Hardy Weinberg Equilibrium
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Hardy Weinberg Equilibriump2 + 2pq + q2 = 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.
PopulationA 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 IA, IB, and i.
Allele frequency % or proportion of that allele in the population
ConditionsEvolution will NOT occur and Hardy-Weinberg equilibrium will be met if the following conditions are met:
Conditions2.The population is infinitely large- laws of probability must apply
Conditions3. All members of the population breed
Conditions4. All mating is totally random
Conditions5. Everyone produces the same number of offspring
Conditions6. There is no migration in or out of the population
EquationEquation used to find genotype frequencies:p+2pq+q=1
p + q = 1
p is the frequency of the dominant allele
q is the frequency of the recessive allele
p2 is the frequency of the homozygous dominant genotypes
q2 is the frequency of the homozygous recessive genotypes
2pq is the frequency of the heterozygotes
ExampleAlbinism 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.
QuestionCalculate the frequencies of the alleles and all three genotypes in this population.
SolutionSynthetic Theory of Evolution: Sample Hardy-Weinberg Problem
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 Ideashttp://w3.dwm.ks.edu.tw/bio/activelearner/18/ch18summary.html
Disturbances to EquilibriumThere are some situations that may make H-W equilibrium of alleles more likely to change:
1) 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.
2) Non-random Mating Individuals are often attracted to one another because they value specific traits. Ex. In humans, wolves, elk
-will reduce genetic diversity, thus decrease frequency of some alleles3) Inbreeding
4) Genetic Drift - a reduction in the gene pool variation caused purely by chance. Usually in small populations. If a specific allele doesnt reproduce (by chance) it may be lost entirely.
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.
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: 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 timeMeans 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.
SpeciationDivergence producing new species, two types:Allopatric speciation: physical separation of species drives the splitting of one species into two (or more)Eg. Grand Canyon SquirrelsDarwins Finches
May not be immediately obviousEg. Anole lizards in Cuba not physically separated now, but were 5 million years agoAllopatry Animation
2. SympatryDivision of one species into two or more in absence of physical barriersDisputed by some
H-W Equilibrium - SummaryDoes not change unless a force is acting upon itThis force is often natural selection leads to evolution