Professor Bill London on the Science of Sex, Gender, and Evolutionary Biology

Author(s): Scott Douglas Jacobsen

Publication (Outlet/Website): The Good Men Project

Publication Date (yyyy/mm/dd): 2025/08/18

Bill London is a Professor Emeritus of Public Health at California State University, Los Angeles (Cal State LA). He has a background in biology and public health and is known for his work in consumer health, critical thinking in health science, and skepticism toward pseudoscientific claims. His academic and professional focus includes evaluating health claims, health education, and the importance of evidence-based medicine. London in total has six degrees (two in health education, one in educational psychology, one in public health, one in biological science and one in geography). He successfully completed all coursework but not the thesis toward a master of science in clinical research degree.

In a wide-ranging interview, London contrasts this long-standing scientific understanding with modern sociopolitical discourse, particularly around intersex conditions, gender identity, and medical terminology. He explores hypotheses for the evolution of sexual reproduction, including the Red Queen, Tangled Bank, and Vicar of Bray theories. London critiques recent shifts in medicine that conflate sex and gender, arguing for evidence-based definitions and practices. He emphasizes that while human variation is real, the binary nature of sex remains foundational in biological sciences. His book review appeared in the June/July issue of Free Inquiry in the special section on evolution

Scott Douglas Jacobsen: Today, we are here with the Bill London, Professor Emeritus at Cal State LA. You recently wrote a review of The Evolution of Sex: Mating Strategies of Males and Females (2024) by Kevin Lee Teather, published by Oxford University Press. Let us begin with the fundamental definition presented in that book: What is meant by “sex” scientifically?

Prof. Bill London: The biological definition of sex, which has a long history of use and has only recently been challenged in specific contexts, is gamete-based. It applies broadly across species, including both animals and plants. The key term here is anisogamy—”aniso” meaning “unequal”—which refers to the existence of two distinct types of gametes.

One type is small and motile, produced in large quantities. The other is large and non-motile. In animals, these are sperm and egg (or ovum). In most plants, the small gamete is the pollen grain, which produces sperm cells, and the large gamete is the egg. This definition of sex—based on the type of gamete an organism produces—applies across all sexually reproducing eukaryotes.

Jacobsen: So we define sex based on gametes, but determining someone’s sex is a separate issue.

London: Defining sex is about the biological role in reproduction—what kind of gamete an organism produces. Determining sex is about how that biological role manifests—genetically, anatomically, developmentally, behaviorally—and that can vary within and between species. However, the core definition based on gamete type stays the same across species.

Jacobsen: And this definition goes way back, right? How far are we talking—when did sexual reproduction originate?

London: It goes way back. Sexual reproduction is estimated to have originated over one billion years ago, likely around 1.2 to 2 billion years ago, in early eukaryotic lineages. Still, yes, it is more than a billion years of evolutionary history.

Jacobsen: So over that billion-year history, what are the key evolutionary milestones?

London: That is a tricky question to answer in a simple list, but I can mention a few primary concepts. First, the transition from asexual to sexual reproduction was a critical turning point. Early on, organisms reproduced by simple division or budding. However, with sex came recombination, which increases genetic diversity and may have provided advantages in adapting to changing environments or escaping parasitic threats.

There are several hypotheses for why sex evolved despite its costs, like the Red Queen hypothesis, which suggests that ongoing adaptation is needed to stay ahead of parasites, and Muller’s Ratchet, which proposes that sexual reproduction helps eliminate harmful mutations. However, the evolutionary reasons for sex are still debated.

Jacobsen: So it is controversial?

London: Yes, in the sense that there are multiple competing hypotheses, and the evidence is not conclusive in favour of just one. Some explanations are more relevant in specific contexts than others. It is a rich area of evolutionary biology.

Jacobsen: There is also that part of the book discussing why there are only two types of gametes. What is going on there?

