Genetic Drift

The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure.

Another way a population’s allele and genotype frequencies can change is genetic drift (Figure 19.4), which is simply the effect of chance. By chance, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting).

Visual Connection

Figure 19.4 Genetic drift in a population can lead to the elimination of an allele from a population by chance. In this example, rabbits with the brown coat color allele (B) are dominant over rabbits with the white coat color allele (b). In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of 0.5. Only half of the individuals reproduce, resulting in a second generation with p and q values of 0.7 and 0.3, respectively. Only two individuals in the second generation reproduce, and by chance these individuals are homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele is lost. This effect can be seen by observing the number of individuals in the population of given genotypes. As shown in the table, with each successive generation, individuals with a recessive b allele decreases as a percentage of the population.

Generation	Individuals with Genotype BB	Individuals with Genotype Bb	Individuals with Genotype bb
1	22	53	25
2	108	118	25
3	633	0	0

Table 19.1

Explain why small populations are more vulnerable to the forces of genetic drift than large populations.

1. The fewer individuals in a population, the more room there is for new individuals to migrate into it.
2. The more individuals in a population, the more alleles are present in its gene pool.
3. The fewer individuals in a population, the more likely it is to go extinct.
4. The more individuals in a population, the larger and more stable is its gene pool.

Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a young age before it leaves any offspring to the next generation, all of its genes—1/10 of the population’s gene pool—will be suddenly lost. In a population of 100, that’s only 1 percent of the overall gene pool; therefore, it is much less impactful on the population’s genetic structure.

Link to Learning

Go to this site to watch an animation of random sampling and genetic drift in action.

Describe an example of genetic drift.

1. Immigration of new individuals can cause genetic drift. For example, if several white rabbits migrate into a population of mostly brown rabbits, the allele for white fur will increase within the population.
2. Introduction of new alleles through mutation can cause genetic drift. For example, if there are two alleles for fur color in a rabbit population, and a mutation in one of them produces a third allele, the gene pool changes to incorporate the new allele.
3. Chance events such as a natural disasters can cause genetic drift. For example, if the only white rabbits in a population get killed by a storm, the allele for white fur will diminish or disappear in the population.
4. Differential survival and reproduction can cause genetic drift. For example, if all the white rabbits in a population get eaten by wolves because their white fur stands out and is more visible, the proportion of the allele for white fur in the population will decrease.

Genetic drift can also be magnified by natural events, such as a natural disaster that kills—at random—a large portion of the population. Known as the bottleneck effect, it results in a large portion of the genome suddenly being wiped out (Figure 19.5). In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population.

Figure 19.5 A chance event or catastrophe can reduce the genetic variability within a population.

Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is likely due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital abnormalities.

Link to Learning

Watch this short video to learn more about the founder and bottleneck effects.

Compare and contrast the bottleneck and founder effects.

1. Both the bottleneck and founder effect are examples of gene flow. However, the bottleneck effect occurs after a cataclysmic event, whereas the founder effect occurs when mutations introduce new alleles into a population.
2. Both the bottleneck and founder effect are examples of genetic drift. However, the bottleneck effect is a process in which a large portion of a genome is wiped out, whereas the founder effect occurs when members of a larger population migrate to establish their own population.
3. Both the bottleneck and founder effect change the genetic structure of a population. However, the bottleneck effect reduces or eliminates alleles within a population, whereas the founder effect introduces or increases alleles.
4. Both the bottleneck and founder effect change the genetic structure of a population. However, the bottleneck effect occurs when inbreeding depression kills off part of a population, whereas the founder effect relies on nonrandom mating.

Nonrandom Mating

If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice; for example, female peahens may prefer peacocks with bigger, brighter tails. Traits that lead to more matings for an individual become selected for by natural selection. One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves.

Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.

Environmental Variance

Genes are not the only players involved in determining population variation. Phenotypes are also influenced by other factors, such as the environment (Figure 19.7). For example, sun exposure is an environmental factor, as a person who spends more time in the sun will likely have darker skin than a person who spends most of their time indoors (assuming both people had similarly colored skin to start with). Some major characteristics, such as gender, are determined by the environment for some species. For example, some turtles and other reptiles have temperature-dependent sex determination (TSD). TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range.

Figure 19.7 The sex of the American alligator (Alligator mississippiensis) is determined by the temperature at which the eggs are incubated. Eggs incubated at 30 °C produce females, and eggs incubated at 33 °C produce males. (credit: Steve Hillebrand, USFWS)

Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Such geographical variation is seen between most populations and can be significant. One type of geographic variation, called a cline, can be seen as populations of a given species vary gradually across an ecological gradient. Species of warm-blooded animals, for example, tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. This is considered a latitudinal cline. Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of a mountain, known as an altitudinal cline.

If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, on the other hand, can lead to abrupt differences, even speciation.