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Copyright Pearson Prentice Hall

16-1 Genes and Variation16-1 Genes and Variation

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16-1 Genes and Variation

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How Common Is Genetic Variation?

How Common Is Genetic Variation?

Many genes have at least two forms, or alleles.

All organisms have genetic variation that is “invisible” because it involves small differences in biochemical processes.

An individual organism is heterozygous for many genes.

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Variation and Gene Pools


A population is a group of individuals of the same species that interbreed.

A gene pool consists of all genes, including all the different alleles, that are present in a population.

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The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

Relative frequency is often expressed as a percentage, and it is not related to whether an allele is dominant or recessive.

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Gene Pool for Fur Color in MiceSample Population Frequency of Alleles

allele forbrown fur

allele forblack fur

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In genetic terms, evolution is any change in the relative frequency of alleles in a population.

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Sources of Genetic Variation


The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

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Mutations

A mutation is any change in a sequence of DNA.

Mutations occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment.

Mutations do not always affect an organism’s phenotype.

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Gene Shuffling

Most heritable differences are due to gene shuffling.

Crossing-over increases the number of genotypes that can appear in offspring.

Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.

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Single-Gene and Polygenic Traits


The number of phenotypes produced for a given trait depends on how many genes control the trait.

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A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.

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Single-Gene and Polygenic Trait

Many traits are controlled by two or more genes and are called polygenic traits.

One polygenic trait can have many possible genotypes and phenotypes.

Height in humans is a polygenic trait.

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Single-Gene and Polygenic Trait

• A bell-shaped curve is typical of polygenic traits.

• A bell-shaped curve is also called normal distribution.

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16-2 Evolution as Genetic Change

Natural selection affects which individuals survive and reproduce and which do not.

Evolution is any change over time in the relative frequencies of alleles in a population.

Populations, not individual organisms, can evolve over time.

16-2 Evolution as Genetic Change

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Natural Selection on Single-Gene Traits

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

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Natural Selection on Single-Gene Traits

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Natural Selection on Polygenic Traits

Natural Selection on Polygenic Traits

How does natural selection affect polygenic traits?

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Natural Selection onPolygenic Traits

Natural selection can affect the distributions of phenotypes in any of three ways:

• directional selection

• stabilizing selection

• disruptive selection

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Directional Selection

When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.

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Stabilizing Selection

When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.

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Disruptive Selection

When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.

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Genetic Drift

Genetic Drift - A random change in allele frequency

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Genetic Drift

Genetic drift may occur when a small group of individuals colonizes a new habitat.

Individuals may carry alleles in different relative frequencies than did the larger population from which they came.

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Genetic Drift

Genetic Drift

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Genetic Drift

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Genetic Drift

Descendants

Population A Population B

When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect.

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Evolution Versus Genetic Equilibrium


The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

When allele frequencies remain constant it is called genetic equilibrium.

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Five conditions are required to maintain genetic equilibrium from generation to generation:

• there must be random mating,

• the population must be very large,

• there can be no movement into or out of the population,

• there can be no mutations, and

• there can be no natural selection.

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Random Mating

Random mating ensures that each individual has an equal chance of passing on its alleles to offspring.

In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits.

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Large Population

Genetic drift has less effect on large populations than on small ones.

Allele frequencies of large populations are less likely to be changed through the process of genetic drift.

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No Movement Into or Out of the Population

Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population.

The population's gene pool must be kept together and kept separate from the gene pools of other populations.

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No Mutations

If genes mutate, new alleles may be introduced into the population, and allele frequencies will change.

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No Natural Selection

All genotypes in the population must have equal probabilities of survival and reproduction.

No phenotype can have a selective advantage over another.

There can be no natural selection operating on the population.

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16-1

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16-1

Which of the following statements is TRUE?

a. The relative frequency of an allele is not related to whether the allele is dominant or recessive.

b. Mutations always affect an organism's phenotype.

c. Crossing over decreases the number of different genotypes that appear in an offspring.

d. Evolution does not affect the frequency of genes in a gene pool.

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16-1

Most inheritable differences are a result of

a. gene shuffling.

b. frequency of alleles.

c. mutations.

d. DNA replication.

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16-1

The main sources of inherited variation are

a. gene shuffling and mutations.

b. gene pools and frequencies.

c. single-gene and polygenic traits.

d. genotypes and phenotypes.

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16-1

A widow's peak in humans is an example of a(an)

a. invariable trait.

b. single-gene trait.

c. polygenic trait.

d. mutation.

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16-1

A graph of the length of the little finger on the left hand versus the number of people having fingers of a particular length is a bell-shaped curve. This indicates that finger length is a

a. single-gene trait.

b. polygenic trait.

c. randomly inherited trait.

d. strongly selected trait.

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