Hardy-Weinberg Flashcards
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?
never
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
Kuru
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.
fitness
The contribution of a genotype or phenotype to the genetic composition of subsequent generations, relative to the contribution of other genotypes or phenotypes.
RXFP2
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.
WILL NOT CHANGE ALLELE FREQUENCIES
inbreeding
) 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
mutation
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.
Ara-3
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.
Horns of Texas Longhorn cattle
have evolved through directional selection; predation was the selection pressure
Longer horns were advantageous for defending young calves from attacks by predators, so feral herds of Spanish cattle developed much longer horns between the early 1500s and the 1860s
Baleen whales
directional selection
) underwent a clade-wide shift in their mode of body size evolution during the Plio-Pleistocene.
temporally linked to the onset of seasonally intensified upwelling along coastal ecosystems.
stabilizing selection is often called
purifying selection because there is selection against any deleterious mutations.
black-bellied seed crackers
its bill size is influenced by disruptive selection
Birds with large bills can crack the hard seeds of a sedge. Birds with small bills feed more efficiently on soft seeds of a different sedge species.
Birds with intermediate sized bills cannot use either kind of seed efficiently and survive poorly.
soapberry bugs
use their “beak” to feed on seeds within fruits
feed most effectively when their beak length is similar to the depth of the seeds within the fruit
In southern Florida, soapberry bugs feed on the larger fruit of balloon vines; they have longer beaks
In central Florida, they feed on the smaller fruit of introduced goldenrain trees; they have shorter beaks
frequency dependent selection
A polymorphism can be maintained when fitness depends on its frequency in the population.
Example: A scale-eating fish in Lake Tanganyika. “Left-mouthed” and “right-mouthed” individuals are both favored; the host fish can be attacked from either side.
