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We’ll take a look at the types of natural selection that can occur. From flying hamsters to moths, you’ll start to grasp the different paths organisms can take as they respond to their changing environments over time.

Directional Selection

Directional selection favors characteristics at one end of the phenotypic spectrum
Directional Selection Graph

We’ve come up with an interesting theory on how flying hamsters could have evolved. Our data and research has prompted us to propose that an extra limb could have slowly evolved into a wing. Hamsters that could fly were more fit than those that couldn’t, and, thus, the flight adaptation evolved.

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To write up our senior thesis, though, I think we need a more complete understanding of evolution. We’ve proposed that over time, a rudimentary wing may have increased the fitness of an ancestral hamster enough to set off a directional evolutionary change.Suppose there were many different rudimentary wing sizes. If we were to graph the size of the rudimentary wing, we might get a distribution like this where most of the hamsters have a rudimentary wing that is average in size. The rudimentary wings that were the largest worked the best for gliding or slowing falls.

If the increase in fitness for larger rudimentary wings is significant at each generation, hamsters with larger rudimentary wings could survive better and end up contributing a greater proportion of the progeny in the next generation due to the lower survival rate of their counterparts in the population. Eventually, this could result in the evolution of a true wing that allowed the hamsters to fly.When characteristics that fall at one end of the phenotypic spectrum are favored over the others, the selection is directional. Directional selection is a common force in an evolutionary trend, like we are proposing for the hamster wing adaptation.

Disruptive Selection

Disruptive selection favors traits at either end of the phenotypic spectrum
Disruptive Selection Graph

However, directional selection is not the only type of selection. A second type of selection is known as disruptive selection.

Although we think we’ve identified the type of natural selection that might have led to the evolution of a flying hamster, let’s just consider these alternatives.Consider the example of peppered moths living outside London. Oddly, the moths living near the urban areas were dark gray while the moths in rural areas were light gray. Scientists hypothesized that the dark moths were more fit in the urban areas and the light moths were more fit in the rural areas. But why?An examination of the habitat provided the answer.

In the rural areas, tree bark was covered with lichen that was a light-gray peppered color; therefore, darker colored moths would be easier for birds and other predators to identify and eat. However, coal-burning factories covered the trees near urban areas with black soot. Now, the lighter colored moths are at a disadvantage while the dark colored moths are camouflaged.If we consider the mortality of different shades of moths in the whole area, dark colored moths could survive well in the urban area and lightly colored moths fared well in the rural area, but moths of intermediate color were easy prey in both areas. Thus, disruptive selection favors individuals with traits on either end of the phenotypic spectrum.

Stabilizing Selection

Stabilizing selection favors traits found at the middle of the phenotypic spectrum
Stabilizing Selection Graph

A third type of selection is known as stabilizing selection. Whereas directional selection favored a trait on one end of the trait spectrum and disruptive selection favored either end of the spectrum, stabilizing selection favors the middle of the spectrum or those individuals with an average measurement of the phenotype.Human birth weight is an example of a trait controlled by stabilizing selection, particularly births before the advent of modern medicine. Babies with an average weight tend to survive better than babies that are particularly small or large.

The smallest babies are often small because of a premature birth. Not only could they suffer from maladies because of incomplete development, but they also have trouble maintaining body heat and are more susceptible to infection. In contrast, larger babies often meant a difficult birth. Particularly before the advent of modern medicine, difficult births often increased the mortality rate. As a result, stabilizing selection reduces phenotypic variation; in this case, favoring average-sized babies.

Heterozygote Advantage

Directional, disruptive and stabilizing selections are terms usually reserved for more complex quantitative traits; however, we can also describe the selection of simple traits.

For instance, let’s revisit the disease sickle cell anemia. We’ve learned that people who are homozygous for the sickle cell anemia allele develop the disease. Obviously, people afflicted with the disease are at a disadvantage because of the sickle-shaped red blood cells; however, evidence suggests that the sickle-shaped cells confer resistance to malaria.

The sickle cell alleles actually have an incompletely dominant relationship.So, let’s reassign + to the allele that produces normal red blood cells and to the allele that produces sickle-shaped red blood cells. That means that the heterozygote possesses both normal and sickle-shaped red blood cells. The sickle-shaped red blood cells help to disrupt the malarial pathogen life cycle.

Although -/- individuals are more resistant to malaria, their fitness is still adversely affected by the sickle cell disease. However, since the phenotype of this gene is incompletely dominant, a person heterozygous at this gene has enough normal red blood cells to avoid suffering from sickle cell anemia but just enough sickle-shaped cells to exhibit an increased resistance to malaria. Since + / + individuals are susceptible to malaria and -/- individuals are susceptible to sickle cell anemia, the heterozygotes have an advantage over both.

Heterozygotes have an advantage when it comes to developing malaria or sickle cell anemia
Heterozygote Advantage Table

When there is a heterozygote advantage, the population can be said to be carrying a balanced polymorphism. A polymorphism is simply a fancy name for alternate versions of a trait, such as a round red blood cell versus a sickle-shaped one. Therefore, a balanced polymorphism is simply a situation in which two alleles of a gene are maintained in a population because heterozygotes are more fit than their homozygous counterparts.

Lesson Summary

In summary, directional selection is a type of natural selection in which individuals with a trait at one end of a phenotypic spectrum are most fit.Disruptive selection is a type of natural selection in which individuals at either end of a phenotypic spectrum are most fit.Stabilizing selection is a type of selection in which individuals in the middle of a phenotypic spectrum are most fit.Heterozygote advantage is a case in which the heterozygous genotype has a higher relative fitness than either homozygous genotype.A balanced polymorphism is a situation in which two alleles of a gene are maintained in a population because heterozygotes are more fit than their homozygous counterparts.

Lesson Objectives

After watching this lesson, you should be able to:

  • Define the three types of natural selection
  • Identify a heterozygote advantage with genotypes
  • Explain what a balanced polymorphism is

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