My understanding is that a neutral variation does not have to remain neutral. When an environmental stress occurs, that previously-unimportant trait can suddenly become a life saver (or the opposite). Also, multiple traits might be propagated as a unit. So if a formerly neutral trait happens to become fatal (maybe it makes you especially delicious to a newly-arrived predator) other, genuinely useful traits that were closely associated in the genome might be lost as well. Bad luck!
For a neutral mutation that occurs in one person, wouldn't the probability of fixation be approximately zero? The trait is certainly lost when that genetic line ends, but also has a 50% chance of being dropped every time the line extends. Without advantage to add weight in its favor, the probability the trait gets anywhere seems tiny.
Yes. The probability that a new mutation goes to fixation is 1/2N - with N being the population size. And yes all the rest of your intuition is correct.
Happy I discovered your Substack! I want to know if a neutral mutation can achieve equilibrium with the original if you just introduce a reproductive divide in the finite population—group A and B very rarely interbreed, and the starting rate of the mutation is 50/50 among the two groups. Does this extend the generation time until fixation/disappearance? Or does it literally have no effect?
It can’t create a true equilibrium, but it can make the polymorphism persist for a very long time. Each subgroup drifts independently and resolves fast (fixation or loss within have the original expected number of generations). Half the time, the two groups land on opposite outcomes: one fixes, the other loses. At the species level, the allele frequency if near 0.5, looking stable, but it really isn’t. In your scenario, migration will eventually break the stalemate, but if interbreeding is rare, “eventually” could mean tens of thousands of generations. Population structure doesn’t change where drift ends up.
My understanding is that a neutral variation does not have to remain neutral. When an environmental stress occurs, that previously-unimportant trait can suddenly become a life saver (or the opposite). Also, multiple traits might be propagated as a unit. So if a formerly neutral trait happens to become fatal (maybe it makes you especially delicious to a newly-arrived predator) other, genuinely useful traits that were closely associated in the genome might be lost as well. Bad luck!
For a neutral mutation that occurs in one person, wouldn't the probability of fixation be approximately zero? The trait is certainly lost when that genetic line ends, but also has a 50% chance of being dropped every time the line extends. Without advantage to add weight in its favor, the probability the trait gets anywhere seems tiny.
Yes. The probability that a new mutation goes to fixation is 1/2N - with N being the population size. And yes all the rest of your intuition is correct.
Happy I discovered your Substack! I want to know if a neutral mutation can achieve equilibrium with the original if you just introduce a reproductive divide in the finite population—group A and B very rarely interbreed, and the starting rate of the mutation is 50/50 among the two groups. Does this extend the generation time until fixation/disappearance? Or does it literally have no effect?
It can’t create a true equilibrium, but it can make the polymorphism persist for a very long time. Each subgroup drifts independently and resolves fast (fixation or loss within have the original expected number of generations). Half the time, the two groups land on opposite outcomes: one fixes, the other loses. At the species level, the allele frequency if near 0.5, looking stable, but it really isn’t. In your scenario, migration will eventually break the stalemate, but if interbreeding is rare, “eventually” could mean tens of thousands of generations. Population structure doesn’t change where drift ends up.
Another excellent piece! I’m really happy the algorithm threw your substack my way, I feel like it’s going to be a new favourite