Goals: the goals of this lecture are to introduce the causes and results of two forms of evolution: genetic drift and gene flow.
Related Textbook Material: Freeman and Herron (2001) Chapter 6
Lab Manual Questions over this material are in Lab Manual Chapter V
An important point to emphasize, to start out with, is that natural selection is NOT the same thing as evolution. Since natural selection was the first form of evolution discovered, people sometimes make the mistake of thinking it is the same as evolution, but as you have seen there are four other ways that evolution can occur.
Remember that genetic drift refers to evolution occurring through random changes in allele frequency over time.
Let's consider why genetic drift occurs (why allele frequencies would change at random.) When individuals in a population reproduce, they do not necessarily reproduce their alleles in the exact proportions in which they have them. When gametes are made through meiosis, each contains one of the two alleles for each trait, and which of those alleles gets passed on to the offspring produced during reproduction depends on which gamete, by chance, goes into forming the offspring.
Consider for example a human family. Since females are chromosomally XX and males are chromosomally XY, one would expect, if reproduction were NOT random, that half of the children in a family would be girls and half would be boys. Obviously, there are lots of families with more boys than girls, and lots with more girls than boys -- this is because of the random joining of gametes; by chance, sometimes more boys get produced than one would expect, and other times more girls get produced than one would expect. This same kind of randomness affects all the other genetic traits in the family.
Now consider a whole population of reproducing organisms. Since none are passing on exactly the alleles they have for any gene -- because of random union of gametes sometimes some alleles are passed on more than would be expected, other times less than would be expected -- when the whole population reproduces, the allele frequency will fluctuate -- go up or down at random -- from generation to generation. This is genetic drift.
Now let's consider the situations in which we expect to see most effects of genetic drift. The effects of genetic drift are strongest in small populations -- the fewer individuals in the population, the more genetic drift affects the population. It is generally true that random events (like drift) are most noticable in small samples. To see why, suppose first you had a very large population. Consider a gene with two alleles, B and b. In a very large population, suppose one family happened by chance to pass an unusually large number of B alleles to their offspring, causing an increase in B (and decrease in b) in the next generation. Since the population is so large, it's almost certain that somewhere else there would a be a family that happened by chance to pass an unusually large number of b alleles to their offspring, causing an increase in b, and decrease in B in the next generation. So the effect of an increase in B in one family would be cancelled out by a decrease in B in the other family, and allele frequencies would not change very much. In large populations like this, then, there is little effect of genetic drift.
Now consider a small population. With few individuals, it's much more likely that if there was, by chance, an unusually large number of one allele, such as B, reproduced by one family, there would be no other family producing a small number of B's to compensate. There would be a random increase in B in that generation. So we expect that the smaller the population, the larger the effect of genetic drift.
Now let's consider the effects of genetic drift on a population.
In the short term, over a few generations, we would expect allele frequencies to increase and decrease in a random, unpredictable way, as a result of genetic drift.
In the longer term, the main result of genetic drift is loss of genetic variation. This occurs because over time, at random, there will be a generation in which one allele (which has become rare by chance) will not get passed at all to the next generation. Given enough time, this will always be the effect of genetic drift -- by chance, alleles will be lost. The smaller the population, since genetic drift has a stronger effect in small populations, the more quickly genetic variation will be lost.
Note that, in general, if there are two alleles for a gene, when one is lost from the population the other becomes the only allele for that gene left in the population. This is refered to as being fixed: we say that an allele is fixed when it is the only allele for a gene left in the population. The opposite of fixed is lost -- if an allele is no longer present in a population, it has been lost.
Genetic drift also results in different populations becoming genetically different from each other because by chance, different alleles will become fixed in different populations.
