Causes of Microevolution: Natural Selection, Gene Flow & Genetic Drift

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  • 0:01 Species-Level Evolution
  • 1:20 Natural Selection
  • 2:49 Gene Flow
  • 4:05 Genetic Drift
  • 6:27 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Environments are dynamic, which causes populations to be as well. In this lesson, you'll learn about microevolution, as well as the mechanisms behind it that cause changes in allele frequencies within populations.

Species-Level Evolution

With the exception of humans, most organisms are not found throughout the entire world but rather only in certain areas. And this is no accident. Each habitat on Earth has conditions that support certain organisms better than others; things like temperature, sunlight, precipitation, terrain, etc.

Populations have, over thousands or millions of years, adapted to fit their environments. That is, over long periods of time, the genetic makeup of each population has shifted so that the physical traits that interact with the environment are the most functional for that type of habitat.

But what if the environment doesn't remain stable? It's rare that it does, and both small- and large-scale changes in the environment can impact population gene pools. When changes occur in the allele frequency of a population, this is called microevolution.

These changes occur at or below the species level. So, these are differences in allele frequencies within or among populations but not to the point where we get drastically different organisms, like a sheep turning into a fish. It'll be more like a population of mostly short-horned sheep over time becoming a population of mostly slightly longer-horned sheep.

Natural Selection

One mechanism of microevolution is natural selection. This process by which individuals with certain traits are more or less likely to survive and reproduce acts like an editor for allele frequency in populations. Natural selection does not act on individuals, but it is a mechanism where traits are either selected for or against, depending on environmental conditions.

For example, let's say you have a population of tree frogs. Green tree frogs blend in to their environment best, which helps them avoid predation. But say within a population of frogs, you have some that are bright purple. This provides very poor camouflage, making these individuals easy targets. If you are more likely to be eaten by a predator, then you are less likely to reproduce and pass on your purple frog genes.

But let's say that the trees suddenly become afflicted with a disease that turns them all bright purple. Now, our green frogs are the ones at a disadvantage because they are the ones that stand out against their environment. This environmental change will have a dramatic effect on our frog population because as the green frogs get eaten by predators, the purple frogs will produce more purple offspring. This will in turn change the overall gene pool of the population, making alleles for purple more prevalent.

Gene Flow

Sometimes gene pools within populations can change because individuals leave that population or new individuals come in. We call this movement of alleles between populations gene flow. Like water flowing in a river, genes can also 'flow' from one place to another.

Migration is one way that genes are transferred from population to population. Many migratory animals, like birds, congregate in certain areas along migration routes. Members of the same species but from different populations may breed and produce offspring, which can change the gene pool of one or both populations.

Populations can be quite large, or they may be as small as your backyard. Say, for example, that you and your next-door neighbor both have gardens in your backyards, and you are both growing the same species of tomato. You each have a separate population of tomatoes, but if a bee comes along and lands on your tomatoes, then flies over to your neighbor's tomatoes, the pollen from your tomatoes gets transferred to his tomatoes. That bee has transferred genetic information from one population to the other, potentially altering the allele frequency of one or both tomato populations.

Genetic Drift

Not as common but definitely more impactful to population gene pools is genetic drift. This is when chance events change allele frequencies unpredictably. Genetic drift is the most dramatic in small populations because there tends to be less genetic variety when there are fewer individuals.

There are two situations where genetic drift is most impactful on populations. First is the bottleneck effect. This is a drastic reduction in the size of the population. Think of the checkout lines at a store on Black Friday. There are hundreds of people trying to purchase their items, but only so many checkout lines available for people to go through. The number of people who have paid and are leaving the store is far fewer than the number of people still waiting to pay because not everyone can get through the checkout lines at once.

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