Lesson Overview

17.2 Evolution as Genetic Change in Populations

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How Natural Selection Works

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How does natural selection affect single-gene and polygenic traits?

Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.

Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.

Lesson Overview

Evolution as Genetic Change in Populations

How Natural Selection Works

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Evolutionary fitness is the success in passing genes to the next generation.

Evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.

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Lesson Overview

Evolution as Genetic Change in Populations

Natural Selection on Single-Gene Traits

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Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution.

For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

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Lesson Overview

Evolution as Genetic Change in Populations

Natural Selection on Single-Gene Traits: The example of Lizard Color

Lesson Overview

Evolution as Genetic Change in Populations

Natural Selection on Single-Gene Traits

If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common.

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Lesson Overview

Evolution as Genetic Change in Populations

Natural Selection on Single-Gene Traits

Single-Gene Traits: The allele for red coloring might not become common.

Lesson Overview

Evolution as Genetic Change in Populations

Natural Selection on Single-Gene Traits

Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

Lesson Overview

Evolution as Genetic Change in Populations

Natural Selection on Single-Gene Traits

Single-Gene Traits: The allele for black color might become more common.

Lesson Overview

Evolution as Genetic Change in Populations

Natural Selection

• Polygenic Traits can produce a broad range of phenotypes (ex: height)). In most cases, the graphs of variation make a bell shaped curve (normal distribution).

Short

Tall

# of Individuals

If fitness varies, natural selection can act in three ways…

• 1. It can stabilize the distribution.
• Select for the mean.
• 2. It can shift the distribution.
• Select for one extreme.
• 3. It can disrupt the distribution.
• Select for both extremes.

Stabilizing Selection

• This can occur when organisms in the middle of the curve are more fit than those organisms at the ends of the curve.
• Example: Mass of human infants at birth.
• Too big or too small = higher death rate
• AVERAGE= GOOD!
• Selects for the mean (average).

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Under 5lbs

Over 10lbs

# of individuals

Directional Selection

• Occurs when organisms at one end of the curve have higher fitness than organisms in the middle or the end. This means a change in the allele frequency (normal) will occur (evolution).
• Selects for one extreme.
• Ex: Birds with bigger beaks get more food.

Small Beak

Big Beak

# of individuals

Disruptive Selection

• Occurs when organisms at both ends of the curve are more fit than organisms in the middle. If the pressures are strong enough and last long enough then the curve will split into two different curves (species).
• Selects for both extremes!

Light color

Dark Color

# of Individuals

Genetic Drift

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What is genetic drift?

In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.

Lesson Overview

Evolution as Genetic Change in Populations

Genetic Bottlenecks

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The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population.

For example, a disaster may kill many individuals in a population, and the surviving population’s gene pool may contain different gene frequencies from the original gene pool.

Lesson Overview

Evolution as Genetic Change in Populations

The Founder Effect

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The founder effect occurs when allele frequencies change as a result of the migration of a small subgroup of a population.

Lesson Overview

Evolution as Genetic Change in Populations

The Founder Effect

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Two groups from a large, diverse population could produce new populations that differ from the original group.

Lesson Overview

Evolution as Genetic Change in Populations

Evolution Versus Genetic Equilibrium

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What conditions are required to maintain genetic equilibrium?

According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium: (1) The population must be very large; (2) there can be no mutations; (3) there must be random mating; (4) there can be no movement into or out of the population, and �(5) no natural selection.

Lesson Overview

Evolution as Genetic Change in Populations

Evolution Versus Genetic Equilibrium

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A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve.

Lesson Overview

Evolution as Genetic Change in Populations