32.4:
Genetic Drift
A general misconception about evolution is that it requires natural selection to occur. However, this is not always the case.
Genetic drift is one mechanism by which evolution can occur without natural selection. It is defined as a change in the allele frequency of a population due to chance.
To envision this, let's use a giraffe population as an example and imagine their alleles of tan and brown being represented by marbles of two different colors. We will assume here that each color starts out equally abundant.
If we were to start a new generation out of this population, we would need to breed pairs of individuals and thus select from four alleles per pair. If we select a breeding pair at random, then we might end up with two marbles of each color. However, by chance alone, some pairings will have only marbles of one color, or three of one color and one of the other.
These chance deviations over multiple pairings might mean that the next generation no longer has an equal mixture of each allele. It's this variation of relative allele frequencies over time that defines genetic drift.
Some forms of genetic drift can drastically change allele frequencies. The bottleneck effect and the founder effect are two such examples of extreme genetic drift.
The bottleneck effect occurs when a population’s size is significantly reduced for one generation or more. It can be explained using a metaphor—a bottle containing marbles of different colors, i.e., different alleles.
When the bottle is upturned—a bottlenecking event occurs—only a few marbles randomly fall out. This collection of marbles, aka the surviving population, then creates a new population that is likely not representative of the original population.
The new population demonstrates a significantly reduced genetic diversity, which is an extreme genetic drift. Events like natural disasters and over-hunting can cause such a bottleneck effect.
Sufficiently large populations can usually withstand such events without dramatic loss of diversity. Small populations, however, can be disrupted for generations or even permanently.
In the second kind of extreme genetic drift, the founder effect, small segments of a population relocate, becoming isolated, creating new “founder populations.” The consequences are reminiscent of the bottleneck effect. The new populations are likely nonrepresentative of the original population because they are less genetically diverse.
Therefore, unlike adaptive evolution, where allele frequency changes to select for traits that are fit for the environment, like ladybugs with a greater amount of melanin surviving better in colder climates because of an improved ability to absorb heat, genetic drift represents a type of evolution that is purely due to stochastic change. For example, the random removal of a section of a population through a catastrophic event or migration.
32.4:
Genetic Drift
Natural selection—probably the most well-known evolutionary mechanism—increases the prevalence of traits that enhance survival and reproduction. However, evolution does not merely propagate favorable traits, nor does it always benefit populations.
Life is not fair. A deer grazing contentedly in a field can have her meal cut tragically short by a bolt of lightning. If the doomed doe is one of only three in the population, 1/3 of the population’s gene pool is lost. Random events like this can indelibly affect a population, sometimes for generations. This evolutionary mechanism is called genetic drift.
Genetic drift is a shift in population allele frequencies due to chance events. Alleles are variations of a gene, and their frequency is the portion, or percentage, of the population with that allele. Genetic drift can alter the frequencies of advantageous, neutral, and harmful alleles alike.
Genetic drift does not dramatically impact sufficiently large populations; this is because it does not occur in isolation, but alongside other evolutionary mechanisms, like natural selection. In large populations, many individuals can be lost, and the remaining gene pool is still diverse enough for natural selection to act.
However, genetic drift can sharply reduce genetic diversity in small populations, creating a sampling error. A sampling error occurs when a sample is not representative of the population from which it is derived. When part of a population is eliminated, the remaining members may represent only a fraction of the original population’s genetic diversity. Larger samples are typically more representative, which is why scientists maximize sample size for their experiments.
Two extreme examples of genetic drift are the bottleneck effect—caused by catastrophic events, like natural disasters—and the founder effect, a result of colonization. In both cases, smaller populations derived from larger ones create a sampling error that leads to evolution, sometimes from less-than-favorable traits.