17.2 How Natural Selection Works

Example of natural selection on single-gene traits

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.

The _____ of individuals may vary from one end of the curve to the other.

Fitness

Large population size helps maintain ____ _____.

Genetic equilibrium

How do you know if a population is in genetic equilibrium?

If allele frequencies in the population remain the same.

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.

What does disruptive selection act against and what can it create?

It acts against individuals of an intermediate type and can create two distinct phenotypes.

Genetic drift has less effect on ____ populations.

Large

If mutations occur, ___ ____ may be introduced into the gene pool, and ___ _____ will change.

New alleles, allele frequencies

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

Populations, differ

Natural selection can affect the range of phenotypes and hence the ___ of the ___ ___.

Shape of the bell curve

The Hardy-Weinberg principle states what?

That allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.

The Hardy-Weinberg principle describe what?

The conditions under which evolution does not occur.

What happens to the range of phenotypes in directional selection?

The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others.

What is evolutionary fitness?

The success in passing genes to the next generation

What happens to the graph during stabilizing selection?

This situations keeps the center of the curve at its current position, but it narrows the overall graph.

When does the founder effect occur?

When allele frequencies change as a result of the migration of a small subgroup of a population.

Genetic drift occurs in small populations when what happens?

When an allele becomes more or less common simply by chance.

When does directional selection occur?

When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.

When does disruptive selection occur?

When individuals at the upper and lower end of the curve have higher fitness than individuals near the middle.

When does stabilizing selection occur?

When individuals near the center of the curve have higher fitness than individuals at either end.

How does natural selection affect single-gene and polygenic traits? (2)

1. Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies. 2. Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.

What conditions are required to maintain genetic equilibrium? (5)

1. The population must be large 2. No mutations 3. Random mating 4. No movement into or out of the population 5. No natural selection

What is the bottleneck effect?

A change in allele frequency following a dramatic reduction in the size of a population.

Example of genetic bottlenecks

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.

What principle talks about the conditions required to maintain genetic equilibrium?

According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium

Genetic drift can cause changes in ___ ____ in small populations.

Allele frequencies

What is evolutionary adaptation?

Any genetically controlled trait that increases an individual's ability to pass along its alleles.

Polygenic traits have a range of phenotypes that often form a __ _____.

Bell curve.

What can natural selection for a single-gene trait lead to?

Changes in allele frequencies and then to evolution

If allele frequencies don't change, the population will not ____.

Evolve

Genetic drift is a random change in ______ ______.

allele frequency