Rapid population growth decreases the efficiency of purifying selection.
I don’t think that’s true, unless we’re thinking about “efficiency” in different ways.
In general we think of selection in terms of gene frequencies, not raw abundance. Which means that all that is relevant is relative fitness, and if your fitness advantage remains the same, doubling everyone’s reproduction rate doesn’t change the force of selection.
Unless you’re talking about the time until a deleterious, entirely recessive allele goes extinct? Since in that case drift is the only force that will push it from “very rare” to “non-existent”, increasing population size will decrease time to extinction of the deleterious recessive allele (drift is faster/stronger in smaller populations).
I don’t think that’s true, unless we’re thinking about “efficiency” in different ways.
I think what the OP means is that conditions which allow rapid population growth are conditions which reduce the natural selection pressure.
An extreme version of this is the observation that if everyone survives and breeds, there is no fitness advantage to any gene and the gene frequencies do not change.
I’m probably being overly anal here, but this something I work with on a professional basis.
Preface: when I’m talking about ‘fitness’, I mean the (slightly simplified) biological definition of the term, which means “number of offspring you have before you die”
I think what the OP means is that conditions which allow rapid population growth are conditions which reduce the >natural selection pressure.
This doesn’t need to be true at all—see below.
An extreme version of this is the observation that if everyone survives and breeds, there is no fitness advantage to >any gene and the gene frequencies do not change.
Also not necessarily true (because “everyone survives and breeds” is very different from “everyone survives and has an identical number of offspring).
Selection occurs due to differences in relative fitness, which can be calculated as (personal fitness)/(average fitness of everyone). If everyone has 2 offspring, everyone has a relative fitness of 1. If we have a good year, and everyone has 3 offspring, we have the same relative fitness.
You and OP seem to be thinking about situations in which some sort of environmental limits on fitness have disappeared, and now everyone is limited by some trait for which there is less/no variation. That’s actually a really special situation. Certainly the statement “Some events reduce variation in relative fitness while also increasing average fitness” is true. But so is the statement “Some events increase variation in relative fitness while also increasing average fitness”. Any time conditions increase the fitness of above-average fitness individuals, average population growth increases and selection becomes STRONGER. This is something you would definitely expect in organisms for which individuals actively compete for patchy resources—in a good year, the alpha/owner/whatever of a given territory will get most of the increased value of said patch, and individuals who were without territory may not gain anything at all. (it really depends on the situation, though)
Anyways, average fitness and variation in fitness are two largely independent things. “Rapid population growth”, or even “nobody dies before breeding” doesn’t inherently mean less selection/slower evolution. Sometimes just the opposite (see http://en.wikipedia.org/wiki/Adaptive_radiation)
This is what I get for replying shortly after waking up while thinking not all that clearly in terms of the rate of extinction of new variants over the rate of appearance of new variants.
I don’t think that’s true, unless we’re thinking about “efficiency” in different ways.
In general we think of selection in terms of gene frequencies, not raw abundance. Which means that all that is relevant is relative fitness, and if your fitness advantage remains the same, doubling everyone’s reproduction rate doesn’t change the force of selection.
Unless you’re talking about the time until a deleterious, entirely recessive allele goes extinct? Since in that case drift is the only force that will push it from “very rare” to “non-existent”, increasing population size will decrease time to extinction of the deleterious recessive allele (drift is faster/stronger in smaller populations).
I think what the OP means is that conditions which allow rapid population growth are conditions which reduce the natural selection pressure.
An extreme version of this is the observation that if everyone survives and breeds, there is no fitness advantage to any gene and the gene frequencies do not change.
I’m probably being overly anal here, but this something I work with on a professional basis. Preface: when I’m talking about ‘fitness’, I mean the (slightly simplified) biological definition of the term, which means “number of offspring you have before you die”
This doesn’t need to be true at all—see below.
Also not necessarily true (because “everyone survives and breeds” is very different from “everyone survives and has an identical number of offspring).
Selection occurs due to differences in relative fitness, which can be calculated as (personal fitness)/(average fitness of everyone). If everyone has 2 offspring, everyone has a relative fitness of 1. If we have a good year, and everyone has 3 offspring, we have the same relative fitness.
You and OP seem to be thinking about situations in which some sort of environmental limits on fitness have disappeared, and now everyone is limited by some trait for which there is less/no variation. That’s actually a really special situation. Certainly the statement “Some events reduce variation in relative fitness while also increasing average fitness” is true. But so is the statement “Some events increase variation in relative fitness while also increasing average fitness”. Any time conditions increase the fitness of above-average fitness individuals, average population growth increases and selection becomes STRONGER. This is something you would definitely expect in organisms for which individuals actively compete for patchy resources—in a good year, the alpha/owner/whatever of a given territory will get most of the increased value of said patch, and individuals who were without territory may not gain anything at all. (it really depends on the situation, though)
Anyways, average fitness and variation in fitness are two largely independent things. “Rapid population growth”, or even “nobody dies before breeding” doesn’t inherently mean less selection/slower evolution. Sometimes just the opposite (see http://en.wikipedia.org/wiki/Adaptive_radiation)
This is what I get for replying shortly after waking up while thinking not all that clearly in terms of the rate of extinction of new variants over the rate of appearance of new variants.
I’ve definitely made the same mistake before. More than once. Which is why I felt confident enough on this to offer a correction.