Chapter 25 - Population Genetics Flashcards
What is genotypic frequency and how do you determine it?
Must know equation
The proportion of a particular genotype within a population
Add number of individuals with a specific genotype and divide it by the total number of individuals in a population (N)
f(AA) = # of AA individuals/N
f(Aa) = # of Aa individuals/N
f(aa) = # of aa individuals/N
f(AA) + f(Aa) + f(aa) = 1
What is allelic frequency and how is it determined?
Must know these equations
The proportion of a particular allele within a population
- Always fewer alleles than there are genotypes
- Number and types of alleles have more continuity from one generation to the next
- Alleles not necessarily changing but combinations of alleles to express a phenotype changes
Calculate in 2 ways:
Number of genotypes in population
- p = f(A) = 2nAA + nAa/2N
- q = f(a) = 2naa + nAa/2N
- p + q = 1
Frequencies of genotypes in a population
- p = f(A) = f(AA) + 1/2 f(Aa)
- q = f(a) = f(aa) + 1/2 f(Aa)
- p + q = 1
What is the Hardy-Weinberg Law?
A way to segregate the alleles in gamete formation and combine alleles in fertilization to determine how it influences the gene pool, in a theoretical manner
Assumption: the population is large, randomly mating, not affected by mutation, migration, or natural selection
Prediction 1: the allelic frequencies of the population do not change
- Allele frequencies determine the frequencies of the genotype
Prediction 2: the genotypic frequencies will stabilize after one generation
What is Hardy-Weinberg equilibrium?
When assumptions are met, reproduction alone does not alter allelic or genotypic frequencies, and the allelic frequencies determine the frequencies of genotypes
p^2 + 2pq + q^2 = 1
What are the assumptions of the HW law?
HW assumes that the population is infinitely large at the theoretical level
- In practice, many large populations have genotypes that are predicted within HW
- Deviations from HW tend to occur in small populations
Members of the population randomly mate with respect to genotype; therefore, each genotype mates relative to its frequency
- If the frequency of AA is 0.6, AA mating to AA will occur at a frequency of 0.6
- 0.6 x 0.6 = 0.36
- There is a 36% chance that an AA will mate with an AA
Allelic frequencies of the population are not affected by natural selection, migration, or mutation (microevolution forces)
- Rate of mutation is so low that it tends not to affect HW
- While natural selection and migration are regular in populations, the HW law only examines the effect of reproduction in the gene pool
- When that effect is known, other factors like selection and migration are examined
- HW can be used as a baseline/control to see why populations are out of HW equilibrium
HW law applies only to a single locus
- Nearly all populations don’t mate randomly for all loci and natural selection affects nearly all populations at some loci
What is microevolution?
Changes in an allele frequency in a population over time
What are the implications of HW law (6)?
Within HW equilibrium, if allele frequencies are stable, a population cannot evolve
When a population is in HW equilibrium, the genotypic frequencies are determined by the allelic frequencies
The frequency of the heterozygote is at a maximum when allelic frequencies are equal (at 0.5)
When the frequency of one allele is low, the homozygotes for that allele will be rare
- Most of the copies of that allele will be present in the heterozygote
When the frequency of one allele is high, most of the individuals are homozygotes
When a population is not in HW equilibrium, one of the assumptions has not been met
What are the proportions of genotypic frequencies of X-linked female genotypes?
p^2
2pq
q^2
What are the proportions of genotypic frequencies in X-linked male genotypes?
p and q
What are the genotypic frequencies for genes with 3 alleles?
p^2
2pq
q^2
2pr
2qr
r^2
How can you estimate allelic frequencies with the HW law?
Take square root of a given frequency for homozygote (p^2 or q^2)
Subtract that from 1 to determine the other frequency
Calculate other frequencies as above
What is the effect of nonrandom mating?
Affects the way in which alleles combine to form genotypes and alters the genotypic frequencies of a population
Does not affect frequency of alleles
What are the 2 types of nonrandom mating?
Positive assortative mating – tendency for like individuals to mate
Negative assortative mating – tendency for unlike individuals to mate
What is inbreeding and what is its effect out of HW equilibrium?
Preferential mating between two related individuals
Positive assortative mating for relatedness
Affects all of the genes in an individual
Effect: tends to lead to an increase in homozygotes in a population and a decrease in heterozygotes
What is outcrossing?
Preferential mating between two unrelated individuals
What is the inbreeding coefficient?
Measures the probability that two alleles are identical by descent
F = 0 - mating is occurring randomly
F = 1 - all alleles are identical by descent
Decreases proportion of heterozygotes by 2Fqp in each subsequent generation
- f(AA) = p^2 + Fqp
- f(Aa) = 2pq - 2Fqp
- f(aa) = q^2 + Fqp
What are the implications of inbreeding?
Inbreeding depression - harmful to outcrossing species because it increases homozygotes and frequency of lethal or deleterious alleles in a population
Favorable - if inbreeding for many generations, all individuals will eventually be homozygous and all deleterious alleles will be weeded out, leaving the population homozygous for only beneficial alleles
Co-adapted gene complexes - at molecular level, can help preserve groups of genes that can exhibit different gene interactions that work well together in different environments
What are the 4 processes that change allelic frequencies (microevolutionary forces)?
