dimorphism—known as ‘anisogamy’—is a remarkable case of convergent evolution. It has arisen independently in multiple distantly related eukaryotic lineages, including in animals [1]; dikaryotic fungi [2]; various groups of green algae, including the ancestors of land plants [3,4]; red algae [5]; brown algae [6,7]; yellow-green algae (Xanthophyceae: Vaucheria) [8]; diatoms [9]; oomycetes [10]; dinoflagellates [11]; apicomplexans [12]; and parabasalids (Trichonympha) [13]. By contrast, the gametes of most unicellular and some multicellular eukaryotes are isogamous, with a unimodal distribution of gamete sizes. Anisogamy is often taken as the defining difference between ‘male’ and ‘female’ sexual strategies: males produce only sperm; females produce only eggs; and hermaphrodites have the potential to produce both gamete types, either simultaneously or at different life stages. Isogamous species lack sexes by this definition. However, their gametes can often be classified into two, or occasionally more, ‘mating types’, such that fertilization only occurs between gametes of unlike types [2,14–16].

If you add up these groups, you get at least 11 evolutionarily independent origins of anisogamy: the production of “larger eggs and smaller sperm.”  The independence is probably inferred via a “cladistic” method by looking at the family trees of these groups, seeing that the ancestors were either asexual or isogamous, and noting that anisogamy appeared on a later-appearing derived branch.

If the authors are indeed correct, then what we have here is a remarkable example of evolutionary convergence: eleven separate groups independently evolving binary sex with large eggs and small sperm.  There are of course evolutionary theories showing why an ancestral condition of sex with equal-size gametes would split into a derived condition with two sizes of gametes, but that is a theoretical result.  Here we see that this has actually happened in nature nearly a dozen times, so the theories may hold some water.

I’ll add one thing. Not only has anisogamous sex apparently evolved eleven times independently, but, even in the one group of animals the determinants of sex—the features that trigger the development of two types of animals producing different-sized gametes—have also evolved independently. Luana and I pointed this out in our paper, “The ideological subversion of biology” (bolding is mine):

We can see the stability of the two-sex condition by realizing that what triggers the development of males versus females varies widely across species (Bachtrog et al. 2014). Different sexes can be based on different chromosomes and their genes (e.g., XX vs. XY in humans, ZW vs. ZZ in birds, individuals with like chromosomes being female in mammals and male in birds); different rearing temperatures (crocodiles and turtles); whether you have a full or half set of chromosomes (bees); whether you encounter a female (marine worms); and a host of other social, genetic, and environmental factors. Natural selection has independently produced diverse pathways to generate the sexes, but at the end there are just two destinations: males and females. And so we have an evolved and objectively recognized dichotomy—not an arbitrary spectrum of sexes.

Now I’m not smart or diligent enough to figure out why once there are two sexes—which is the case in animals, and must thus have been true in our common ancestor)—how you can evolutionarily traverse from one determinant of sex (say a gene on a chromosome) to something like temperature-dependent sex determination or social sex determination (e.g. the famous clownfish, used by miscreants to claim that there are more than two sexes).  It’s a mystery waiting to be solved.  But so even here, in one group, we have convergent evolution—of the factors that cause the two sexes to diverge.

I find all this fascinating, and it shows the power of Orgel’s Second Rule: “evolution is cleverer than you are.”

16 thoughts on “Apparent independent and multiple evolution of binary sex”

  1. The unicellular green alga I study, Chlamydomonas reinhardtii, is isogamous in size but the gametes of the two mating types are anisogamous in ultrastructure: plus gametes send out a microvillus-like protrusion to contact minus gametes and carry specific genes encoding this trait. Hence I would argue that there’s more to “morph” than size. Chlamy has close unicellular relatives that make gametes of two different sizes. Lots of options.

    1. That’s cool about Chlamydomonas! The emphasis on gamete size is from the theory by Parker: selection favours both big gametes and lots of gametes, but an individual can’t maximize both quantities, so anisogamy by size is a sort of compromise. At least that’s how I understand the theory.

    3. Here‘s a paper on „Gamete dimorphism of the isogamous green alga (Chlamydomonas reinhardtii)“: https://www.nature.com/articles/s42003-022-04275-y

      „The gametes of isogamous species are of similar size and appearance. Therefore, two gametes belonging to opposite mating types generally cannot be distinguished from each other based on their size or morphology in isogamous species. Such features of the gametes are crucial because they define mating type and sex. Even so, bipolar sexual differentiation is present in isogamous species, e.g. cytoplasmic inheritance, gamete recognition and adhesion mechanisms and prefusion mating behaviour, and so these dimorphisms must have preceded the evolution of two sexes.

      Studies of chlorophyte algae indicate that two gametes of opposite mating types can be distinguished based on their morphology, irrespective of the gamete size difference. This morphological feature is a mating type- or sex-specific asymmetric positioning of the mating structure (cell fusion apparatus of green algae) and/or cell fusion site of the gamete, which occupies different positions between the opposite mating types or sexes (Fig. 1). The gamete can be divided into two morphological types (α and β) based on this difference.“

  2. I had no idea that anisogamy has originated several times. I suppose, in retrospect, I’m not surprised, as modeling suggests that isogamy may be an unstable state.

  3. Hi Jerry ; from your post:

    “Now I’m not smart or diligent enough to figure out why once there are two sexes—which is the case in animals, and must thus have been true in our common ancestor)—how you can evolutionarily traverse from one determinant of sex (say a gene on a chromosome) to something like temperature-dependent sex determination or social sex determination (e.g. the famous clownfish, used by miscreants to claim that there are more than two sexes). It’s a mystery waiting to be solved. ”

    There is quite a bit of good work on both these questions [ selection on alternative sex determining mechanisms and dioecy( separate sexes) vs sex-change or simultaneous hermaphroditism].
    For recent papers/books on the topics, use Google Scholar to find recent references that cite the 2 classic books.
    1]Bull, James J. 1983 EVOLUTION OF SEX DETERMINING MECHANISMS
    2] Charnov, Eric L. 1982 THE THEORY OF SEX ALLOCATION

    Both Bull and I are on Google Scholar.

    Ric Charnov

      1. And for non-mathematical evolutionary overviews of anisogamy, sex determination, sex allocation, and alternative reproductive tactics, see either of my books on Hermaphroditism, or Pregnancy (both books are also on Google Scholar, where recent references that cite them can likewise be found)

        1. I really enjoy the professionals and even wise amateurs we have commenting at WEIT. That, and the fact the boss edits out the wild loud mouth trolley crazies (not the case in most places like twitter and even below my own articles) makes WEIT top notch.

          D.A.
          NYC

  4. I once considered writing a Science Fiction story where a race had 3 sexes, two motile and one sessile. It made my brain hurt trying to provide an evolutionary explanation of how this came to be. I came to the conclusion that an organism of two sexes might ‘enjoy’ enough extra fitness to make the whole sex business work but three sexes would probably reduce fitness and so fail. I wish my maths was up to the task of proving it.

    2. A main character in Asimov’s novel The Gods Themselves is of a three-sex species (Rational, Emotional, Parental).

  5. I was pretty sure that its been known for many years that anisogamy evolved independently many times. But maybe that was not known for sure? Anyway, I did not know there were fungi that could do this!

    I am very glad the authors are not tempted to equate mating types with the sexes. I see that from time to time and it chafes, especially when referring to species with multiple mating types.

    Finally, it is so good to not suffer the mental gymnastics that certain professors must go thru in order to sustain incompatible beliefs about basic biology and moral ideology. It is good to not have two masters.

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