London: That is a great question. In the early evolution of sex, it is thought that gametes were originally the same size—a condition called isogamy. However, over time, many lineages evolved anisogamy, with two distinctly sized gametes. This divergence likely arose from evolutionary trade-offs between producing many small gametes to maximize fertilization chances versus investing in fewer, larger gametes with more resources to support early development.

The reason we only see two types of gametes in nearly all sexually reproducing organisms is due to the instability of systems with more than two types—it is evolutionarily stable to have one “mobile and numerous” and one “large and resource-rich” gamete type. That is why we do not see three or more gamete types persisting in nature.

Jacobsen: So sexual reproduction is not free—it has real costs?

London: There is a classic idea in evolutionary biology that “there is no such thing as a free lunch.” Producing males, for example, is costly in many species because males often do not directly contribute to offspring production beyond fertilization. That is known as the “two-fold cost of sex”. If every individual were capable of producing offspring alone, population growth would double compared to systems where only females produce offspring.

Still, the benefits of sex—like genetic variation seem to outweigh the costs, at least under many conditions.

Jacobsen: The meiotic division that produces gametes has a cost. Mating has costs. So why does sexual reproduction happen at all?

London: Right, there are definite drawbacks to sexual reproduction compared to asexual reproduction, but there are also various hypotheses that try to explain its evolutionary advantage. I did not cover those in detail in the review because they can get pretty technical.

Jacobsen: Fair enough. I am not sure how much general interest there is in that anyway.

London: True—but sexual reproduction does increase genetic variability, and that is central to most of the leading hypotheses. The book discussed three main ones: the Tangled Bank hypothesis, the Red Queen hypothesis, and the Vicar of Bray hypothesis.

Jacobsen: Would you be open to going over those?

London: Sure, though I am not immersed in this field—I would just be reading from the book. However, I am open to it if it is helpful.

Jacobsen: I will let you in on a big secret—I am not deeply ensconced in this research either.

London: Fair enough! So, here is the Vicar of Bray hypothesis, also sometimes called the Fisher–Muller model, though that is for more technical reasons. It is based on the idea that one significant advantage of sexual reproduction is increased genetic variability. The hypothesis suggests that offspring in a sexual population will be genetically diverse and, as a result, better able to respond to environmental changes.

Natural selection would then favour the individuals with gene variants—or alleles—that best suit the environment. In theory, this gives sexually reproducing populations a long-term advantage. The issue is, however, that local environments do not typically change much from one generation to the next. If the parents thrive in that environment, then genetically identical offspring—like in asexual reproduction—should do just as well. So, the hypothesis faces a problem: it works better as a long-term strategy, but natural selection operates in the short term, generation by generation. Evolution does not plan. Also, this hypothesis leans on group selection, which most evolutionary biologists reject. Selection acts on individuals, not groups.

Another hypothesis was proposed in 1974 by Michael Ghiselin. It is called the Tangled Bank hypothesis because it references Darwin’s imagery from On the Origin of Species, where he describes a “tangled bank” full of diverse organisms interacting. In such a structurally diverse environment, genetically varied offspring might be better suited to exploit different ecological niches. If siblings differ genetically, they may be less likely to compete directly for the same resources, which could benefit the species overall in a crowded or complex habitat.

However, again, there are problems. Suppose this was the main reason for the evolution of sex. In that case, we would expect to find sexual reproduction most common in species that live in densely populated, highly competitive environments. However, that pattern does not always hold up in nature. So, the hypothesis has its limitations.

That leaves the Red Queen hypothesis. This one has the most empirical support. It is also the title of a book by Matt Ridley—The Red Queen. The name comes from Through the Looking Glass by Lewis Carroll, where Alice meets the Red Queen, who tells her, “It takes all the running you can do to stay in the same place.” It is a metaphor for the evolutionary arms race, especially between hosts and parasites.

The idea is that organisms must constantly evolve to maintain their current level of fitness in a changing environment, especially as parasites and pathogens also evolve. Sexual reproduction, by continually shuffling genes and creating variation, helps organisms stay ahead of their biological enemies. This hypothesis explains why sex persists despite its costs—it enables faster adaptation.

