Mechanisms of Evolution
Topics in Depth
The Theme of Gene Flow and Genetic Drift in Mechanisms of Evolution
Just Goin' With the (Gene) Flow
Remember Gilligan's Island? Gilligan's crew is part of a very small human populations separated from the rest of the human species. If they never got off the Island, their gene pool would be considered to be shallow.
Although it happens that some populations of the same species are geographically isolated, this is not usually the case. One population of squirrels could live near another population of squirrels. All they'd need to do is hitch a ride in the back of a pickup truck to end up in the middle of the first population's territory. Once there, the alien squirrels might reproduce with a member of the first population. When genes are exchanged due to the mixing of populations, the result is gene flow. Picture genes literally flowing from one population to the next.
Some strains of Staphylococcus aureus strains (gold) have acquired antibiotic resistance, a scary trait that can be passed from one population of bacteria to another.
Gene flow can happen quickly in bacteria. The CDC (along with the general population) is nervous about the rise in antibiotic resistant bacteria. Because they can multiply at ridiculously fast clips, some bacteria are evolving antibiotic resistance at a rate that medicine can't keep up with. Strains of bacteria with antibiotic resistant genes can pass on their genes to other bacteria when they hang out together.
Gene flow also often happens to populations that migrate, or move from one location to another location at a specific time year after year. Imagine that several populations of geese migrate to the same sunny Florida getaway every winter. Lucky birds. While vacationing in the Keys, this species has plenty of opportunity to interact with other populations and acquire some genetic diversity.
Do You Catch My Genetic Drift?
In addition to flowing from one population to another like birds in the wind, alleles can change frequency within their own population. This is genetic drift.
The bottleneck effect describes genetic drift that occurs when a population goes through a traumatic event where a random majority of the population doesn't make it. We know. This sounds awful. It's like one of those crazy scenes on National Geographic where a sudden flood takes out a bunch of lizards sunbathing on river rocks. This makes a big difference on the resulting gene pool. Maybe most of the lizards had long tongues excellent for catching fast flies, but that deluge left only a few KISS-like lizards in the surviving individuals. The next generation would have a lower frequency of long-tongue alleles.
In a bottleneck effect, a population goes through an event where only a few random individuals survive. Here, marbles represent an allele in the population that can be blue or yellow. These alleles may have had the same frequency in the parent population, but after an even that destroyed most of the population, random surviving alleles may not be equally represented in the next generation. In this example, most of the surviving individuals carry the blue allele. This alters the allelic frequency in the next generation.
The founder effect describes genetic drift that occurs when a few members of the population break away from the pack and are "found", or start their own population. Because a small number of individuals start a new population, the allele frequency in this new gene pool can be dramatically altered.
In the founder effect, genetic drift happens because a few individuals broke off and started their own population.
Both gene flow and genetic drift alter allele frequencies. Ixnay on the Hardy-Weinberg rinciplepay.
Flying south isn't the only way to migrate. Some plankton migrate to the other side of the world simply by catching a ride in a ship's hull. True story.
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