Natural selection (in which individuals that are better adapted to their environment survive and reproduce more successfully than others) is not the only mechanism of evolution. Other processes, such as genetic drift, can cause a change in gene frequency between generations. Genetic drift is the process by which random events cause random fluctuations in gene frequencies. Gene frequency is a measure of the proportion of available slots at a given locus that have a particular allele. A locus (plural, loci) is the physical location of a gene on a chromosome. An allele is a version of a gene. Random fluctuations in gene frequency can occur through chance alone. Certain individuals may reproduce more than others through sheer luck. These changes are not related to fitness or adaptation, including traits that improve survival or reproduction in a particular environment.
In a very large population (interbreeding group), random drift has little effect on long-term gene frequencies. One human eye color gene has two alleles, B and b, leading to brown eyes (BB or Bb) or blue eyes (bb). Imagine a human couple in which both parents are heterozygous for eye color (Bb). If they have only one child, that child has a 3/4 chance of having brown eyes and a 1/4 chance of having blue eyes. If they have lots of children, chances are that the children will display the genotypes BB, Bb, and bb in a 1:2:1 ratio. Over generations, if every heterozygous couple has a large family, both B and b alleles will be found in subsequent generations.
However, in a very small population—for example, an endangered species such as the Florida panther—random fluctuations and events can have a huge impact and may even lead to one allele becoming fixed (the only variant that exists for the gene) or lost, further decreasing genetic variation. Returning to the example of a human couple heterozygous for eye color (Bb), if each couple has only one child, there is a chance that their child will have blue eyes (bb). If there are only a few couples in the population, many of them have only one child, and that child has blue eyes, the frequency of the brown allele (B) will decrease and the frequency of the blue allele (b) will increase. Thus, as a result of random assortment and a small population, allele frequencies can change from one generation to another.
There are two main forms of genetic drift: the bottleneck effect and the founder effect.
Bottleneck Effect
An example of genetic drift (genetic change due to random events) is the bottleneck effect. The bottleneck effect is the result of a sharp decrease in population (interbreeding group) size. The gene pool of the remaining population may not reflect that of the original population. The gene pool is the total set of alleles of all individuals in a population. Imagine a population of organisms living near an ocean shore. If a hurricane arrives and the area becomes flooded, most of the population may be washed out to sea. The few survivors may not represent the full genetic variation of the original population. Previously rare alleles may now be more common in the new population, for example.
Flooding, earthquakes, or the sudden appearance of a new geographic barrier, such as a large area devastated by fire or a new road that bisects a wilderness area, may cause a genetic bottleneck in local populations. The individuals that remain following such an event were not selected based on favorable traits, as occurs with natural selection. They were selected by random chance. For example, a fire takes unpredictable paths as it moves through a wilderness area, and organisms run for safety in every direction. Some animals perish and some survive. Of those that remain, what was once a single population may be two (or more) different populations, each on a different side of acres of scorched land that will no longer be habitable by that particular animal, possibly for decades. Which animals (and which alleles they carry) end up on a given side of the area through which the fire moved may be random. The result may also be that the full genetic variation of the former population is not represented within each new population.Bottleneck Event
Founder Effect
Another example of genetic drift (genetic change due to random events) is the founder effect. The founder effect occurs when a few individuals start a new population. The gene pool (set of all genes) of the new population may not reflect that of the original population. For example, a small sample of individuals from one population migrates and begins (founds) a new, isolated population away from the possibility of gene flow with the original population. If there are very few founders (individuals beginning a new population), the effect is stronger because of the limited number of alleles involved and hence the limited possible variations. If one of the founders is carrying a rare trait, such as black fur instead of brown, that trait may become common in the new population as a result of random chance. If, for instance, the black-furred founder has many offspring and the majority of them have black fur, that trait will become more prevalent in the new population.
An experiment tested the founder effect using Caribbean lizards following a devastating hurricane. The researchers repopulated recently emptied islands with randomly chosen lizard pairs from a single island of survivors. Over four years, natural selection (in which individuals that are better adapted to their environment survive and reproduce more successfully than others) shaped the traits, such as limb length, of the new populations of lizards. However, many generations later, each population still showed genetic characteristics of their randomly chosen founders.