So far we have looked at what will happen to allele frequencies over time when genetic drift is occurring. Now let's consider genotype frequencies. We have seen that in a population with no evolution occurring (Hardy-Weinberg Equilibrium) that if allele frequencies are p and q then genotype frequencies will be p2, 2pq, and q2. We have seen that in a population in which natural selection is occurring, that if the allele frequencies are p and q, then genotype frequencies will start out, in zygotes, as p2, 2pq, and q2, but will change in adults to frequencies that also depend on differences in fitness, because some individuals are surviving disproportionately more than others. Genetic drift occurs because of randomness in which alleles are passed from generation to generation -- there is nothing to cause some individuals to survive disproportionately more than other, so within a generation, if allele frequencies start out p and q, there is nothing to change the genotype frequencies from those that are randomly produced, which, as we've seen, are p2, 2pq, and q2. So if genetic drift is the main, or only, form of evolution occurring, and we measure the allele frequencies within a generation to be p and q, we expect genotype frequencies of p2, 2pq, and q2.
Now let's make sure we understand how this is different from Hardy-Weinberg Equilibrium. In Hardy- Weinberg Equilibrium, while genotype frequencies are also p2, 2pq, and q2, nothing changes allele frequencies from generation to generation -- they stay the same throughout time. When genetic drift is occurring, allele frequencies (and therefore genotype frequencies) DO change from generation to generation. Evolution is occurring. So if, for example, the allele frequencies in one generation were p=0.4 and q=0.6, then the genotype frequencies for just that generation would be (0.4)2, 2(0.4)(0.6), and (0.6)2. However, when reproduction occurred, the allele frequencies for the next generation would be changed by random chance -- they would not be 0.4 and 0.6 any more. Suppose by chance they change to 0.3 and 0.7. Then in this new generation, the genotype frequencies would be (0.3)2, 2(0.3)(0.7), and (0.7)2. From generation to generation, then, allele and genotype frequencies change through genetic drift.
We've now seen several situations (Hardy-Weinberg Equilibrium, zygotes in populations undergoing natural selection, and populations undergoing genetic drift) in which genotype frequencies are p2, 2pq, and q2. These genotype frequencies are often called the Hardy-Weinberg Proportions. Clearly, they are named for the Hardy-Weinberg Equilibrium, but when we talk about the proportions, rather than the equilibrium, we're not necessarily talking about a situation of no evolution as we are with Hardy-Weinberg Equilibrium. We are just talking about a relationship between allele and genotype frequencies that occurs in several different situations. We'll use this more in the next lecture to help to distinguish between different forms of evolution such as genetic drift and natural selection.
Now that we've seen the effects of genetic drift, let's consider how genetic drift affects whether or not a population evolves through natural selection. It turns out that genetic drift tends to make natural selection less likely, and can counteract the effects of natural selection. The reasons for this are as follows.
First, remember that the main impact of genetic drift on a population is that it decreases genetic variation. Now remember Darwin's four postulates -- the four things that must be true in a population for natural selection to occur. Two of these are that there must be variation, and it must be heritable (there must be genetic differences among individuals.) Genetic drift tends to get rid of genetic variation. It makes all individuals genetically similar. As a result, there are few differences and little potential to have some higher fitness forms and some lower fitness forms, so there is little potential for natural selection to occur.
Genetic drift can also cause increases in alleles that lead to low fitness, just by chance. These alleles would presumably die out or become very rare through natural selection, but if by chance those individuals who carry them happen to reproduce a lot of them, then (through drift) they become more common than they would otherwise. Genetic drift thus can counteract the effects of natural selection (note that it doesn't have to have this effect -- by chance, natural selection and drift could also decrease the same alleles -- but it will sometimes have this effect.)
Because genetic drift decreases the potential for natural selection and can counteract natural selection, we generally predict populations subject to high levels of genetic drift (small populations) will have lower levels of adaptation than will large populations (since natural selection is what causes adaptation.) Now let's consider situations in which we expect small populations to exist, that is, situations in which there will be strong effects of genetic drift.
Some populations are small and remain small over long periods of time simply because they naturally occur in small areas of good habitat. For example, there are fish called desert pupfish that occur in small ponds in the desert. The ponds are too small to hold many fish, so populations are small. Each pond has a separate population from each other pond since, clearly, fish are not able to get out of the pond and walk through the desert to another pond. These pupfish populations are subject to large amounts of genetic drift and have been for a very long time.