“The Queen suddenly grabs Alice and begins running very rapidly, but they never get anywhere. Perplexed, Alice wonders why they are not passing anything—and why they remain in the same place despite all the running.”

“Now you see,” says the Red Queen, “it takes all the running you can do to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that.”

This phrase came to be used to describe the coevolutionary process between organisms that interact with one another. Initially, it was applied to relationships like predator–prey or parasite–host dynamics. Suppose an adaptation evolves in one organism to improve its survival, say, a toxin in prey that makes it less palatable to predators. In that case, there is selective pressure on the interacting organism to evolve a counter-adaptation. So, it is essentially an evolutionary arms race.

Jacobsen: You mentioned that much of the focus in this area has been on pathogens, and that links to your background. Your expertise is in public health, right?

London: Correct. I have a biology degree, but I do not do evolutionary biology research. My work has been more in health science and public health broadly.

Jacobsen: To come back to something you said at the beginning of the interview—you mentioned the longstanding biological definition of sex, centered on gametes. Could you expand on why that definition is considered so fruitful?

London: It’s a productive definition because it leads us to ask meaningful questions about differences between the sexes—about male and female reproductive strategies. The subtitle of the book I reviewed is Strategies of Males and Females. These strategies vary by species, but within a given species, they tend to be distinct.

Mating strategies differ. Parental investment differs. Sometimes, even body morphology differs. That leads into the important concept of sexual selection, which Darwin highlighted—for example, in the case of the peacock. The elaborate feathers seem less useful for survival.

London: The peacock’s ornamental plumage doesn’t help it avoid predators. It might be a liability in that sense. But it attracts peahens. That’s the core of sexual selection—traits that increase reproductive success, even if they come at a cost to survival.

Jacobsen: So what’s the competing definition of sex that’s been put forward recently? How does it differ from the traditional, gamete-based definition?

London: There isn’t a replacement definition, per se. What’s happening is that people are drawing attention to biological exceptions or variations, often in the context of humans. But biological sex, as defined by gamete type, is still the framework that works across all sexually reproducing species.

The controversy is anthropocentric, mainly. That is, it’s focused on humans and human social concerns. In biological research on animals and plants, the gamete-based definition remains central and useful. Just because there’s variability or ambiguity in some individual cases doesn’t mean the definition breaks down. It just means biology—as always—is complex.

They’re referring to males and females, but often, the discussion is motivated by a desire to make everyone feel included. People point out that some individuals have conditions that are atypical for either male or female. These conditions are rare, but they’re often used to challenge the idea of sex as a binary.

The argument is that describing sex as binary excludes some people. But I would suggest that this framing doesn’t deal with biological reality. Every person—every human being—has one biological mother and one biological father. That’s an inescapable fact of human reproduction. It took a sperm and an egg. It’s that simple.

Now, one alternative idea that’s been promoted is a continuum model of sex. The claim is that sex isn’t binary because a small number of individuals don’t fit neatly into the male or female categories. But suppose you examine these so-called intersex conditions closely. What matters—biologically—is the type of gonads an individual has. There are only two types: gonads that produce sperm (testes) and gonads that produce eggs (ovaries).

That’s the biological basis for defining sex: gamete type. Some people use the term differences rather than disorders, but medically, these are called disorders of sexual development (DSDs). These are cases where a person’s physical traits may appear ambiguous or not typical of their chromosomal or gonadal sex. However, the key determinant is still the type of gonad present.

Some rare individuals may have both types of gonadal tissue, but even then, one is usually underdeveloped or non-functional. So while it seems ambiguous, it isn’t so on a biological level.

Take the condition called Complete Androgen Insensitivity Syndrome (CAIS), for example—something neurologist Steven Novella often refers to. In this condition, the body doesn’t respond to androgens like testosterone, which are responsible for developing male characteristics. So genetically male individuals (with XY chromosomes) who have this condition develop a body that appears female in every external way—but they still have testes.

