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As we have noted, evolution occurs at a population level and relies on variability in the genetic composition of the individuals within the population. It is dependent on the balance of genotypes within a population rather than an individual's phenotype. Genetic differences do not need to be large to cause evolution in a short period of time. Only a small, slight fitness advantage is needed to cause selection to occur. Mutations are a mechanism that provides some of this genetic variation that natural selection acts on. However, not all evolution occurs through natural selection. In addition, the size and genetic diversity of a population play roles in the effects of external forces on a population.
Genetic Drift
Genetic drift is a random shift in the frequency of alleles in a population. While genetic drift is random, natural selection is not. It is a non-selective process, meaning it does not usually produce a better-adapted population or individuals. Genetic drift is more pronounced in small populations because when there are fewer individuals, the chance of an allele (or a version of a gene) becoming lost or fixed is greater. Imagine you have a population of three individuals, each with a different form of a gene – alleles A, B, and C. If you lose one of these individuals, you would lose a third of the genetic diversity. Now imagine you have a population of 30 individuals. Ten of these have allele A, ten have allele B, and ten have allele C. If you were to lose a third of the population, the same proportion we lost in the first example, it is unlikely that all these individuals would be carrying the same allele. Because of this, the population would still retain all three alleles. There are two main ways genetic drift occurs in small populations – bottlenecks and the founder effect.
Bottlenecks
A bottleneck occurs when the size of a population shrinks in response to an event such as a natural disaster or disease outbreak. This loss of individuals within a population leads to a loss of genetic diversity. The gene pool of the surviving population is smaller and allele frequency can change in a non-adaptive way.
Founder Effect
The founder effect occurs when a small number of individuals from a population leave and establish a new population somewhere else. Since this new population consists of only a small proportion of the original population, it also contains only a subset of the genetic variation. This results in this new population having a different allele frequency and certain alleles may become fixed or not be present.
Variations in Populations
The previous section focused on how the genetic diversity of a population can change rapidly and randomly. But why is genetic diversity so important to a population? There are several reasons and they all center on the fact that having more alleles in a population allows it to adapt to change and withstand environmental pressure without going extinct. When species evolve, they evolve under specific conditions. When these conditions change, they may now be less adapted to their environment, resulting in a reduction in fitness. In some instances, alleles that are adaptive in one scenario, may be detrimental in another. Species and populations with little genetic diversity are at risk because they lack the flexibility in their genetics to adapt. Genetically diverse populations, on the contrary, are more resilient. They are more likely to contain individuals that will survive new conditions and reproduce.
