16-1 Genes and Variation

16-1 Genes and Variation

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.

Variation and Gene PoolsA 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.

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.

Variation and Gene Pools

Gene Pool for Fur Color in MiceSample Population Frequency of Alleles

allele forbrown fur

allele forblack fur

Variation and Gene Pools

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

Sources of Genetic Variation

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

Sources of Genetic Variation

MutationsA 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.

Sources of Genetic Variation

Gene ShufflingMost 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.

Single-Gene and Polygenic Traits

Single-Gene and Polygenic TraitsThe 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.

Single-Gene and Polygenic Trait

• A bell-shaped curve is typical of polygenic traits.• A bell-shaped curve is also called normal distribution.

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

Natural Selection on Single-Gene Traits

Natural Selection onPolygenic Traits

Natural selection can affect the distributions of phenotypes in any of three ways:• directional selection• stabilizing selection• disruptive selection

Natural Selection onPolygenic Traits

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.

Natural Selection onPolygenic Traits

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.

Natural Selection onPolygenic Traits

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.

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.

Genetic Drift

Genetic Drift

Genetic Drift

Genetic Drift

Genetic DriftDescendants

Population A Population BWhen 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

Evolution Versus Genetic EquilibriumThe 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.

Five conditions are required to maintain genetic equilibrium (aka Non-evolution) 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.

Hardy Weinberg Conditions• Allelic / genotypic frequencies in a

population remain constant from generation to generation unless specific disturbing influences are introduced.

A Model Gene Pool• Imagine a gene pool containing alleles R,r.

• Frequency of R = .9• Frequency of r = .1

Rr RRRRRRRRRRRRRRRRR

r

Allele Frequencies

• The sum of the frequencies, by definition, must equal 1:

• This can be stated

p + q = 1• Or…

R + r = 10.9 + 0.1 = 1

Genotype Frequencies• When dealing with alleles, we are selecting two

at a time. The probability of which pair will be realized is equal to the product of their individual probabilities:

p + q = 1p2 + 2pq + q2 = 1

(.9)2 + 2(.9)(.1) + (.1)2 = 1RR + Rr + rR + rr = 1

Evolution Versus Genetic Equilibrium

Random MatingRandom 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.

Evolution Versus Genetic Equilibrium

Large PopulationGenetic 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.

Evolution Versus Genetic Equilibrium

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.

Evolution Versus Genetic Equilibrium

No MutationsIf genes mutate, new alleles may be introduced into the

population, and allele frequencies will change.

Evolution Versus Genetic Equilibrium

No Natural SelectionAll 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.