They won’t have a uterus or ovaries. To anyone encountering them, they would appear female. Still, biologically, their bodies followed the male developmental pathway until it failed to respond to testosterone.

So what do we do with that? Some people propose a spectrum, with male and female at either end and intersex individuals somewhere in between. But if you use that model, you’re effectively suggesting that some males are “less male” and some females are “less female” than others. That’s not necessarily inclusive.

The more straightforward and more scientifically grounded way to view it is that sex evolved as a binary system—two sexes—for sexual reproduction. The exceptions, while honest and worthy of understanding, don’t negate the underlying biological structure.

The fact that people don’t always engage in sex for reproductive purposes—that it brings pleasure, intimacy, and other benefits—is undoubtedly true. But from an evolutionary standpoint, sex evolved because it leads to reproduction. The pleasurable aspect exists because it encourages behaviors that result in reproduction. 

Of course, humanists and secular thinkers would say people are free to have sex for reasons beyond reproduction—and that’s fine. No one has to subscribe to a reproductive-only framework. But when we’re talking about definitions in biology, particularly across species, the reproductive role is central.

I’m not even sure how some people are defining sex these days. They seem to treat it as a vague or holistic concept, saying it can be defined in many ways. But the book The Evolution of Sex spends much time arguing why we shouldn’t use definitions based on subjective traits or piecemeal anatomical features.

Some arguments focus on individuals with mixed anatomical features—someone with certain aspects of male anatomy and others of female anatomy. But those arguments tend to be anecdotal and very human centered. They don’t offer a simple, consistent, or biologically functional definition that can apply across species.

The gamete-based definition—classifying organisms by whether they produce sperm or eggs—is objective and applicable across the biological world. When we look closely at intersex conditions through that lens, we usually find it’s not ambiguous. In most cases, the gonads indicate whether someone is male or female biologically. But people often shift the discussion to include other traits.

Jacobsen: So what’s the purpose of the newer language—terms like “spectrum,” or “differences in sexual development”?

London: From a scientific standpoint, these newer terms did not emerge from [new] research. They’re more about appreciating human variation, which is fine. I would argue that we can understand human differences—without discarding a robust biological framework—by recognizing that, just like any other body system, the reproductive system can develop atypically.

Some people prefer the term differences in sexual development rather than disorders, because it’s seen as less stigmatizing. It’s similar to using “differently abled” instead of “disabled.” It may sound kinder, but whether it changes perception or helps people is another matter.

Sometimes, this softer language minimizes the seriousness or medical relevance of certain conditions. Whether we call them disorders of sexual development, differences, or anomalies, the biological facts don’t change. It’s a matter of terminology, but the underlying science remains the same.

Another tactic people use when arguing that sex is a spectrum is to shift focus to other traits—like height, strength, or voice pitch—and show that there’s overlap between males and females. For instance, males tend to be taller, but many females are taller than many males.

That kind of argument confuses dimorphism—statistical differences in traits—with binary classification, which is about reproductive role. Just because there’s trait overlap doesn’t mean sex itself isn’t binary. So I don’t even follow the logic of saying, “Some women are taller than some men; therefore, sex isn’t binary.” It conflates unrelated concepts.

Jacobsen: I see. But that kind of thinking reflects an alternative framework. Within a professional setting—and based on your reading and experience—would you say the gamete-based definition of sex is still the one overwhelmingly used in academic and research contexts?

London: That’s a tricky question to answer directly, because it depends on the field. In biology, yes—the gamete-based definition remains standard. When biologists study animals, plants, fungi, or any other sexually reproducing species, they define sex based on whether an organism produces small gametes (sperm) or large gametes (eggs). That’s the operational definition across most of the biological sciences.

