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Mechanisms of Evolution

Mechanisms of Evolution

Gene Flow and Genetic Drift

Just Goin' With the (Gene) Flow

Remember Gilligan's Island? Probably not…and since we do, that probably makes us a fossil. But watch some TV Land or YouTube videos. When you're up to speed we'll continue.

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…and in Technicolor.

Although it happens that some populations of the same species are geographically isolated, it's actually quite rare. One population of squirrels could live near, but separate from, another population of squirrels, and all they'd need to do to pay the other tribe a visit is hitch a ride in the back of a pickup truck. Once there, those hitchhiking squirrels might reproduce with their new friends. 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. Ewww…

Colorized scanning electron micrograph of E. coli.
(Source)

Bacteria are great models for studying evolution. They multiply quickly, are easy to grow, and they don't moan and complain when you throw bleach on them. Plus, gene flow happens quickly in bacteria.

The Centers for Disease Control and Prevention (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 germy superpowers to other bacteria when they hang out together. Which is like always. Whoever heard of one bacterium? 85 million, sure. But one?

Gene flow is also popular among the migrating crowd. Those that migrate, or move from one location to another at a specific time of the year (year after year) interact with a lot of different species when they make it to their vacation spots. 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 Our Genetic Drift?

In addition to flowing from one population to another, like a plastic grocery sack in the wind, alleles can change frequency within their own population. This is lovingly referred to as genetic drift.

Genetic drift occurs thanks to random sampling within a population that leads to changes in allele frequency. And yeah, by "sampling" we mean the passing on of genes the old fashioned way: via reproduction.

There's typically a good range of diversity for alleles within a population, but any given allele might only exist in the DNA of a limited number of individuals. If those individuals reproduce more or less successfully by sheer happenstance, then we get genetic drift: those alleles become suddenly more or less prominent in the gene pool.

This effect is amplified when there are fewer individuals carrying the gene for a specific trait. For example, if only a handful of individuals carry a gene for a trait, and randomly none of them ever reproduce, that gene disappears. Conversely, if they all manage to reproduce, and for some random reason individuals without the trait don't reproduce, that trait suddenly becomes much more prevalent.

That's genetic drift for you, nature's version of roulette for passing on alleles.

There are also two mechanisms for sudden, usually drastic, genetic drift: the bottleneck effect and the founder effect.

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. They should have moved faster.

As you can imagine, this makes a big difference on the resulting gene pool, and the end result will depend, in part, on chance events. As an example, maybe most of the lizards had long tongues excellent for catching fast flies, but that the deluge left only a few Mylie Cyrus-like lizards in the surviving individuals. Now all their lizardy descendants will have a lower frequency of long-tongue alleles.

A schematic illustrating the bottleneck effect. A starting 'population' of red, orange, and green balls is in a glass bottle. Balls pour out the neck of the bottle, and the few individuals in the surviving population do not share the same proportion of colors as the original population.
Must have been some event. The reds didn't even make it out the other side. (Source)

In a bottleneck event, a population goes through an event where only a few random individuals survive. Here, marbles represent survivors, and the color of the marble represents an allele for a trait in the population. Let's imagine that it's a gene that codes for either red, orange, or green skin. Because why not?

These alleles had a particular distribution in the parent population (orange and green are relatively common, with some reds sprinkled throughout), but after some serious stuff went down 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 orange allele, and none of the reds survived. This alters the allelic frequency in the next generation. Hopefully that red trait isn't very useful…

The founder effect describes a type of genetic drift that seems pretty similar to the bottleneck effect. This occurs when a few members of the population break away from the pack and "found" 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. The catch here is that the parent population wasn't just obliterated when a meteorite collided with their swamp.

A schematic illustrating the founder effect.
"You're not the boss of me!" When alleles run away… (Source)

Again, the main result here is that the alleles in the founding group will have a huge impact on subsequent generations. By chance, the alleles will be different in the new population than they were back home.

Brain Snack

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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