Mutation, migration, genetic drift, and natural selection
How does mutation affect allelic frequencies?
Affects the rate at which one genetic variant increases at the expense of another
Amount of change dependent on mutation rate and amount of first allele in the population
- Amount of first allele is large = large amount of change
- As allele frequency of first allele decreases as a result of mutation, the change in frequency due to mutation will also decrease
Reverse mutations take 2nd allele back to first allele – occurs at a different rate than forward mutations
- Allele frequency of 2nd allele decreases and frequency of 1st allele increases
The effect of typical mutation rates on HW equilibrium is negligible and requires many generations
- If mutation is the only force acting on a population for long periods of time, mutation rates will eventually determine allele frequency (normally isn’t the only force)
What is migration?
AKA: Gene flow
Movement of genes from one population to another
Changes allele frequencies of a population
How does migration affect genetic variation?
Decreases genetic differences between populations
Increases genetic variation within populations
What is genetic drift?
Change in allelic frequencies in a population from generation to generation due to random chance/events
What is sampling error?
The deviation from an expected ratio due to low sample size
Arises when gametes unite to produce progeny, especially when population size is small
Chance influences which alleles are present in the next generation
Direction is unpredictable but the magnitude is predictable
How is the magnitude of genetic drift determined?
By examining the change in allelic frequencies within a single population or by examining the magnitude of genetic difference that accumulate among populations
Within a single population, the frequency of an allele can increase or decrease by random chance
Between multiple populations, the allele frequencies will become different between the populations over time due to random chance
Estimated from the variance in allelic frequency (s^2)
- Drift is maximal when p and q are equal
- Drift is maximal when population size is small
- s^2 = pq/2N
What is population size?
The number of “breeding” individuals within a population
AKA: Effective population size
What are breeding individuals?
Those that contribute genes to the next generation
What are the 3 causes of genetic drift?
Population reduction
Founder effect
Genetic bottleneck
What is population reduction?
Decrease in population size due to limited food, space, or other resources in the population’s environment
Those who survive can have a greater impact on the allele frequency of the population
What is the Founder effect?
Establishment of a population by a smaller number of individuals
If a few individuals break off from the main population and start a new population, it will be random what their allele frequencies will be in the new populations
While it can eventually grow large, the genes carried by all of its members are derived from very few genes present in the founders
Random chance plays a role in which genes are found in founders of the new populations
What is a genetic bottleneck?
Develops after a population undergoes a drastic reduction in population size
It’s by chance which individuals are left in the population that survive bottleneck event
What are the effects of genetic drift (3)?
Change in allelic frequencies within a population
- Because random, frequency is just as likely to increase or decrease
- Over time, allele frequency will wander
Reduction in genetic variation within populations (fixation of allele)
- Over time, allele frequency may be 1 or 0 (homozygous for one allele or the other)
- 0 = allele no longer exists in population
- 1 = all individuals in population have the allele
Population divergence between populations
- Because drift is random, allele frequencies don’t change the same in each population, which allows populations to individually acquire genetic differences over time
All of these occur simultaneously in a population - result of sampling error
What is natural selection and when does it occur?
The differential reproduction of genotypes
Occurs if individuals with adaptive traits produce a greater number of offspring than individuals without the trait
- If adaptive advantage has some sort of genetic basis, it will be inherited by offspring
- When inherited, they’re going to appear with greater frequency in the next generation
What is fitness (W)?
Reproductive success of a genotype relative to that of other genotypes in a population
How is fitness calculated?
Must know this equation
W = mean number of offspring produced by genotype / mean number of offspring produced by the most prolific genotype
What is the selection coefficient (s) and how is it determined?
Relative intensity of selection against a genotype
s = 1 - W
How does fitness impact allelic frequencies?
Differential fitness among genotypes leads to changes in the frequencies of genotypes over time, which in turn leads to changes in allelic frequencies that make up the genotypes
What is the general selection model used for and what are the required components?
Can determine changes in allele frequency due to natural selection
Requires: knowledge of initial allele frequencies of population and the fitness values of the genotypes
Assumes mating is random and that only force is natural selection
Can be used to calculate allele frequency after any type of selection is acting on a population
Can also determine if change in allele frequency after selection is against recessive, dominant, or codominant traits, and traits in which heterozygotes have a larger fitness
What are the 3 types of selection?
Directional selection - one allele/trait is favored over another
- Expressed on graph as peak moving to the left or right of the bell curve
- Selection can be against recessive, dominant, or codominant traits
Overdominance - heterozygote has a higher fitness than either homozygote (stabilizing selection)
- Expressed as peak in the middle of bell curve
- Called heterozygous advantage
Underdominance - heterozygote has a lower fitness than either homozygote (disruptive selection)
- Expressed as 2 peaks - one on left and one on right with a dip in the middle
What are the long term effects of each microevolutionary force?
Mutation - equilibrium reached between forward and reverse mutations
Migration - equilibrium reached when allelic frequencies of source and recipient population are equal
Genetic drift - fixation of one allele
Natural selection
- Directional selection - fixation of one allele
- Overdominant selection - equilibrium reached
- Underdominant selection - unstable equilibrium
