Sex ratio. Fisher's principle. Evolutionarily stable strategy

Fisher's principle is an evolutionary model that explains why the predominant sex ratio of species of living organisms in nature is approximately 1:1.
The theory was thoroughly outlined by Ronald Fisher in his 1930 book, The Genetic Theory of Natural Selection.
In the video we will make this model on the computer and play with different parameters.

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[ShawnHCorey]

An argument against hermaphroditism in animals is to avoid self-fertilization. The purpose of sex is to mix up the genes for the species to better resist diseases and parasites. Animals are more mobile than plants and more exposed to different diseases and parasites. Avoiding self-fertilization would mean it is less likely a significant portion of the population would be wiped out by new exposure to diseases and parasites.

[TheAgamemnon]

The explanation for hermaphrodism in plants (and lack thereof in animals) is actually relatively simple: Since plants can conquer new (or recently leveled) habitats only by seed transport or growing into - and not by migrating into them, they must utilize this advantage for rapid recolonisation and mitigate the negative effects as best they can. (see other commenters) Higher numbers of individuals and sum of total biomass helps a lot with the lack of genetic diversity from self-pollination, too.

[kylemorris]

Wild to think that a guy born in the 19th century came up with these theories long before anyone had access to computers they could use to model the results or even check the math.
Great video!

[kenthartig]

If I had to make a guess about why hermaphrodites don't appear in animals, it would be that producing both sets of gametes just uses too much energy per organism. In the case of mammals, who give birth to live young, this makes a lot of sense to me. However, I'm less confident in this hypothesis applying to egg-laying animals. Maybe it's a detererrent from the sorts of issues incest produces? Self-fertilization is the most extreme form of this. If I recall correctly, plants are also more resilient to genetic defects (i.e. abnormal gene counts), which could explain why they are able to handle it.

[OneCharmingQuark]

I believe there are many hermaphroditic animal species in the ocean, and also some non-insect land invertebrates. It might be related to mobility. Snails, barnacles, clownfish might be more tied to specific locations where you might not find a mate of the opposite sex. The same applies to flowering plants, which tend to be hermaphrodites.

[somnvm]

the explanation for rarity of hermephraditism in the animal kingdom that I've heard, is that it takes a lot of energy to support such a complicated reproduction system. Also, there are some groups of animals who are more likely to be hermaphradite, such as slugs.

[ArthurWolf]

If somebody gives you the solution to the question about hermaphrodites in the animal kindom, make sure to put it in a future video, I (and I'm sure others) would like to know.

[fronk]

I believe hermaphroditism is less viable when organisms have more complex mate selection and behaviors (plants and snails for example are limited in their mate selection/behavioral strategies and are often hermaphroditic). Bateman's principle suggests two opposing strategies in offspring production: energy investment caters for either more quantity or quality in offspring. The fact that females produce eggs, seeds, or gestate their young classifies them into the quality-optimizing strategy. When two organisms (regardless of sex type) parent one set of offspring, there's a natural competition between them for who will provide the necessary energy for the offspring to succeed. It would be fair for both parents to provide an equal amount of energy but because of what amounts to selfishness (informally), this role may end up being deferred to whatever organisms take up that role (persisting in equilibrium due to their importance). Thus a hermaphroditic species that has mate selection and mate competition would generally differentiate into separate rolls and ultimately biological sexes. But I'm not an expert and I could be way off. I'm sort of basing this off of how snails have "love darts" that play a role in mate competition for the male roll.

[KindOfWitch]

Its interesting to think that If we were to evolve in these ways that we wouldn't feel so attached to people, it may even result in us not being the clade to evolve sapience

love your content keep it up

[KindOfWitch]

do you use commas for floating ponts? I'm asking cause I was wondering about the way you represent the ratio

[TheOrian]

I think an important element was missing for the 1:6 ratio and why 1:1 is almost the norm.
It's the factor of importance; for survivability if you have 1 male for 6 females, that makes one male as important as 6 females(very broadly), and if an accident or whatever happens to kill that male, then it would be almost equivalent to losing 6 members instead of 1.

I believe(I'm not a scientist after all) that this factor of importance makes the system too vulnerable to alea, while 1:1 ratio has every member of approx equal value, losing one is just losing one, and having more fills the gap without additional cost. This would give a lot more stability to survive in time, while unbalanced ratio would more easily go extinct from events.

[Darkgon]

Hermaphrodites actually do exist in nature, but most we commonly think of are specifically under a conditional sense of either being both, or either sex depending on the situation. Some fish species will change sex to fill reproductive voids in their communities, slugs will fight to impregnate each other (the loser becoming female), and most flowering plants are actually both sexes and exchange pollen.

