Evolution acts upon the genetic variety within a population, and genetic variety is driven by random occurrences. Mutations are a leading factor in genetic variety. While a large portion of mutations are neutral (they don’t change the fitness of the organism and/or cannot be observed), some can affect species fitness.
Some mutations are beneficial (such as the peppered moth example) while some can actually be harmful. Either way, mutation definitely helps guide evolution by contributing new phenotypes.
Genetic Drift and Gene Flow
Genetic drift and gene flow are two other common mechanisms for evolution. Genetic drift is any random event, usually a natural disaster, that greatly affects a population. For example, a volcanic eruption may kill 40% of a population, and if it happens to kill all the individuals that carry a specific allele, then that phenotype can no longer exist in that population.
These natural events will have more of an impact on smaller populations than larger ones, as it is more likely to kill *all* the individuals with a certain allele in a smaller population than in a larger one.
The Founder’s Effect and The Bottleneck Effect
Other types of genetic drift could be the founder’s effect and the bottleneck effect. The founder’s effect is when a small group breaks away from their population and colonizes a new area, which leads to the possibility that a new allele could be overrepresented in this new group, and could soon become the dominant trait.
This would also change the allele frequency of the original group, which is the bottleneck effect, or, more specifically, when the resulting population is a lot smaller and not representative of all the alleles and frequencies of the original population.
Image courtesy of WikiMedia Commons.
Gene flow is grounded in a similar concept, but with a less extreme outcome. It simply describes the phenomenon of how immigration (movement of individuals into a population) and emigration (movement of individuals out of a population) can affect allele frequencies as well. Gene flow is more common and less disastrous, and helps to explain how alleles and phenotypes can be added and removed from populations.
Mechanisms like genetic drift and gene flow not only have the ability to reduce genetic variation in populations, but therein lies the possibility of separating them as well. Especially in an example with a founder’s effect, the two populations may begin to reproduce and evolve separately, each with their own unique favorable traits.
If the two species do not reproduce with each other, it can lead to speciation, or the development of two entirely different species. At that point, the two species would be unable to reproduce with each other, and have vastly different allele frequencies and genetic variety.
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