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What Are Factors That Lead to Evolutionary Change?


As we’ve already learned, evolution in its simplest form is just the change in allele frequency within a population of organisms over time. But how and why do these changes occur?


According to the Hardy-Weinberg principle, a population that is not experiencing any outside evolutionary forces will not see any change in the frequency of alleles. This state is known as “Hardy-Weinberg equilibrium”, and there are generally understood to be four main evolutionary factors that would cause a population to leave this state.


The first way that allele frequencies can change in a population is through mutation. Although mutations are rare, they provide the raw material of variation on which natural selection can act. New mutations change the allele frequency of a population by adding a new allele, even if it’s only a very slight change.


Another factor that tends to have a relatively small impact is genetic drift. Genetic drift can best be understood as changes in allele frequency due to randomness. For example, the only organism in a population with a particular rare allele might be struck by lightning and die before it could reproduce; thus that allele would be removed from the population out of sheer randomness.



(PublicDomainImages, 2014)

Genetic drift is usually only an important factor in very small populations or populations with very low genetic diversity. For example, at one time there were only seven black-footed ferrets left alive on earth. Conservationists brought the species back from the brink of extinction through captive breeding, but back when there were only seven ferrets genetic drift posed a real threat to the species. The loss of even one ferret could have removed certain alleles from the population forever.


The next evolutionary factor is gene flow, which refers to the movement of individuals in and out of a population. Gene flow can remove alleles from a population (in the case of individuals leaving) or bring new alleles into a population (in the case of individuals joining the population). A male lion leaving his birth pride in one valley and moving to take over another pride in the next valley over would be an example of gene flow.


The last evolutionary factor that drives change in alleles is one you are already very familiar with: natural selection. While mutations, genetic drift, and gene flow can have an impact on allele frequency, natural selection is the main driver behind evolution.


Image Citation:


PublicDomainImages (photographer). (2014). Picture Ferret Footed [photography]. Retrieved from

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