Genetic drift question

Question

With random genetic drift a population of finite size eventually evolves to hit either p=1 or p=0. Why is further evolutionary change not possible (without mutation or migration)?

Answer 1

If p is the frequency of an allele within the population, when p reaches 0 (meaning no individuals have the allele) or 1 (meaning all individuals have the allele), then the genes within the population will be the same for all subsequent populations unless new individuals with different alleles enter the population, or mutation changes the alleles that appear in new generations.

Evolution is defined as change over a period of time, and in biology, evolution specifically refers to a change in the types and frequency of heritable traits (usually traits that are genetically inherited) found within the population between generations. Evolution can be long-term, on the order of millions of years, or short-term like from one generation to their offspring. If all individuals within the population have the exact same genes, which can happen with genetic drift, then each new generation will have the exact same genes as their parents. That trend will continue indefinitely as long as the population survives, unless new individuals with different genes enter the population (migration) or mutation leads to new heritable alleles. Different frequencies of alleles in one generation compared to previous generations means that evolution has occurred, whereas generations remaining the exact same means no change has occurred over time in the population, and therefore the population is said to not have evolved.

In real-world scenarios, evolution will pretty much always occur over long periods of time due to mutation at least, or else the population will go extinct. However, this concept is important when you are thinking about how quickly a population is evolving, because if there are a lot of alleles in the gene pool, population change can occur frequently in a relatively short span of time due. In very small populations with a more limited gene pool, inbreeding is common, and genetic drift can cause some alleles to go extinct by chance, which means all members of the population can end up with very similar genes, and the population will tend to evolve much more slowly.

Answer 2

Hi Rose! Random genetic drift, mutation, and migration (which facilitates gene flow) all help to maintain genetic variation within a population. But in order for natural evolution to occur, the variation has to be associated with a change in fitness (it has to change how competitive an individual is).

Our key words here are "random" in "random genetic drift" and "finite" in the "population of finite size". Imagine tossing a coin. If we tossed a coin an infinite number of times, we would get heads half the time and tails half the time. But in reality, we can only toss the coin a certain number of times. Maybe we toss the coin only 10 times before we get bored and stop. Would we get exactly 5 heads and 5 tails from tossing the coin 10 times? Maybe, but probably not. Instead we would likely get some random variation, like maybe 7 heads and 3 tails. The more times we tossed the coin, though, the closer we would get to that 1/1 ratio where heads appear half the time.

Random genetic drift and finite population sizes work the same way. Imagine that the number of gametes in a population is the number of times we toss the coin, and the allele frequency is the times that the heads appear. If a population has 50,000,000 individuals, genetic drift would result in an allele being passed on roughly half the time. But if a population has only 50 individuals, maybe the allele is passed on 7 out of 10 times, like our coin toss, due to random variation. If this happens, then the next generation will have an allele frequency of p=.7. So we "toss the coin" again, and this time the allele is passed on 8 out of 10 times. So the next generation has an allele frequency that is slightly higher. Over many generations, the allele will either become fixed (p=1) or disappear from the population (p=0) because no other forces of evolution, like mutation or migration, have acted to influence the allele frequency. Even if a population has 5 million individuals and the allele remains present for a million or more generations, as long as no other forces of evolution act to influence the allele frequency, eventually it will hit p=1 or p=0.

This is also why genetic drift has a larger effect on allele frequencies in smaller populations. Fewer gametes means more variation due to chance, just like how fewer coin tosses means more variation in the proportion of heads we get vs tails.

Here is a nice article that can provide more information:

https://www.nature.com/scitable/knowledge/library/natural-selection-genetic-drift-and-gene-flow-15186648/

Best,

Nicholas

Answer 3

Genetic drift is the process by which favorable combinations of alleles are fixed in a subpopulation through natural selection. So in a finite population, an allele (p) will eventually be determined to be either favorable or unfavorable through natural selection. If the allele is favorable, natural selection will result in this allele being passed along until every member of this finite population has this allele (p=1). If it's unfavorable, eventually natural selection will weed this allele out until no members of the population have this allele (p=0). So genetic drift can be thought of as eliminating genetic variability. Whereas mutation introduces genetic variability to a population, thus interrupting the genetic drift towards either p=0 or p=1. Similarly, the migration of a population may make alleles previously selected as favorable, now unfavorable or vice versa. This can interrupt that genetic drift, and thus prevent p=0 or p=1.

Hope this helps!