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How Gene Pool Diversity Affects a Group's Survival Potential

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  • 0:02 Diversity Is Maintained
  • 0:59 Why Diversity Matters
  • 3:05 Heterozygote Advantage
  • 6:05 Frequency-Dependent Selection
  • 7:46 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.

Humans are genetically the same yet very different. These differences are important, not just for humans but for all species because it makes it possible for populations to adapt to changing environmental conditions, which is essential to their survival.

Diversity Is Maintained

There are billions of people on Earth, but you can easily pick your friends out of a crowd because they are unique individuals. We are all humans, but genetically, we are quite diverse. Unless they are clones, other organisms are also similar yet different. For example, all dogs have a specific genetic code that makes them dogs, but each dog is a unique individual. It's the same with each tree in the forest, each mosquito biting your arm and each bird singing in your backyard.

This genetic diversity is no accident. In fact, it's critical to a species' ability to survive and evolve through generations of environmental changes. Natural selection is the process that acts on variants within a population, but there are processes in place that prevent it from removing all types of variation present in the gene pool. Let's take a look at why genetic diversity matters and how it's maintained.

Why Diversity Matters

Before we get into the actual mechanisms that maintain a population's diversity, let's take a look at why this diversity is important. Even if a population has been living in an environment for a long time and seems well adapted to it, that environment itself does not necessarily remain stable. Both small- and large-scale changes can occur in that environment, and if a population isn't genetically diverse, it may not be able to adapt and survive. Things like hurricanes, drought, habitat destruction or even a good winter cold spell can have dramatic effects on local populations.

When environmental changes occur, the population has to adjust if it wants to survive. Individuals do not adapt to fit their environment, but the population can be modified over time. Let's say, for example, that you have a population of elephants. Some of the elephants have very long trunks, some have very short trunks and others have trunks that are somewhere in the middle.

For now, this population is doing just fine. The elephants with the long trunks can pick leaves off the taller trees, while the elephants with short trunks do fine eating leaves that are closer to the ground. But one day, someone comes through and cuts down all of the short trees. Now, all of the elephants with short trunks are out of luck - they have no food to eat because they can't reach the higher leaves!

However, the elephants with mid-sized and long trunks are still okay. They can reach the leaves that are mid-way and high up the trees, and the population continues to thrive even though the elephants with short trunks die out.

But imagine that all of the elephants in the population had short trunks. The whole population would be wiped out when the short trees were cut down! But because of the diversity in trunk sizes, the population continues to survive. In this case, the variety of individuals allowed the population as a whole to adapt and persist.

Heterozygote Advantage

Okay, so now that you have seen an example of why diversity is so important, let's look at how it's maintained. Most eukaryotic organisms are diploid. This means that they have two sets of chromosomes. This provides an incredible advantage when it comes to maintaining genetic diversity within a population. Much of the genetic information you carry is not expressed physically. This 'hidden' genetic information is carried along through generations, like little stowaways, only showing itself once in a while or when it becomes necessary in a given environment.

Take your ears for example. Some people have earlobes that are attached to the side of their face while others have earlobes that are detached and free-hanging. The attached earlobe is a recessive trait, which means that if the allele for the detached earlobe is present, this is the trait that will be physical expressed. However, you can still 'carry' the attached earlobe allele but it will only show itself if you have two attached earlobe alleles instead of one of each. It's in this way that the genetic information for attached earlobes is preserved even if it's not expressed very often.

Your ear shape isn't necessarily an important factor in your environment, but a lot of other important genetic information follows this same logic. Individuals who have two different alleles for a trait are called heterozygous. This would be someone with both the attached and detached earlobe alleles (one of each). Someone who had two detached or two attached alleles is homozygous, because these are individuals that have two of the same alleles for a trait. 'Hetero' means different, and 'homo' means the same, which can help you remember the difference between the two.

Natural selection favors heterozygote individuals because this maintains genetic diversity within the population. This mechanism is called the heterozygote advantage. A well-documented example of why heterozygotes are favored can be seen with the sickle-cell allele. Sickle-cell disease in humans is when red blood cells are deformed and occur in a sickle shape. These cells are destroyed by the body, so if an individual has many of them, this can seriously lower their red blood cell count, making them anemic.

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