Unit 14: Population Genetics and Evolutionary Genetics
How allele frequencies behave across a whole population under mating, selection, drift, migration, and inbreeding, and what that reveals about evolution.
10 topics · 38 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Allele and Genotype Frequencies in a Population
- How it worksHow do genotype counts in a population get converted into allele frequencies?
- Worked exampleFind the allele frequencies of A and a given a population of 100 individuals with 64 AA, 32 Aa, and 4 aa.
- Common mistakeWhy doesn't a dominant allele have to be the more common one in a population?
- Worked exampleFind the frequency of heterozygous carriers in a population where the recessive allele frequency is 0.1 under Hardy-Weinberg.
Your turn1 of 2
A population has 62 AA individuals, 42 Aa individuals, and 28 aa individuals. What is the frequency of the recessive allele a?Hardy-Weinberg Equilibrium
- How it worksHow does random mating combine allele frequencies into the p squared, 2pq, q squared genotype ratio?
- Worked exampleFind the expected genotype frequencies under Hardy-Weinberg given an allele frequency of 0.3 for the recessive allele.
- Common mistakeWhy doesn't Hardy-Weinberg equilibrium describe evolution happening, but rather evolution's absence?
- Going furtherWhat happens to Hardy-Weinberg genotype ratios in the very first generation after one of its five assumptions is violated?
- Worked exampleFind the frequency of affected homozygous recessive individuals given a carrier frequency of 1 in 50 for a recessive disorder.
Your turn1 of 2
In a population at Hardy-Weinberg equilibrium, the recessive allele frequency is 0.363. What is the expected frequency of homozygous recessive (aa) individuals?Testing Whether a Population Is in Hardy-Weinberg Equilibrium
- How it worksHow does a chi-square test compare observed genotype frequencies to Hardy-Weinberg expected frequencies?
- Worked exampleFind whether a population of 500 with 320 AA, 160 Aa, and 20 aa individuals is in Hardy-Weinberg equilibrium using chi-square.
- Going furtherWhat happens to a population's chi-square value across several generations of continuous inbreeding?
Your turn1 of 2
A population of 339 AA, 147 Aa, and 38 aa individuals is tested against Hardy-Weinberg expectations. First compute the allele frequencies from these observed counts, then the expected genotype counts under Hardy-Weinberg, then the chi-square statistic (sum of observed minus expected, squared, over expected, across all three genotype classes). What is the chi-square value?Natural Selection Changing Allele Frequencies
- How it worksHow does differential survival and reproduction shift allele frequencies away from Hardy-Weinberg equilibrium each generation?
- Worked exampleTrace an allele frequency's trajectory over 10 generations given a selection coefficient of 0.2 against the recessive homozygote.
- Common mistakeWhy doesn't selection against a recessive allele ever manage to eliminate it completely from a population?
- Going furtherWhat happens to an allele frequency trajectory under selection when heterozygotes have the highest fitness of all three genotypes?
- Ties togetherHow does an allele frequency trajectory under selection connect back to the Hardy-Weinberg equilibrium it disrupts?
Your turn1 of 2
A recessive allele has frequency 0.448 in a population. Homozygous recessive individuals (aa) have a selection coefficient of 0.619 against them (their relative fitness is 1 minus s, while AA and Aa both have fitness 1). Using q' = q(1 - s·q) / (1 - s·q2), what is the allele frequency after one generation of selection?Genetic Drift
- How it worksHow does random sampling of gametes in a small population cause allele frequencies to wander toward fixation or loss?
- Worked exampleTrace several repeated small-population samples of an allele starting at 0.5 frequency wandering toward fixation or loss over generations.
- Common mistakeWhy doesn't genetic drift require a trait to provide any survival advantage at all?
- Going furtherWhat happens to the strength of genetic drift as a population's size increases from 10 to 10,000 individuals?
Your turn1 of 2
A population of 699 individuals currently has an allele frequency of 0.619. Genetic drift's expected magnitude per generation is measured by the variance in allele frequency change, p(1 - p) divided by 2N. What is this variance for this population?The Founder Effect and Population Bottlenecks
- How it worksHow does a small group of founders carrying only a random sample of alleles reshape a new population's genetic makeup?
- Worked exampleTrace how a bottleneck reducing a population from 10,000 to 20 individuals changes its allele frequency distribution.
- Ties togetherHow do the founder effect and a population bottleneck both work through the same genetic drift mechanism at different scales?
Your turn1 of 2
A population of 14232 individuals experiences a bottleneck, crashing down to just 133 survivors, both before and immediately after having the same allele frequency. Using the drift variance formula p(1-p)/2N, how many times LARGER is the post-bottleneck generation's drift variance compared to the pre-bottleneck population's drift variance?Gene Flow Between Populations
- How it worksHow does migration of individuals between populations transfer alleles and homogenize their frequencies?
- Worked exampleFind the new allele frequency in a population of 900 after 100 migrants with a different allele frequency arrive and interbreed.
- Going furtherWhat happens to two populations' allele frequency divergence when gene flow between them stops completely?
Your turn1 of 2
A resident population of 1545 individuals has an allele frequency of 0.424. A group of 281 migrants with an allele frequency of 0.323 arrives and interbreeds completely with the residents. What is the new allele frequency in the combined population?Inbreeding and the Inbreeding Coefficient
- How it worksHow does the inbreeding coefficient measure the probability that two alleles at a gene are identical by descent?
- Worked exampleFind the inbreeding coefficient for the offspring of a first-cousin mating.
- Common mistakeWhy doesn't inbreeding create new harmful alleles, only expose the ones already present?
- Going furtherWhat happens to the frequency of homozygous recessive genotypes in a population as its average inbreeding coefficient rises?
Your turn1 of 2
An offspring's parents share 3 distinct common-ancestor paths connecting them, each path passing through 6 separate meioses. Using the path-counting formula (each path contributes (1/2) raised to the number of meioses in that path, summed over all paths), what is this offspring's inbreeding coefficient?Molecular Evolution and the Neutral Theory
- How it worksHow does genetic drift, not natural selection, fix most neutral mutations in a population over time?
- Worked exampleFind the expected substitution rate at a neutral locus given a mutation rate of 1 in 100 million per generation.
- Ties togetherHow does the neutral theory of molecular evolution connect genetic drift to the steady ticking of a molecular clock?
Your turn1 of 2
At a neutral genetic locus, the mutation rate is 6.99e-7 new mutations per gene copy per generation. According to Kimura's neutral theory, the rate at which new mutations get fixed (substituted) in the population equals the mutation rate itself, regardless of population size. What is the expected substitution rate at this locus?Phylogenetics and Molecular Clocks
- How it worksHow does building a phylogenetic tree from shared derived DNA sequence changes reveal evolutionary relationships?
- Worked exampleFind the divergence time between two species given a molecular clock rate of 2 percent difference per million years and an observed 8 percent difference.
- Common mistakeWhy doesn't a molecular clock tick at exactly the same rate in every gene and every lineage?
- Worked exampleFind which of two lineages evolved faster given one shows 12 percent sequence divergence and the other shows 4 percent over the same time span.
Your turn1 of 2
A molecular clock accumulates sequence differences at a rate of 2.24 percent per million years. Two species show an observed sequence difference of 14 percent. How many million years ago did these two species diverge?