HARDY WEINBERG EQUILIBRIUM p 2 2 pq q

HARDY WEINBERG EQUILIBRIUM p 2 + 2 pq + q 2 = 1

Two scientists independently derived the basic principle of population genetics called the Hardy. Weinberg Principle. This principle states that • If all factors remain constant, the gene pool in a population will have exactly the same composition generation after generation. This condition is called genetic equilibrium. • If the genetic equilibrium of a population is upset, the population is said to be evolving.

Evolution • “The sum total of the genetically inherited changes in the individuals who are the members of a population’s gene pool” • Evolution is simply a change in frequencies of alleles in the gene pool of a population.

Population • A group of the same species living in the same place at the same time

Gene Pool • All of the genes/alleles that occur in a population. • Ex) Human gene pool for blood type are I A, IB, and i.

Allele Frequency • % or proportion of the allele in the population.

Conditions • Evolution will NOT occur and Hardy-Weinberg equilibrium will be met if the following conditions are met:

Conditions • 1. No MUTATION

Conditions • 2. The population is infinitely large • Laws of probability must apply

Conditions • 3. All members of the population breed

Conditions • 4. All mating is totally random

Conditions • 5. Everyone produces the same number of offspring

Conditions • 6. There is no migration in or out of the population

Equation • Equation used to find genotype frequencies: • p 2 + 2 pq + q 2 = 1 • AND • p + q = 1

• p is the frequency of the dominant allele • q is the frequency of the recessive allele • p 2 is the frequency of the homozygous dominant genotypes • q 2 is the frequency of the homozygous recessive genotypes

Equation • 2 pq is the frequency of the heterozygotes

Example • Albinism is only expressed in the phenotype of homozygous recessive individuals (aa) • The average human frequency of albinism in North America is only about 1 in 20, 000.

Question • Calculate the frequencies of the alleles and all three genotypes in this population.

Example • In a population that is not evolving, 21% of the individuals are homozygous dominant, 49% are heterozygous. What percentage of the next generation are predicted to be homozygous recessive?

Example • 2. 16% of a population is observed to have a continuous hairline (recessive). What percentage of the population possesses the dominant allele? If there are 500 members in the population, how many would be heterozygous?

Example • 3. A recessive genetic disorder occurs in 9% of the population. What percentage of the population will be carriers for the disorder? What percentage will be homozygous dominant?

Disturbances to Equilibrium • There are some situations that may make H-W equilibrium of alleles more likely to change.

Mutations • Whether a mutation is good or bad, often depends on the environment. A harmful mutation can turn out to have a selective advantage if the environment changes over time.

Non-Random Mating • Individuals are often attracted to one another because they value specific traits. Ex: Humans, elk, peacocks

Inbreeding (breeding of closely related individuals) • Will reduce genetic diversity, thus decrease frequency of some alleles

Genetic Drift • A reduction in the gene pool variation caused purely by chance. Usually in small populations. If a specific allele doesn’t reproduce (by chance) it may be lost entirely.

Genetic Drift Example

Gene Flow • Migration – is the movement of genes into (immigration) / out of (emigration) the population. Some genes may migrate more readily than others.

Bottleneck Effect • Occurs when a part of the population is eliminated by chance

Founder Effect • Occurs when the founders of a new population have a specific genotype. Ex. Polydactyl hands in Amish in Pennsylvania

Natural Selection • Selective Advantage: the most important reason for changes to H-W equilibrium • New mutations may arise that give the organism an advantage over others of the same species

• These alleles become more common with time • Means that some alleles are helping individuals to survive and reproduce

Stabilizing Selection • Atypical phenotypes are eliminated an average is favored. Ex. Birth weight (humans) or coloring of butterflies.

Directional Selection • An atypical phenotype is selected for because of a progression of change in the environment. Ex. Horse evolution, peppered moth

Disruptive Selection • Two ore more phenotypes are selected due to different characteristics within a habitat. Ex. Fish that feed on bottom vs fish that feed on top

Speciation • Divergence producing new species, two types: • Allopatric Speciation: physical separation of species drives the splitting of one species into two (or more) • Ex Grand Canyon Squirrels, Darwin’s Finches

• May not be immediately obvious. • Eg. Anole lizards in Cuba – not physically separated now, but were 5 million years ago. • https: //www. youtube. com/watch? v=c. Sgulsyds. QU

Sympatry • Division of one species into two or more in absence of physical barriers • Disputed by some scientists • Orcas?

Summary – Hardy-Weinberg Equilibrium • Does not change unless a force is acting upon it • This force is often natural selection – leads to evolution