Thinking about evolution at the gene level, suppose you had some gene on the Y chromosone (or whatever it is in insects and the like) which made you have more offspring but also made them all male.
That gene should be selective for itself or whatever the term is, much more than a normal gene on the Y chromosone, because it is passed on to all offspring rather than half of them.
So in theory this gene should spread amongst the entire population fairly quickly after a few generations.
But of course if all the offspring are male, the species will eventually die out!
Has this ever happened that we know of? And why doesn’t it happen more often? You could obviously have a female equivilant too.
Well, let’s look at it. I don’t think there’s anything implausible about your ‘Male fecundity trait’ appearing as a random mutation, and under the right circumstances, it might spread fairly far.
Suppose the females are not capable of recognizing males with MFT or influencing their mating accordingly. If there are some that recognize and prefer to mate with males without MFT, then their daughters will tend to succeed in following generations, and when fewer females will mate with MFT males, their numbers will decline again.
But otherwise, the proportion of males with MFT among the males of the species will always tend to rise. As that raises, so to will the proportion of males within the species. Any social behavior in which males will tend to sacrifice themselves for the survival of the females they have mated with (and their own children) will delay that latter trend, but cannot halt it entirely.
Females will become rare and valuable, and also, to a much less obvious extent, would be the remaining non-MFT males who could give them daughters, (though fewer children overall.) But we’re stipulating that it’s impossible for a female to find a non-MFT mate other than by chance, and the odds are stacked against them more and more. As the surviving female bloodlines become fewer and fewer, there’s a greater chance that some unrelated disaster could wipe them, and the species, out. (Or that one manages to evolve a way of finding the non-MFT males.)
I don’t see any other way out for our poor species. Am I missing something?
I think it’s the stipulation that’s the issue. If there’s a MFT mutation, there’s no reason to assume that a female couldn’t have a ‘daughters only’ mutation, or an anti-MFT mutation (males could also have a daughters-only mutation). And since females are so valuable, this mutation is going to spread very, very quickly – far more quickly than the MFT mutation spread initially.
In the long run, a daughters-only mutation is going to have the same issue as the MFT mutation, so things generally settle out at a 50-50 ratio.
At least with human males, passing the Y chromosome to one’s offspring already ensures that they will be male.
And passing the X chromosome ensures that the offspring will be female.
And the whole idea of Survival of the Fittest is that some mutation causes you to have “more” (surviving and reporducing) offspring.
Well, the simple answer is that several species are female only, but can reproduce parthenogentically, as anyone who ever kept certain species of stick insect will be painfully aware.
I interpreted the question as boiling down to ‘can a single mutation that is beneficial to individuals but upsets the gender balance contribute to the extinction of an entire species?’ It’s an interesting one and relates to the whole philosophy of evolution, as a naive assumption would be that all mutations that don’t benefit the species would die out at an individual level.
Saying that the gene Y chromosome isn’t really relevant (though plausible.) It’s just hypothesizing a mutation that can lead to greater quantities of healthy offspring, but only of the same sex as the parent.
A daughters-only mutation could have two forms: Somehow the ovum is resistant to y-carrying sperm, or the mother’s body somehow rejects male zygotes at some point in the pregnancy.
However, the male-only mutation proposed takes the form of only producing y-carrying sperm, and no x-carriers. If this male mates with the daughter-only female, the only result is no offspring. This doesn’t end up protecting the daughter-only gene, unless she finds males without the son-only gene.
The first thought that comes to mind is that this isn’t all that likely to wipe out a whole species. Very few species have fully interbreeding populations on a global scale. Some are separated geographically and others by herding, pack or other reproductive strategies. So this gene might wipe out the wolves from forest A, but not forest B, or the salmon from watershed A, but not watershed B. Having extinguished itself, the gene ends even though the species does not.
The faster and more effective the gene is, the more likely it is to wipe out only a regional presence rather than the entire species.
The second thought is that I’m not sure this mutation is even possible in many groups of species. The X and Y chromosome trick pretty much eliminates this possibility in humans, at least. Crocodilians (among others) have the gender of their offspring determined by the temperature of the nest, not genetics. Other species have a way for individuals to change sex as they age or in response to environment. So these are at least three examples that should make the mutation you describe ineffective or impossible.
