Evolution in Genetic Terms

Genes and Variation

Alleles = different versions of a gene (a gene codes for a trait).

Genetic variation is studied in populations.Population = an interbreeding group of individuals

of the same species that occupy the same geographical area.

Members of a population share a common group of genes called a gene pool.

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

Evolution in Genetic Terms

In genetic terms, evolution is any change over time in the relative frequency of alleles 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 of Alleles

The sources of genetic variation are mutation and sexual reproduction.

Only mutation can add new genes and new phenotypes to a gene pool.

Sexual reproduction can increase the number of phenotypes by “shuffling the genetic deck” producing many different combinations of genes in an offspring. This is especially true with polygenic traits (traits controlled by two or more genes).

Single-Gene TraitsA single-gene trait is a trait controlled by only one gene. Many single-gene traits create only 2 or 3

phenotypes. Examples: Widow’s peak in humans. Seed shape in pea plants. Genotypes for Coat Color Flower color in plants.

100

75Genotype Frequencyas % of Pop. 50 25 If: A = Black coat color and a = Brown coat color 0 Draw a phenotype graph for the Genotypes genotype information on the left

Polygenic Traits

Polygenic traits, often being controlled by many genes, can create many different phenotypes.

Examples: Human height.

Human skin, hair, and eye color.

Polygenic Traits

Polygenic Traits

Polygenic Traits ContinuedThe range of phenotypes of a polygenic trait

typically create what is known as a normal distribution on a graph (also known as a bell curve).

Frequency

Mean (avg.)

Value of Variable ----------

Natural Selection on Polygenic Traits

Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce one of three types of selection:

1. Directional Selection

2. Stabilizing Selection

3. Disruptive Selection

Directional Selection

Directional selection is a type of selection in which the mean value for a trait shifts in a particular direction. Example: Giraffe neck length.

Stabilizing Selection

When individuals near the center of the curve have higher fitness than individuals at either end. Example: Human baby birth weight.

Disruptive Selection When individuals at the outer ends of the curve have higher

fitness than individuals near the middle of the curve.Example: Diverging beak lengths in populations of finches

due to different types of available food resources.

Disruptive selection can lead to speciation (where one ancestral species splits into two separate species).

Genetic Drift

Genetic drift is random change in allele frequencies as a result of chance alone. There are no selective pressures at work in genetic drift (no natural selection).

In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave by chance alone.

This is due to the chance variation in survival and/or reproductive success that can create changes in allele frequencies in small populations.

Genetic Drift Cont.

*** Important***Reproductive success (or lack thereof) in these flowers is not due

to varying fitness levels. What is it due to?

Genetic BottlenecksA genetic bottleneck is a drastic decrease in the

size of a population with a resulting decrease in genetic variability within a population.

Disaster, disease, and human hunting are known causes of bottlenecks.

The full range of genetic variation of the original population is never fully represented in the few surviving individuals.

Bottlenecks can increase genetic drift. Why?

Genetic Bottlenecks

Scenario A Scenario B Scenario C

Does selective breeding of plants and animals create a sort of genetic bottleneck?

Why or why not?

Which scenario(s) on the left show a change in the allele frequencies of the new surviving population?

Which scenario shows a total loss of genetic variability for the trait?

Bottleneck ExamplesNorthern Elephant Seals were hunted down to

around 30 individuals in the 1890’s. They now number in the hundreds of thousands.

Cheetahs show little genetic variation due to a genetic bottleneck in their past. Cheetahs are so similar to each other genetically that skin grafts between unrelated cheetahs are never rejected.

It is believed that around 70,000 years ago the Toba supervolcano erupted in Indonesia bringing the global human population down to around 15,000 individuals. The 6.6 billion people alive today are believed to have descended from these 15,000 individuals.

Founder Effect

Genetic drift may also occur when a few individuals colonize a new habitat. These founding individuals may carry alleles that differ in relative frequencies from those of the main population, just by chance.

The allele frequencies of this offshoot population will not be representative of the population they came from.

This type of genetic drift is known as founder effect.

Founder Effect Cont.

Original Population Founder Population

Example of Founder Effect

The Amish of Pennsylvania have a high incidence of Ellis-van Creveld syndrome. Polydactyly is a common symptom. This is an example of founder effect.

Evolution vs. Genetic Equilibrium

If a population is not evolving, allele frequencies in its gene pool do not change, which means that the population is in genetic equilibrium.

The Hardy-Weinberg PrincipleThe Hardy–Weinberg principle states that allele frequencies

in a population remain constant—that is, they are in equilibrium—from generation to generation unless specific disturbing influences are introduced into the population.

The Hardy-Weinberg principle predicts that five conditions can disturb genetic equilibrium and cause evolution to occur:

The 5 conditions:

• Nonrandom Mating.• Small Population Size.• Immigration and Emigration.• Mutations.• Natural Selection.

SpeciationSpeciation is the formation of a new species.A species is a population or group of populations

whose members can interbreed and produce fertile offspring.

For a new species to develop, the existing gene pool must be split into at least two groups. When this happens, reproductive isolation has occurred.

Reproductive isolation is the key for a new species to develop.

Reproductive Isolation

When reproductive isolation occurs, isolated gene pools now have a chance to change over time.

As the allele frequencies of these new populations evolve independently of each other, eventually the two groups can differ so much genetically that they can no longer mate successfully. Speciation has then occurred.

Reproductive Isolation

There are three ways reproductive isolation can occur.

• Geographic Isolation: This is when two populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

• Behavioral Isolation: This is when two populations that are capable of interbreeding develop differences in courtship or other behaviors.

• Temporal Isolation: This happens when two or more species reproduce at different times of the year.