Patterns of Natural Selection

1. Patterns of Natural Selection Learning Objectives Students will be able to: -identify graphs of directional, stabilizing, and disruptive selection -describe the outcomes of directional, stabilizing, and disruptive selection Today, we will learn about: -natural selection in single-gene traits -natural selection in polygenic traits, including -directional selection -stabilizing selection -disruptive selection -genetic equilibrium

2. Natural Selection on Single Gene Traits Review • A single gene trait is a trait that is controlled by only one gene. Recall that organisms with two of each chromosome will have two copies of each gene, but those copies may be different alleles • Evolution can be defined as changes in allele frequency in a population In a species of lizard, skin color is a single gene trait. The diagram below shows changes in lizard skin color over time due to natural selection. Natural selection on single gene traits can lead to changes in allele frequency and thus to evolution

3. Natural Selection on Polygenic Traits • Polygenic traits are traits that are controlled by more than one gene. In many polygenic traits, there is a range of phenotypes instead of just a few. One example of a polygenic trait is skin color in humans • Natural selection can affect the distribution of polygenic phenotypes in three ways: directional selection, stabilizing selection, and disruptive selection • Think of a distribution as the way phenotypes are spread out over a population

4. Directional Selection • Directional selection occurs on polygenic traits. • In directional selection, one extreme trait is favoredover the others, meaning the trait one extreme causes the organism to be more fit and therefore have more offspring that survive. • An example of this is running speed in rabbits. The faster rabbits can outrun predators easier, so they are less likely to get eaten, and more likely to survive and produce offspring. Directional selection favors the trait of fast running.

5. Stabilizing Selection • In stabilizing selection, the traits that are the most average are selected for, and the extremes are selected against. • One example of a trait that has experienced stabilizing selection is birth weight. Babies that are very small are often not healthy enough to survive, while babies that are too large may get stuck in the birth canal, causing death of the baby and frequently death of the mother as well. • The result of stabilizing selection is that most individuals will have a phenotype that is within a narrow range.

6. Disruptive Selection • In disruptive selection, the extreme traits are selected for, and average traits are selected against. • One example of this is beak sizes in birds. If the only seeds available in an environment are small seeds and large seeds, natural selection will favor birds with either small or large beaks. The birds with medium sized beaks will not be very effective at feeding, so medium beaks will be selected against.

7. Genetic Equilibrium • When the allele frequencies in a population are not changing, we say that the population has reached genetic equilibrium. This can happen in populations of organisms that are very well adapted to their environment AND the environment is not changing