Why so? For males - you could have an effect that supercharges Y chromosome sperm, at the expense of the X sperm. For females, it would be harder, as they’d have to selectively accept only X sperm from the males.
I’m not sure about that. In theory, there could exist a mutation on the Y chromosome that causes its carriers to only produce Y sperm. It would seem to me that in absence of other factors, this gene would become widespread in a population and eventually cause extinction by having no females born.
If some males have a gene that means they only have male offspring, then the ratio of males to females will increase, and a large portion of those males will have the male-only gene.
But once you reach the point where there are a lot more males than females, then the males who are still capable of producing female offspring have a large evolutionary advantage.
Imagine it reaches the point where 90% of the population is male. Then virtually every female will get to pass on her genes to the next generation. But on average only 1/8 of the males will get to do so. Now assume each mating opportunity produces 16 offspring. (It makes the math easier.)
If I’m a male-only male and I’m lucky enough to mate, my partner will have a litter of 16 males. So on average only two of my sons will get to mate and pass on their genes.
However, if I’m a normal male and I’m lucky enough to mate, my partner will have a litter of 8 males and 8 females. On average only one of my sons will get to mate and pass on his genes. But all 8 of my daughters will. So, on average 9 of my offspring will get to mate and pass on their genes.
2 offspring that reproduce vs. 9 offspring that reproduce = huge advantage for normal males.
This is why most species have a 50/50 division between male and female. If a mutation ever shifts population distribution in favor in one sex, there’s a strong competitive advantage to being the other sex or being able to produce offspring that are the other sex.
First, there’s a third possible form of a daughter-only gene: the mutation could cause the zygote to develop female regardless of the presence of a Y chromosome (and this is probably actually easier to imagine, biochemically).
But I agree that for the two forms you stated, the female would need to find additional males to mate with. Depending on the species this could be a small cost or a large one. But remember, as the proportion of females in the population gets smaller, the benefit increases (while the cost stays the same or even declines).
First off, you’re not accounting for the fact that male-only males have more children overall. So maybe a normal male’s ‘litter’ is only 4 and 4, or 6 and 6. But that doesn’t change the ‘huge evolutionary advantage’ that you mention.
But the thing is, that huge advantage only exists by crossing into the female world, as it were, and getting out of the tremendous competition that exists among males. There’s no way, as I see it, that the normal males can use this tremendous advantage to actually restore the balance between themselves and the male-only males, or even to keep the imbalance from getting worse in each generation. Only the females can do that, by choosing or developing some other mutation that offsets the original one.
Their advantage is only that they can be fathers of females, and females are rare and valuable now - but though the male-only males cannot sire females, they can flood the market with ‘cheap knockoff males’ and make it hard, even impossible to find the normal males.
I think that dracoi’s got a good point that this is probably more likely to effect a smaller ecosystem, (though if just one male-only-male was able to migrate to another habitat, he could infect it with his progeny too. ) And I suppose that though it’s plausible, this may not be a very likely mutation to develop spontaneously.
I created a simple Python script to model the population dynamics. The script assumes that the population is made up of normal males, normal females, and mutant males which only produce male offspring. In each generation the script goes through the pool of females, picks a random male for each to mate with, and then generates a new population of normal males, normal females, and mutant males for the next generation based on those matings.
When I run it, I see a clear progression. At first, the number of mutant males in each generation rises, mostly at the expense of the normal males. As they do the overall male-to female ratio rises with each generation, and the overall population falls. Inevitably the number of females in the population decreases, and with it the overall population falls, and eventually goes extinct. I’m not seeing the restoring effect The Hamster King mentions at all.
I’ve never heard of one for males that does that and increases fertility too. However, I understand that it’s a fairly common problem with insects that a single mutation can create male eggs that produce a poison that kills everything lacking the mutant gene; including all female eggs naturally. The result being that the species tends to suffer periodic population crashes as the mutation appears, spreads, makes the population overwhelming male, then the crash comes due to a lack of females. Presumably this has killed off species in the past; all it would probably need to happen is a one-two punch of the mutation and some natural disaster hitting the species in succession.
