Hardy Weinberg Equilibrium p 2 + 2pq + 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 (changed), the population is said to be evolving.  They showed that genetic recombinaton, which occurs in each generation, does NOT by itself change the overall composition of the gene pool.

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. allelesgene poolpopulation

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, I B, and i.

Allele frequency – % or proportion of that 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 MutationMutation

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 random

Conditions 5. Everyone produces the same number of offspring

Conditions 6. There is no net movement/migration in or out of the population 7. All alleles are equally viable

Equation Equation used to find genotype frequencies: p² + 2pq + q² = 1 And p + q = 1 p²=AA 2pq=Aa q²=aa

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 2pq 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.

Solution Synthetic Theory of Evolution: Sample Hardy- Weinberg Problem Synthetic Theory of Evolution: Sample Hardy- Weinberg Problem

Examples: 1. In a population, 21% of the individuals are homozygous dominant, 49% are heterozygous and 30% are homozygous recessive. What percentage of the next generation are predicted to be homozygous recessive?

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?

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?

Quiz – Theoretical Ideas ch18summary.html ch18summary.html

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

1) Mutations: primary cause of variation 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.

2) Non-random Mating Individuals are often attracted to one another because they value specific traits. Ex. In humans, wolves, elk. OR this is due to selective-breeding for certain traits.

-will reduce genetic diversity, thus decrease frequency of some alleles. This is non- random mating as only a select few genetic combinations get to expresses themselves 3) Inbreeding

4) 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. This usually can occur if small populations exist

Genetic Drift Example

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

6 ) Bottleneck Effect occurs when a part of the population is eliminated by chance. Ex. Genocide, over- hunting, natural disaster Usually see this in species that are endangered or almost extinct.

7) Founder Effect - occurs when the founders of a new population have a specific genotype. Ex. polydactyl hands in Amish in Pennsylvania.

8) Natural Selection Selective Advantage: this is the most important reason for changes to H-W equilibrium. New mutations or adaptations 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

I. Stabilizing Selection: atypical phenotypes are eliminated, and an average is favored. Ex. birth weight or color.

II. Directional Selection – an atypical phenotype is selected for because of a progression of change in the environment. Ex. horse evolution, peppered moth.

III. Disruptive Selection two or 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: 1. Allopatric speciation: physical separation of species drives the splitting of one species into two (or more) Eg. 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 Allopatry Animation

2. Sympatry Division of one species into two or more in absence of physical barriers Disputed by some

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