Genetic Drift and Gene Flow

Evolution & Genetic Drift Evolution and Genetic Drift The variety of human attributes evident in society comes as a result of the variety of alleles that direct the expression of human genotypes. This expression results in very different phenotypic traits that form the basis of human individuality. Based upon these traits, a person may be more or less likely to adapt well to their environment. When viewed over a long enough time period, the frequency of allele distribution results in more or less advantageous phenotypes.

Those phenotypes that are less adaptive to the surroundings make it harder for that individual to survive. This is the basis of natural selection, where desirable traits are selected for based upon the advantages that the phenotype confers. This is also the reason for changing allele frequencies, as those gene variants that are less helpful to the organism become (over time) less common in the population. "These ever-present and interacting forces of.. genetic drift and...natural selection work to differing degrees to shape populations." (Lewis 242).

Although there are only two alleles for a particular gene in any given individual (one on each matching chromosome), there are usually many possible alleles for a trait within a population. When allele frequencies aren't changing, the population is said to be in a state of Hardy-Weinberg equilibrium. This is often not the case, however, as various positive characteristics allow for that allele frequency to grow, and numbers of deleterious alleles to decline (and sometimes vice versa). There are many reasons for changing allele frequencies within a population.

These include nonrandom mating (where one chooses a spouse based upon some nonrandom process), migration and genetic drift, among others. Underpinning evolution and natural selection is the concept of gene flow. When various alleles move through the population based upon migration patterns and mating, the allele variants are spread through the population. Generally, gene flow contributes to greater genetic diversity, however, in some circumstances, genetic diversity may be reduced, as in the case with genetic drift.

Lewis (247) says that "genetic drift occurs when a subset of a population contains different gene frequencies than the larger population because it is a small sample." Over time, evolution and natural selection will work together to make the gene pool of a smaller population different from that of the greater allele distribution. This is especially common in cases where a small community has a lot of intermarriages (as in many religious communities).

This mating within a small sample can lead to a different allele frequency than one would see in the larger sample. The cause of genetic drift is, essentially, the decline in numbers of a small population, leaving the remaining individuals to replenish the genetic stock. Since there is less allelic variation in this subsample, the ensuing generations will have less genetic variation as well.

"Genetic drift occurs when the population size plummets, due either to migration, to a natural disaster that isolates small pockets of a population, or to the consequences of human behavior." (Lewis 244). One type of genetic drift is the "founder effect." This is a change in allele frequencies in a population due to the founding of new settlements. This new settlement consists of a small group of people who usually mate within their community, and perpetuate certain alleles.

When compared against the greater population, the residents of the new settlement have genotypes influenced by genetic drift. Various examples of the founder effect influencing allele frequencies include the Old Order Amish of Pennsylvania, Ashkenazi Jews, the Finns, and Afrikaner communities in South Africa. When a new settlement is established, "the new colony may have different allele frequencies than the original population, amplifying some traits while dimishing or even losing others." (Lewis, 244). The Old Order Amish of Pennsylvania mate within their own community, and seldom involve the outside community.

This loss of genetic diversity has led to a higher incidence of certain inherited diseases within Amish communities. For example, the founder effect in the Amish settlement has caused a variety of conditions, such as dwarfism and Crigler-Najjar syndrome, where the body can't break down bilirubin. Maple Syrup Urine Disease is much more common in the Amish, as is Homocystinuria, where an individual's blood vessels are damaged. In the case of polydactyly, where a person has extra digits, the prevalence among the Amish is much higher.

"Today, thanks to large families and restricted marriages, the number of cases of polydactyly among the Amish exceeds the total number in the rest of the world." (Lewis, 246). The Afrikaner population in South Africa was settled by a small group of immigrants from Holland, France and Germany. This population has very high rates of porphyria variegata.

Due to very precise record-keeping, researchers have been able to determine that "all affected people descended from one couple who came from Holland in 1688!...this couple had many children - who, in turn, had large families, passing on and amplifying the dominant gene." (Lewis, 246). Similar founder effects are also seen in the Ashkenazim, and Finns, who have less genotypic diversity than the larger population, likely because their ancestors were from a small original community.

Another type of genetic drift that occurs to alter allele frequencies is the "population bottleneck." This occurs when much of a particular group is wiped out, and the few remaining individuals are left to build up the colony. The result is a small genetic contribution to subsequent generations. A population bottleneck occurs when the size of a genetically diverse population drastically falls, remains at this level for a time, and then expands again.

The new population loses some of its genetic diversity if different alleles are lost in the bottleneck event. (Lewis, 247). Population bottlenecks have led to genetic drift in both human and animal populations. The cheetah population is.