Evolutionary biologists define sex through phenotype in relation to the gamete type it supports.

Think of phenotype as machinery and gamete type as the end product.

Thus, to determine what the sex of an organism is, we have to understand what function its machinery supports. ImageImageImage
Machinery that supports and produces small gametes is a male phenotype, and machinery that supports and produces large gametes is a female phenotype.

This definition does not change in the presence of infertility or genetic mutations.
[1] Cox, P., Togashi, T. (2011). The Evolution of Anisogamy, A Fundamental Phenomenon Underlying Sexual Selection. New York Cambridge University Press.
[2] Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).
[3] Arnold, A. (2012). The end of gonad-centric sex determination in mammals. Trends in Genetics, 28(2), 55-61.
Additional sources (outside the pictures):

[4] Kumar et al. (2019). Anisogamy. Encyclopedia of Animal Cognition and Behavior.

[5] Kodric-Brown et al. (1987). Anisogamy, sexual selection, and the evolution and maintenance of sex. Evolutionary Ecology, 1, 95-105.
[6] Czaran, T., Hoekstra. R. (2004). Evolution of sexual asymmetry. BMC Evolutionary Biology, 4(34).

[7] Hurst, L. (1996). Why are there only two sexes? Proceedings-Biological Sciences, 263(1369).

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More from @zaelefty

22 Nov
"The differentiation of two types of gametes is at the crux of evolution of sexual reproduction, and thus the gamete-producing tissues (gonads) and tissues involved in gamete delivery and reproduction are fundamental to the definition of an individual's sex."

-- Arthur Arnold
Arnold, A. (2012). The end of gonad-centric sex determination in mammals. Trends in Genetics, 28(2), 55-61.
This biologist, Arthur P. Arnold, argues for a more sophisticated system of sex determination outside of the gonads alone. Some sex differences precede the gonads, such as the differences in the sex chromosomes X and Y, and the subsequent sex-based transcription factors.
Read 7 tweets
21 Nov
With Twitter silencing intersex / DSD advocates such as @intersexfacts, it's more important than ever to continue to follow, support, and learn from people with DSDs.

Below is a list of important accounts to follow and support when it comes to sex development conditions.
I highly recommend following @RaeUK, @ClareCAIS, @soulfoodie, @anubisgatesis, @XO83766320, @XOXYKZ, @IntersexAF, @v281l, @mum_sex, and @dsdfamilies.

These accounts provide a great diversity of perspectives and insight into both the science and experience of living with DSDs.
@EndosexD is also another highly knowledgeable person when it comes to DSDs. He doesn't have a DSD himself, but he went through the complex sex determination process as a parent and understands the complex issues surrounding DSDs from a personal level.

His insight is invaluable
Read 7 tweets
15 Nov
If we view sex as a spectrum of sex characteristics and morphology, and ignore the higher order function of sex (reproduction), there is nothing stopping us from categorizing each person into their own sex category.

This is what we see with queer Theory.
Queer Theory, from which the sex spectrum arguments are derived, argues that the categories of male and female are arbitrary, determined by society, and that sex characteristics can relate to any identity within any grouping. It is all about social / political power, not biology.
If sex is not about male and female (reproductive function) but rather a conglomeration of bodily characteristics detached from function, then there is no male and female. There is just unique individuals, each with their own variations of sex characteristics.
Read 8 tweets
14 Nov
Many queer (theory) activists found this tweet, and as expected, they are conflating sex and gender, claiming sex is a spectrum, and other absurdities. 😜

I'll be quote tweeting some responses.
I'll list my quote tweets below!
Read 13 tweets
25 Oct
Brain differences are great! But they don't prove what you think they prove.
Specific regions have been shown to be different in certain populations, affected by genetics and the environment, but if we take all the traits of someone's brain and aggregate them together, their sex can be almost always identified.
For example, in homosexual transsexual MtFs before cross-sex hormone treatment, their gray matter, white matter, and cerebral spinal fluid volumes all match male controls. We can find subtle differences, but overall, brains can be identified as belonging to male or female.
Read 5 tweets
24 Oct
An excerpt about the biological definition of sex:

"The biological definition of sex is not just one option among many, or a matter of arbitrary preference: the very existence of differentiated males and females in a species depends on the existence of two gamete types."

"Chromosomes and hormones participate in the mechanics of sex determination and sexual differentiation, but do not play the same foundational role."

"Crucially, anisogamy gives rise to a true sex binary at the species level: even if a given individual may fail to produce viable gametes, there are only two gamete types with no meaningful intermediate forms."

Read 5 tweets

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