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Hardy Weinberg equilibrium

a model in which allele frequencies do not change across generations, and genotype frequencies can be predicted from allele frequencies.


allele frequencies

measure the amount of genetic variation


calculations of allele and genotype frequencies in a population

allow biologists to measure evolutionary change


gene pool

sum of all copies of all alleles at all loci in a population.

the sum of the genetic variation in the population


allele frequency

Proportion of an allele in the gene pool.

p=number of copies of the allele in the population/total number of copies of all alleles in the population


genotype frequency

Proportion of each genotype in the population.


Do populations in nature ever meet the conditions for HW equilibrium?


But it is useful for predicting genotype frequencies from allele frequencies

patterns of deviation from the model help identify mechanisms of evolutionary change.


If certain conditions are met

the genetic structure of a population does not change over time.


Conditions that must be met for Hardy–Weinberg equilibrium:

No mutation
No selection among genotypes
No gene flow
Population size is infinite
random mating



heterozygote advantage

Kuru is an acquired prion disease largely restricted to the Fore linguistic group of the Papua New Guinea Highlands

Heterozygosity for a common polymorphism in the human prion protein gene (PRNP) confers relative resistance to prion diseases.

Kuru imposed strong balancing selection on the Fore, essentially eliminating PRNP 129 homozygotes


Malaria heterozygote advantage

People who carry one copy of the S allele are more likely to survive malaria than AA homozygotes.

The higher fitness of AS heterozygotes favors a balance between the 2 alleles; balancing selection.



The contribution of a genotype or phenotype to the genetic composition of subsequent generations, relative to the contribution of other genotypes or phenotypes.



In wild Soay sheep, large horns confer an advantage in strong intra-sexual competition, yet males show an inherited polymorphism for horn type.

Genetic variation in this trait is maintained by a trade-off between natural and sexual selection at a single gene, relaxin-like receptor 2 (RXFP2)

An allele conferring larger horns, Ho+, is associated with higher reproductive success

a smaller horn allele, HoP, confers increased survival, resulting in a net effect of overdominance (that is, heterozygote advantage)


nonrandom mating

The phenomenon in which individuals select mates based on their phenotypes or genetic lineage.




) increases the frequency of homozygotes and reduces the frequency of heterozygotes in each generation.

does not cause evolution, because allele frequencies do not change.

change genotype frequencies


inbreeding depression

a decline in average fitness that takes place when homozygosity increases and heterozygosity decreases in a population


inbreeding depression results from two processes

Many recessive alleles represent loss-of-function mutations

increases the frequency of homozygous recessive individuals and thus the frequency of individuals expressing the mutation.

Many genes—especially those involved in fighting disease—are under intense selection for heterozygote advantage. If an individual is homozygous at these genes, then fitness declines.


Random genetic drift

a change in genetic variation from generation to generation due to random sampling error.

. Allele frequencies may change as a matter of chance.

This is more likely to occur in a small population



origin of genetic variation.
any change in the nucleotide sequences of DNA.
random with respect to the needs of an organism;
selection acting on the random variation results in adaptation.


characteristics of mutation

Most mutations are harmful or neutral.

A few are beneficial; or if conditions change, a mutation could become advantageous.

Mutations can also restore genetic variation that other processes have removed.

can be very high, as in viruses; or quite low as in some eukaryotes.



After 31,000 generations, one strain of bacteria, Ara-3 had evolved a new way to nourish themselves

, they drew on a different energy source in their medium, citrate.

Ara-3 eventually evolved the ability to use it with oxygen present (Cit+ cells


gene flow

a result of the migration of individuals and movement of gametes between populations.

New individuals can add alleles to the gene pool or change allele frequencies

reduces genetic differences among populations


genetic drift

results from random changes in allele frequencies.

Harmful alleles may increase in frequency, and rare advantageous alleles may be lost.

also possible that advantageous alleles may be introduced.
In small populations, genetic drift can be significant.


founder effect

colonizing population is unlikely to have all the alleles present in the whole population.


Brown anole lizard

populations were established on seven small islands in the Bahamas from the same large-island source

generated significant among-island genetic and morphological differences that persisted

All populations adapted in the predicted direction (i.e., shorter hindlimbs) in response to the narrower vegetation on the small islands

Both founder effects and natural selection jointly determine trait values in these populations


bottleneck effect

Environmental conditions result in survival of only a few individuals.

Species with low genetic variation.

greater prarie chickens and california fan palms


stabilizing selection

preserves the average phenotype.

reduces variation but does not change the mean.

Natural selection is often stabilizing (rates of evolution are slow).
ex: human birth weights


Directional selection

favors individuals that vary in one direction

When individuals at one extreme are more successful.

results in an increase of the frequencies of alleles that produce the favored phenotype

positive selection


disruptive selection

favors individuals that vary in both directions from the mean.

variation in a population is increased


If directional selection operates over many generations

an evolutionary trend occurs.