Population Genetics and Hardy Weinberg Equilibrium

Population

A group of individuals of the same species that live in the same area and interbreed to produce fertile offspring

Gene pool

A population’s genetic makeup which consists of all copies of every type of allele

Fixed allele

If there is only one allele present for a particular locus in the population, it is fixed - many fixed alleles means less genetic diversity

Microevolution

small scale genetic changes in a population

Evolution is driven by random occurences

• Mutations

• Genetic Drift

• Migration/Gene Flow

• Natural Selection

Mutations

• Result in genetic variation

• Can form new alleles

• Natural selection can act on varied phenotypes

• Can be harmful, neutral, or beneficial

Mutation rates

• Slow in plants and animals

• Fast in prokaryotes due to a faster generation time

Genetic Drift

Chance events that cause a change in allele frequency from one generation to the next

Traits of genetic drift

• Most significant to small populations

• Can lead to a loss of genetic variation

• Can cause harmful alleles to become fixed

• Does NOT produce adaptations

• Bottleneck and Founder Effect

Bottleneck Effect

When a large population is drastically reduced by a non-selective disaster (floods, famine, fires, hurricanes, hunting, etc.), leading to some alleles to becoming overrepresented, underrepresented, or absent

Found effect

When a few individuals become isolated from a large population and establish a new small population with a gene pool that differs from the large population

• Loss of genetic diversity

Gene Flow

The transfer of alleles into or out of a population due to fertile individuals or gametes

• Alleles can be transferred between populations

Reproductive success is measured by relative fitness

The number of surviving offspring that an individual produces compared to the number left by others in the population

Effects of natural selection can be measured by

examining the changes in the mean of phenotypes

Three modes of natural selection

1. Direction

2. Stabilizing

3. Disruptive

Directional

Selection towards one extreme phenotype

Stabilizing

Selection towards the mean and against the extreme phenotypes

Disruptive

Selection against the mean. Both phenotypic extremes have the highest relative fitness

Sexual selection

A type of natural selection that explains why many species have unique/showy traits

• Males have useless structures because females choose that trait - sometimes harmful to survival

Hardy Weinberg Equilibrium

A model used to assess whether natural selection or other factors are causing evolution at a particular locus

• Determines what the genetic make up of the population would be if it were NOT evolving

• No differences - not evolving

• Differences - may be evolving

Five conditions for Hardy Weinberg Equilibrium

1. No mutations

2. Random mating

3. No natural selection

4. Extremely large population size

5. No gene flow

If conditions are not met

Microevolution occurs