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Other Causes of Evolution
Other Causes of Evolution
Recall that our definition of evolution has become "the change in allele frequencies of a population over time". Natural selection is typically credited with being the primary mechanism of evolution. This is certainly common, but it is not the only way by which evolution occurs.
In reality, any thing that causes a change in those allele frequencies is therefore causing evolution to occur. The primary mechanisms we need to focus on for AP Biology are genetic drift and gene flow.
A. Genetic Drift
A. Genetic Drift
What It Is
What It Is
Genetic drift is a mechanism of evolution that is defined by random fluctuations in the frequency of alleles in a population. The key term here is random. We have discussed randomness (sometimes called stochasticity) before in the context of meiosis.
That same randomness can be the cause of changing allele frequencies in a population. These changes can cause a loss of genetic variation or even deleterious (or harmful) alleles to become fixed in a population.
There are two forms genetic drift can take that you need to be familiar with: the bottleneck effect and the founder effect.
1 - The Founder Effect
1 - The Founder Effect
If a small number of individuals in a population become isolated (perhaps blown to a new island), they may establish a new population.
Depending on which individuals made it to the new area, that new population may have a very different gene pool from that of the original population.
In the image shown, you can see that some butterflies from the ancestral population moved to a new island and colonized it. However, those colonizers did not contain any butterflies with the white phenotype.
This could mean, depending on how the phenotype is determined (dominant, recessive, etc.) that the new population will not ever contain white butterfliess.
2 - The Bottleneck Effect
2 - The Bottleneck Effect
A bottleneck effect is essentially when a large population is suddenly reduced in size often due to a catastrophic event such as a disease, extreme weather, or human action (see greater prairie chicken case study in the textbook).
Essentially a rapid decrease in population size can cause some alleles to be overrepresented, underrepresented, or even absent in the survivors. These alleles are represented by different colors in this image.
The survivors now may have many resources available and begin to reproduce. Even if the surviving population recovers and returns to its original size, however, it may lack genetic diversity for a long period of time.
For example, in the image shown, there are no yellow alleles in the surviving population, blue alleles appear to be overrepresented, and white underrepresented compared to the original allele frequencies. These survivors could reproduce and replenish the population, but there would be no yellow alleles present unless that mutation occurred again independently.
Why Small Populations are Susceptible
Why Small Populations are Susceptible
Randomness has greater impacts on small populations. Think about this in the context of sample sizes. If a couple has 4 kids and they are all male, is that truly so unusual? Normally you'd expect 2 boys and 2 girls, but each child's sex is randomly determined by meiosis in the father. However, if that same couple had 300 children, and they were all male, that would truly be unsual and would indicate something else might be going on.
As the sample gets bigger, randomness tends to cancel itself out. We would see closer to 50% male and 50% female in a large sample than in a small sample. This is why genetic drift can usually be ignored in large populations, but is particularly relevant in small populations. So if you are working in conservation with an endangered species, the randomness of meiosis can be disastrous.
B. Gene Flow
B. Gene Flow
What It Is
What It Is
Allele frequencies of a population can also change through gene flow, the movement of alleles into or out of a population. These alleles can travel from one population to another when fertile individuals (or their gametes) travel there.
Some organisms are stationary, of course, but often their gametes can still travel. For instance, pollen contains the male gametes of plants. This pollen might be wind-carried or carried by small organisms. The parent organism may stay still, but its gametes (and therefore the alleles they carry) can still travel to new populations by pollinating plants there.
Effects on Allele Frequencies and Phenotypes
Effects on Allele Frequencies and Phenotypes
Alleles traveling from one population to another causes those populations to resemble one another more.
For example, in this example shown, one ant population is all homozygous recessive (white) and the other is all homozygous dominant (black).
If an individual from the dominant population travels to the other population, it changes the allele frequencies by introducing a new allele. However, over many generations, that ant may reproduce and cause the dominant allele (A) to spread, causing some individuals to take the dominant phenotype.
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