This evolutionary biology paper on the evolution of two sexes, by Lehtonen & Parker, is the gold standard for explaining why there’s only two sexes in anisogamous species.

I started sharing it in June 2020. It’s great to see it being shared so much!

academic.oup.com/molehr/article…
There’s even a section that tracks every time it’s shared on Twitter and shows the tweets referencing it.
oxfordjournals.altmetric.com/details/280215…
Geoff Parker has been a critical researcher in the evolutionary origins and maintenance of male and female since at least the 1970s.

His papers have been cited thousands of times and make up a critical part of the extensive body of research on the evolution of anisogamy.
He and his colleagues wrote a foundational paper on the evolution of anisogamy and the origins of male and female in 1972, using mathematical models to show how two sexes evolved through gamete dimorphism and how they are maintained in a population.
GA Parker, RR Baker, VGF Smith. (1972). The origin and evolution of gamete dimorphism and the male-female phenomenon. Journal of theoretical biology.
scholar.google.com/citations?view…
Check out his Google Scholar list. It’s impressive.
scholar.google.com/citations?view…
Jussi Lehtonen is also an evolutionary biologist and researcher on the evolution of male and female. He’s authored many excellent papers over the past decade on these very topics. His work is great for explaining the fundamentals of the evolution of sex.
scholar.google.com/citations?user…
On Geoff Parker: "He made and later developed fundamental discoveries concerning the evolutionary mechanism for the origin and maintenance of the two sexes, and pioneered theoretical developments in sexual conflict."
royalsociety.org/people/geoffre…

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More from @zaelefty

14 Nov
I find it interesting to think about how the Sex Spectrum has similarities to Young Earth Creationism (YEC) in its pseudoscientific argument techniques.

One example is through the use of "ad hoc rescue", the use of evidence-less rationalizations to dismiss counter-evidence.
When a person's beliefs are challenged with evidence, each challenge is often met with a specific rationalization to dismiss the evidence.

This rationalization will often change each time a different challenge to their belief is forwarded. The rationale will be baseless.
For example, YEC claims that the universe was created roughly 10,000 years ago.

When cosmologists show that the Hubble Space Telescope can see objects billions of light years away, YEC claims that the light must've been created in transit or sped up to "appear" old [AD HOC].
Read 11 tweets
2 Nov
Sex determination in mammals is binary. Every karyotype in humans results in a male or female.

Genetic errors, such as the unequal distributions of chromosomes (nondisjunction), do not create new sexes.
This shows us that karyotype (the collection of an individual’s chromosomes) is conceptually independent from what sexes are.

It is true that 46:XX and 46:XY are tightly linked to females and males, respectively, but it is not true that this is the defining feature of F and M.
In other words, you can have a karyotype outside of the norm, and you will still be a male or female.

Ex: XXY = male; XO = female
Read 10 tweets
27 Oct
Gender, as described by theorists and activists as an expression of behavior, is practically “personality” at this point.

Are there personality sex differences? Yes. Does this mean a masculine girl is actually a boy? No.
This is what a typical sex difference between males and females in a Big Five personality trait looks like. The averages are very close together.

Traits like this show us there is plenty of variation within males and within females to reject rigid behavioral categorizations.
Trait Agreeableness.

Males and females, on average, are far more alike than different in this trait.

Extremes of the bell curves show little overlap, however (Ex: The MOST disagreeable people are almost always men.)
Read 8 tweets
18 Oct
Across species, it does not matter whether male and female exist in the same individual (hermaphroditism) or in separate individuals (gonochory); sex is binary in any species that has male and female reproductive functions.
For example, in sunflowers, like many other flowering plants, individuals are both male and female, and so the species is hermaphroditic.

In humans, like all other mammals, individuals are either male or female throughout their life cycle, and so the species is gonochoric.
Sex in the sunflower and sex in humans requires only two roles: a male and female, and so sex is binary in both species.
Read 10 tweets
15 Oct
2020 paper on sex differences in gene expression across human tissues.

"Human development, physiology, and disease manifest differently in males and females ... We find sex-biased regulatory network structures in each tissue."
sciencedirect.com/science/articl…
"Even though most transcription factors (TFs) are not differentially expressed between males and females, many have sex-biased regulatory targeting patterns...
Genes that are differentially targeted by TFs between the sexes are enriched for tissue-related functions and diseases."
"Our systems-based analysis identifies a repertoire of TFs that play important roles in sex-specific architecture of gene regulatory networks, and it underlines sex-specific regulatory processes in both health and disease."
Read 5 tweets
14 Oct
“If there is any trait overlap between two categories, then those two categories must be social constructs.”

False.
Males and females overlap significantly in Big Five personality traits and height, among many other things.

Does this mean the categories of male and female are not real?
Humans share large portions of their genome with many other animals. Does this mean humans are not real?
Read 4 tweets

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