Other populations have historically been large but have recently become small. Major examples of this are populations in areas where there were once large areas of habitat, but where much of that habitat has now been destroyed. While such habitat destruction can occur through natural events, such as storms, fires, floods, disease epidemics, etc., by far the most common current cause of habitat destruction is human activity -- through activities such as agriculture, construction, etc. we have taken areas that once had large expanses of habitat and turned them into areas with only small fragments of habitat for many species. By doing this, we have created situations in which a lot of genetic drift occurs. Since genetic drift, as we've seen, decreases the potential for adaptation, this may make these small populations more likely to go extinct, and is one of the concerns of people working to conserve species.
Still other populations may have been historically large, go through a period of time when they are small, and then become large again. Such events when populations become temporarily small are called population bottlenecks. An interesting result is that genetic drift that occurs while the population is small will impact the genetic variation in the population even when it has grown large again. This is called the bottleneck effect: after a population has gone through a period when it is small, there is decreased genetic variation in the population even when it has grown back to its original large size. This is because mutation, which could ultimately add variation back into the population, is a very slow process -- it will take many, many generations for variation to increase through mutation.
This finishes the introduction to genetic drift. At this point, you should look back over it, make sure you understand the main points -- the cause of drift, situations in which drift occurs, the effects of drift on a population, and on the different populations of a species, and the effects of drift on whether a population undergoes natural selection.
First let's look at the effects of gene flow on a population. Gene flow increases genetic variation within a population. This increase occur because individuals from other populations will bring in alleles that would otherwise be absent or rare in the population. Note that the effect of gene flow on genetic variation in a population is the opposite of the effect of genetic drift.
Now consider how gene flow affects similarities and differences among populations. Gene flow tends to make populations genetically similar to each other in that if gene flow occurs, the alleles that occur in one population will be introduced to the other population. The more gene flow occurs, the more similar the populations will become.
If gene flow is the only form of evolution occurring, then, over time, populations will become genetically just like each other -- they will have the same alleles, in the same frequencies.
Now let's consider genotype frequencies. It turns out that gene flow, like genetic drift, typically results in populations with genotypes in the Hardy-Weinberg Proportions of p2, 2pq, and q2. Low levels of gene flow won't be enough to show differences from this, and once individuals from other populations reproduce in a population, mating at random, Hardy-Weinberg Proportions are created by the random mating. High levels of gene flow, as we'll see, result in different populations being effectively the same, like one large population in Hardy-Weinberg Proportions.
It's unlikely that gene flow will really be the only form of evolution occurring. Let's look at how gene flow will affect populations if those populations are also evolving through natural selection and genetic drift.
We have seen that genetic drift tends to make different populations genetically different from each other, by chance. Natural selection also tends to make different populations genetically different from each other, because different populations occur in somewhat different environments, so the traits that have high fitness in one population, and evolve through natural selection, will be different from the traits that have high fitness and evolve through natural selection in another population. So if both drift and selection tend to make populations different, but gene flow makes populations similar, what happens when all occur? The answer depends a lot on how much gene flow is occurring.
If fewer than one individual per generation moves from population to population, so that the amount of gene flow is very low, then populations will develop fixed differences through natural selection. Fixed differences are what they sound like -- differences in which alleles are fixed in different populations. That is, for a gene with two alleles (say D and d) allele D could be fixed (the only allele present) in one population and allele d could be fixed in the other population, and this low level of gene flow would not be enough to change that.
In contrast, if large numbers of individuals move from population to population , so that the amount of gene flow is very high, the populations will essentially be like one single population and will have the same alleles, in the same frequencies, even if they occur in somewhat different environments so that differences might otherwise tend to evolve through natural selection. Large amounts of gene flow will thus mask the effects of other forms of evolutions and make populations similar.
In between these two situations, consider a situation in which more than one individual moves from population to population each generation, but most individuals do not move among populations, so levels of gene flow are fairly low. Movement of at least one individual from population to population each generation will prevent fixed differences -- the alleles found in one population will also be found in the other. However, if levels of gene flow are fairly low, the populations may have large differences in allele frequency -- an allele that is common in one population may be rare in another.
In the next lecture, we will look in more detail at how gene flow and genetic drift, together, affect the potential for natural selection within populations. We'll also consider how to study gene flow, genetic drift, and natural selection, and how to tell which form of evolution has most impact on populations we're studying.
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