Fisher's principle explains why sex ratios in most sexually reproducing species is approximately $1:1$. However, given that in humans and many other animals, males and females differ genetically, it seems that this should cause issues for Fisher's principle.

Say that the Y-chromosome of an animal had a mutation that increased the ratios of males to females produced by the animal. At the extreme, we could imagine a mutation such that the sperm of the animal only carried Y-chromosomes. Although, by the Fisher principle, this would be bad for the overall number of offspring produces by the animal, it seems it would be good for the Y-chromosome itself, since it will now be present in all descendants of the animal (since they are all male), rather than only half the descendants. I would therefore expect that such a mutation would spread, skewing the sex ratios away from $1:1$.

Have scenarios like the one I outlined above actually occurred in species? If not, why not?

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    $\begingroup$ not sure why someone voted to close: this seems like a well-thought-out question, and the answers aren't that easy to find ... $\endgroup$
    – Ben Bolker
    Jul 20, 2020 at 1:03
  • $\begingroup$ @BenBolker & OP — Maybe I'm misunderstanding the question or overlooking some subtlety, but my understanding was that Fisher's principle does a good job of describing the evolutionary forces that generally maintain a 1:1 sex ratio. To me this obviously includes the "selfish"/"driven" behavior of sex chromosomes ... $\endgroup$
    – tyersome
    Jul 21, 2020 at 18:32
  • $\begingroup$ I think sex ratio distorters can at least temporarily push evolution in the other direction. Carvalho, Antonio Bernardo, Michelle Cristina Sampaio, Flavia Roque Varandas, and Louis Bernard Klaczko. “An Experimental Demonstration of Fisher’s Principle: Evolution of Sexual Proportion by Natural Selection.” Genetics 148, no. 2 (February 1, 1998): 719–31. Shows Fisher's principle re-asserting itself in the long run ... $\endgroup$
    – Ben Bolker
    Jul 21, 2020 at 18:46
  • $\begingroup$ Perhaps my question wasn't very well worded, but Fisher's principle as usually stated cannot apply to genes on the sex chromosomes. To understand why, we can imagine a toy example where a Y chromosome prevented the production of sperm with X chromosomes, so that an animal only had male descendants. Although by Fisher's principle this reduces the overall fitness of the animal, for the Y chromosome itself this is adaptive: it now is present in all the animals descendants, rather than half of them, and this factor of 2 more than overcomes the fact that each male has less children on expectation. $\endgroup$
    – djbinder
    Jul 21, 2020 at 23:57
  • $\begingroup$ I think Fisher's principle would imply that other genes should evolve to counteract the driving from the sex chromosomes, as the papers @BenBolker describe, but I don't know in practice if this is why we see animals with 1:1 gender ratios, or if other mechanisms have evolved to prevent runaway gene drives; any species which were suseptible to such a gene drive would need to become parthenogenic or else go extinct. $\endgroup$
    – djbinder
    Jul 22, 2020 at 0:02

1 Answer 1


This sort of thing absolutely happens; useful search terms are "sex ratio distortion", "segregation distortion" (i.e. modifying the ratios with which different chromosomes segregate), and "meiotic drive" (a specific form of segregation distortion).

Your scenario (Y-chromosome genes forcing all offspring to be male) is much less common than the reverse (X-chromosome genes, or other genomic elements, forcing all offspring to be female), because it's easy for an all-female lineage to maintain itself by parthenogenesis. Lyttle (1991) says:

Strong Y drive is of necessity transitory, since drive suppression must either evolve very quickly or the population will be pushed to extinction. This may explain why few Y drive systems have been observed in nature.

However, Lyttle goes on to describe

  • W-chromosome drive in butterflies (in butterflies, females are the heterogametic sex — the sex chromosomes are Z and W, ZZ individuals are male and ZW individuals are female). So this is a little bit like your Y-chromosome example, except that the populations end up all-female rather than all-male
  • male drive in mosquitoes

Jaenike (2001) says

Although several species exhibit Y drive, X drive is far more common.

Table 1 in that paper lists Y-drive examples in houseflies, mosquitoes, medflies, lemmings, and field mice. (I would include an image, but the table extends over 4 pages ...)

As for your final question,

I would therefore expect that such a mutation would spread, skewing the sex ratios away from 1:1 ... Have scenarios like the one I outlined above actually occurred in species? If not, why not?

I haven't dug through all the original literature that Jaenike (2001) cites, to see if there are populations in nature that are male-skewed due to selfish Y chromosomes. The point that Lyttle makes is that, even if you can find evidence of the existence of selfish Y chromosomes hiding in the population, it's very unlikely that these will persist in natural populations for very long, because of the strong selection against them at the population level (and from the rest of the genome); either the population will go extinct, or moderators will evolve that suppress the driving effect of the Y chromosome.

Jaenike, John. “Sex Chromosome Meiotic Drive.” Annual Review of Ecology and Systematics 32, no. 1 (2001): 25–49. https://doi.org/10.1146/annurev.ecolsys.32.081501.113958.

Lyttle, Terrence W. “Segregation Distorters.” Annual Review of Genetics 25, no. 1 (1991): 511–81. https://doi.org/10.1146/annurev.ge.25.120191.002455.

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    $\begingroup$ the one for field mice is so extreme their populations go through boom bust cycles as the gene spread then the population collapses as it starts to run out of females. $\endgroup$
    – John
    Jul 22, 2020 at 1:18
  • $\begingroup$ I took a quick look for papers that clearly described these dynamics, e.g. would be cool to cut & paste a figure showing population size/gene frequency/population sex ratio over time $\endgroup$
    – Ben Bolker
    Jul 22, 2020 at 1:24

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