As for females, it’s actually not that rare in non-mammals; an all-female species has the advantage of not needing to find a mate, and non-mammals can pull off parthenogenesis. The problem is that presumably due to the lack of genetic mixing, female-only species don’t appear to last as long.
It’s a really simple script, and just produces a text output. Here’s an example:
Generation 1: 4999 females 4999 normal males 2 mutant males 10000 total
Generation 2: 4991 females 5003 normal males 4 mutant males 9998 total
Generation 3: 5008 females 4966 normal males 6 mutant males 9980 total
Generation 4: 4996 females 5006 normal males 12 mutant males 10014 total
Generation 5: 4959 females 5009 normal males 22 mutant males 9990 total
Generation 6: 4973 females 4902 normal males 42 mutant males 9917 total
Generation 7: 4946 females 4914 normal males 84 mutant males 9944 total
Generation 8: 4873 females 4849 normal males 168 mutant males 9890 total
Generation 9: 4716 females 4698 normal males 331 mutant males 9745 total
Generation 10: 4395 females 4407 normal males 629 mutant males 9431 total
Generation 11: 3834 females 3854 normal males 1100 mutant males 8788 total
Generation 12: 2979 females 2987 normal males 1701 mutant males 7667 total
Generation 13: 1903 females 1913 normal males 2140 mutant males 5956 total
Generation 14: 895 females 897 normal males 2013 mutant males 3805 total
Generation 15: 268 females 283 normal males 1238 mutant males 1789 total
Generation 16: 52 females 49 normal males 433 mutant males 534 total
Generation 17: 6 females 5 normal males 92 mutant males 103 total
Generation 18: 0 females 0 normal males 11 mutant males 11 total
Population extinct after 18 generations
The same thing, it just takes a little longer:
Generation 1: 4999 females 4999 normal males 2 mutant males 10000 total
Generation 2: 4999 females 4996 normal males 3 mutant males 9998 total
Generation 3: 5012 females 4981 normal males 5 mutant males 9998 total
Generation 4: 5007 females 5008 normal males 8 mutant males 10023 total
Generation 5: 5003 females 4997 normal males 13 mutant males 10013 total
Generation 6: 4986 females 4996 normal males 23 mutant males 10005 total
Generation 7: 5008 females 4920 normal males 43 mutant males 9971 total
Generation 8: 4959 females 4981 normal males 75 mutant males 10015 total
Generation 9: 4888 females 4892 normal males 137 mutant males 9917 total
Generation 10: 4781 females 4747 normal males 247 mutant males 9775 total
Generation 11: 4585 females 4564 normal males 412 mutant males 9561 total
Generation 12: 4300 females 4176 normal males 693 mutant males 9169 total
Generation 13: 3808 females 3689 normal males 1102 mutant males 8599 total
Generation 14: 3135 females 2902 normal males 1578 mutant males 7615 total
Generation 15: 2236 females 2035 normal males 1998 mutant males 6269 total
Generation 16: 1343 females 1126 normal males 2002 mutant males 4471 total
Generation 17: 654 females 481 normal males 1550 mutant males 2685 total
Generation 18: 255 females 156 normal males 896 mutant males 1307 total
Generation 19: 80 females 40 normal males 388 mutant males 508 total
Generation 20: 22 females 6 normal males 131 mutant males 159 total
Generation 21: 5 females 1 normal males 37 mutant males 43 total
Generation 22: 1 females 0 normal males 8 mutant males 9 total
Generation 23: 0 females 0 normal males 2 mutant males 2 total
Population extinct after 23 generations
If I set it so that the mutants produce males 55% of the time (but all their male offspring inherit the mutation) the population goes extinct after about 130 generations. I suspect that this simulation is grossly oversimplifying the dynamics and not taking into account factors that would stop this from happening in the real world.
One good strategy for females in this situation doesn’t require the ability to magically detect mutant males, though it does take some intelligence: only hook up with guys who’ve introduced you to one of their sisters, No sisters, they’re not getting lucky.
) And I suppose that though it’s plausible, this may not be a very likely mutation to develop spontaneously.