[thelordz]

On hermaphroditism, I believe it is the exact opposite of equal male to female ratio in that it would not be genetically stable. If you had a large population of hermaphrodites with 2 genes to contol how masculine and feminine the individual is, the hermaphrodites that are more masculine are more likely to father many children with the more feminine hermaphrodites whereas a more equal hermaphrodite is just as likely to father or mother a child, which would be slower. This would quickly result in increased sexual dymorphism overall.

[Starblazer-ocnt]

For 11:01 I would think the females would pass on more genes if the made more females, based on how the simulation is set up

While the males would not benifit with their gene passing by making more females so they would make more males for the male female balance based on the fishers principle

The reason of the opposite gender being at the opposite genes described above would be from the gene blending of sexual reproduction

All of this is reflected on the chart

This is all very speculative and I’m not to sure why the 2 extremes wouldn’t mix and make a more average one

[Kram]

It seems the bimodality of the case where female phenotypes only can give birth until they give birth to a male phenotype involves a shift where for males the ratio becomes much larger and for females it becomes much smaller, whereas before that shift they are both at about the same 1/3rd ratio.
It might make sense to specifically look at the ratios of only the high or low gene individuals.
By glance it looks like for the very high gene individuals, 2/3rds of those are male and only 1/3rd female, inverting the overall population average. For the low end the ratio instead seems to be about 1:5 or so, and it's inverted in that females dominate.

In this scenario, for *individual* females the best possible gene you can possibly have is a 100% female gene: These individuals can reproduce forever! (or as close to that as possible)
However, of course they *do* need male individuals to actually have reproduction happen. So if the *entire* population were to follow this strategy, they would flat out go extinct within a generation.
Meanwhile, males are *heavily* incentivized to go all in on male genes. They are trying hard to get to that 50/50 ratio that normally would be beneficial.
You seem to end up with a system where, even though *both* parents determine the sex and *both* parents *pass* the sex gene, each parent tries as much as possible to control the outcome, skewing as much as possible towards this outcome. And the offspring of such extremal males and females results in similarly extremal males and females with the potential mutation rate being too low to bridge the gap from one extreme to the other in a single step, perhaps. So you sort of end up with two separate populations like this.

I guess for us with this chromosomal system, it's kinda set that women skew 100% female (they only have X chromosomes to give) but men have a 50/50 split and ultimately determine the chromosomal sex (half the time X, half the time Y)
Though afaik it's not quite as simple as that, as there is a lot of careful weeding-out going on before it comes to the actual moment of fertilization, and there is quite a complex arrangement of competition and cooperation going on at that stage, which may allow for more fine-grained changes than the idealized 50/50 ratio chromosomes dictate and, indeed, from what I've heard, if there is a major imbalance in the population, offspring somehow *will* swing towards that 50/50 split in a subtly *active* way, rather than merely as a result of reaching an eventual far future equilibrium of chromosomes. The mechanisms of that are as yet poorly understood though, afaik.

[alias]

At 7:32, the sex ratios were irelevent, as the population could not shrink. So in no way was it being selected for. What was observed instead of Fisher's principle was infact entropy, with the result being the most likely posibility given that it is equally likely to be 100% male or 0% male. Every succesive posibility is equal in chance and value resulting in an average of 50%.

[principal]

As a homosexual man id be very intrigued to see a simulation that produces the famous 12% homosexuality distribution. Maybe a simple Nash equilibrium can occur like in this example although it’s „against“ our immediate rational conclusion to be „beneficial“ in natural selection

[chilleroheftig]

According to this, it's best to give almost all male one child, for genetic variety, and those we call good genes additional children. There's no reason to forget that one female of very most species can have children with several males. Also, whilst many animals do have 1:1 ratio, the argument that the male genes dominate over the females because they have more offspring doesn't disappear, because unlike you said, one male in most species tries to mate with as many females as possible, and for mammals, partnerships are rarely forever.

[капец]

Как же не повезло англоговорящим, эта говорилка не может повторить ту же атмосферу где ваш голос❤

[quinn]

I believe mammals and larger animals in general have to put an extraordinary investment into their young, in such cases it makes sense to have different roles within a species so one can be specialized to care for their offspring and deal with pregnancy and others can provide, protect, and make riskier moves to support the females. If this theory is correct, we should see that k selective hermaphrodites also have these roles, except they aren’t determined by gender. Let me know if the see anything wrong with this idea!