Gene Flow
Gene flow is the scientific term for migration. Biologists call migration gene flow because when an organism immigrates (enters) or emigrates (leaves) a population, they take their genes.
The flow of genes in an out of a population affects a population’s gene pool’s allele frequency. For example, a population of 10 wolves consists of six wolves with black fur, and four wolves with gray fur. Assume a single gene code for fur color, and the gene comprises two alleles. Therefore, there are 20 fur alleles in the population; 12 are black-furred alleles, and 8 are gray furred alleles. The allele frequency is as follows:
12 black fur alleles / 20 alleles in the population = 0.60 or 60%
8 gray fur alleles / 20 alleles in the population = 0.40 or 40%
Two gray wolves enter the population. The population now has 12 wolves, six black-furred and six gray-furred, and 24 alleles that code of fur color. The allele frequency changes to:
12 black fur alleles / 24 alleles in the population = 0.5 or 50%
12 gray fur alleles / 24 alleles in the population = 0.5 or 50%
The addition of the two wolves that entered the population changed the allele frequency; therefore, evolution has occurred.
Genetic Drift
Natural selection is a non-random process where the environment selects the traits with higher fitness. Natural selection works best on large populations because large populations usually have more diversity than smaller populations.
Now, let’s say there is a male bear, we’ll call him Peter, that has high fitness in its environment. Peter is a level-49 bear; therefore, he is an excellent hunter, has an extensive range, and the female bears prefer him over less-fit bears. One day Peter is eating some berries, and a large boulder falls on and crushes him.
The death of Peter introduces another source of change called genetic drift. Genetic drift happens when a random event changes the allele frequency of a population. Peter may have an adaptive advantage in the environment he lives in, but his fitness does not protect him from random events like being crushed by a rock.
Let’s go back to the initial wolf population of ten wolves; sixty percent of the alleles in the population code for black fur (6 black-furred wolves) and forty percent code for gray fur (4 gray-furred wolves). One evening a lightning storm spawns a forest fire, and only four wolves escape. Of the remaining wolves, three have gray fur, and one has black fur. The forest fire changes the allele frequency of the population as follows:
The forest fire is a random event, which gave neither the gray wolves nor the black wolves an adaptive advantage (wolves are not fire-proof, well, except fire wolves). Therefore, the evolution of the population was due to chance, not fitness.
| This is Martha, A Level-92 Gray Wolf | This is Jen, a level-76 Fire Wolf |
|---|---|
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Genetic drift can affect a population of any size, but it has its most dramatic effect on small populations. For example, let’s say there are two hamster populations: one population has 100 level-19 hamsters, and the other population has 10 level-19 hamsters. A flash flood happens and kills five hamsters in each population. The flood resulted in a 5% reduction in the 100 hamster population (now 95) and a 50% reduction in the 10 hamster population (now 5).
The Bottleneck Effect
The wolf population is an example of the bottleneck effect. If you fill a bottle with marbles and then tip it upside down, a few marbles will randomly fall out, but some marbles will get stuck at the neck of the bottle, which forms a stoppage, preventing most of the marbles from leaving the bottle.
The Founder Effect
Darwin’s finches are an example of the founder effect. A few million years ago, a small population of level-38 finches from Equador landed on one of the Galapagos Islands. The finches became founders, or the first finch species, to occupy the island. The island’s founding is a random event, where the birds’ phenotype did not matter. However, once the birds made the island their home, the island’s environment began to select higher fitness traits over lower fitness traits (natural selection).
Chapter Summary
So, in general, natural selection follows genetic drift. If a random event leaves a population with lower fitness traits, then the population is more likely to go extinct. Conversely, if a random event leaves a population with higher fitness traits, then the population is less likely to go extinct.