But in medicine, there’s been more of a shift. Many medical institutions and professional organizations are moving away from strict binary language. They often use terms like “sex assigned at birth” and avoid emphasizing gametes or reproductive anatomy. There’s been a growing trend toward incorporating gender identity considerations into how sex is discussed, even in clinical contexts.

Jacobsen: That seems to reflect some conflation between different concepts—sex, gender, sexuality.

London: People are increasingly conflating sex with gender, with sexual orientation, and even with sexual behaviour. These are distinct concepts, but they’re often blurred in public discourse.

If someone wants to talk about a gender spectrum, that’s a separate issue. Gender is a subjective, socially influenced identity. People may identify as nonbinary, and society can respect those identities as expressions of personal experience. There’s nothing wrong with recognizing and honouring that.

But many efforts to support transgender individuals go further—they reframe sex itself as a spectrum. And that’s where I think the science is being misrepresented. Most transgender people don’t have any biological anomalies of sexual development. Their transition—whether social, hormonal, or surgical—is from male to female or female to male. They aren’t medically transitioning to some undefined “in-between” sex. So, redefining sex as a spectrum to support gender identity feels more ideological than scientific.

Jacobsen: So you’re saying the biological definition remains central in scientific research, even if it’s being revised or avoided in some clinical or policy settings?

London: Yes. In medical policy, there’s growing acceptance of nonbinary thinking about sex, primarily to accommodate gender-diverse individuals. But in biology, sex is defined—by gametes. That doesn’t mean sex determination is the same across species. In some species, it’s based on chromosomes. In others, it’s triggered by environmental cues like temperature. But the reason we can even ask which chromosomes or conditions determine sex is because we already know what sex is: it’s about which gametes an individual produces, or is structured to produce.

Jacobsen: So, sex determination systems vary, but the definition of sex stays consistent?

London: Whether sex is determined by XY chromosomes, ZW chromosomes, haplodiploidy, or temperature, the underlying concept of sex is tied to reproductive roles. You examine the type of gametes an organism produces. That’s the constant.

In contrast, medicine has drifted somewhat from that clarity. There’s increasing deference to the social and political dimensions of gender identity. I’ll be writing more about this, but as someone focused on consumer health, I’m deeply concerned about evidence-based care, especially in areas like gender-affirming treatment.

Much of what’s promoted to patients and families today is shaped more by ideological commitment than by strong scientific evidence. Suppose you look at systematic reviews on gender-affirming care for youth. In that case, the conclusions are often more cautious than what’s portrayed in public advocacy. That discrepancy matters, especially when making decisions that have lifelong implications.

The evidence just isn’t there, but there’s a strong commitment to specific positions by some activist physicians, who’ve influenced the official statements of various medical groups.

Jacobsen: Appreciate your time. It’s been a pleasure.

London: Likewise. And thank you for the vital work you’re doing. Take care.

Jacobsen: You’re welcome. Bye.

Last updated May 3, 2025. These terms govern all In Sight Publishing content—past, present, and future—and supersede any prior notices. In Sight Publishing by Scott Douglas Jacobsen is licensed under a Creative Commons BY‑NC‑ND 4.0; © In Sight Publishing by Scott Douglas Jacobsen 2012–Present. All trademarks, performances, databases & branding are owned by their rights holders; no use without permission. Unauthorized copying, modification, framing or public communication is prohibited. External links are not endorsed. Cookies & tracking require consent, and data processing complies with PIPEDA & GDPR; no data from children < 13 (COPPA). Content meets WCAG 2.1 AA under the Accessible Canada Act & is preserved in open archival formats with backups. Excerpts & links require full credit & hyperlink; limited quoting under fair-dealing & fair-use. All content is informational; no liability for errors or omissions: Feedback welcome, and verified errors corrected promptly. For permissions or DMCA notices, email: scott.jacobsen2025@gmail.com. Site use is governed by BC laws; content is “as‑is,” liability limited, users indemnify us; moral, performers’ & database sui generis rights reserved.