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What are the consequences for sex ratio theory if human sex ratio is not heritable? This is the subject of a recent exchange in @RSocPublishing #ProcB that started with this paper by Zietsch et al from 2020, using data from the entire Swedish population: https://royalsocietypublishing.org/doi/10.1098/rspb.2019.2849#.YIK0rNG4tNs.twitter
They estimated the heritability of human sex ratio (the proportion of the variation in the probability of having a son versus a daughter that is due to additive genetic variation) to be 0, with an upper 95% confidence interval of 0.002
Based on this finding, they concluded that this renders "Fisher's principle and several other existing hypotheses untenable as frameworks for understanding human offspring sex ratio."
In 2011 I concluded something similar when I found that all variation in sex ratio in song sparrows is due to sampling: the number of heads in a series of coin tosses will vary, even if the probability of heads is a constant 0.5 https://royalsocietypublishing.org/doi/10.1098/rspb.2010.2763#.YIK43TFhjrg.twitter
However, not everyone agreed: last month, @RSocPublishing #ProcB published two comments that accepted Zietsch et al's finding of zero heritability, but not their interpretation
In the first, Lehtonen argues that the heritability of offspring sex ratio at equilibrium is not an assumption of Fisher's principle and that zero heritability is not evidence either for or against it https://royalsocietypublishing.org/doi/10.1098/rspb.2020.2632#.YIK8nXNl__Y.twitter
In the second, Orzack & Hardy agree: "absence of inherited variation is not evidence against the claim that [...] frequency-dependent selection has influenced the human sex ratio" and "if and when this has influenced the human sex ratio remains unresolved" https://royalsocietypublishing.org/doi/10.1098/rspb.2020.2638#.YIK_YRXK_g4.twitter
Nevertheless, Zietsch et al remain unconvinced, and go back all the way to what Fisher actually wrote in his classic 1930 work to reiterate that as the theory cannot explain the data, the theory needs rethinking https://royalsocietypublishing.org/doi/10.1098/rspb.2020.2638#.YILA9qNrr70.twitter
This exchange makes many interesting points, clarifies confusion and make suggestions for future research. And I didn't even touch upon the reviews by an all-star team of reviewers, all of which are available too, thanks to #ProcB's Open peer review policy https://royalsocietypublishing.org/rspb/for-reviewers#question6
I think this would make for an excellent resource for a graduate-level course in #evolution or #behaviorialecology, or a #journalclub (or two), and it would be great to have more exchanges like this
This has been a challenge and a pleasure to handle as an associate editor and I couldn't have done it without @LoeskeKruuk and the @RSocPublishing team, and of course all authors and reviewers. It has been lots of work for everyone, but I hope you agree that it was worth it!
