Hardy-Weinberg Equilibrium Notes

Hardy-Weinberg Equilibrium: Transcript Summary

Core question addressed: Is our population the product of evolution? We compare the expected (predicted) genotype frequencies under equilibrium to the actual frequencies observed.
If actual frequencies equal predicted frequencies: the population is in Hardy-Weinberg equilibrium.
If actual frequencies do not equal predicted frequencies: the population is evolving.
Interpretation of deviation: Something is preventing alleles from manifesting in genotypes exactly as expected under random meiosis and fertilization; the population is not sorting genotypes in a way that matches just meiotic and fertilization processes.
The summary slide emphasizes the goal of HW equilibrium: describe a population in equilibrium as one that is not evolving, with allele frequencies not changing from generation to generation, and where the only influence on allele frequencies at any point in the life cycle is meiosis and random fertilization.
The instructor notes that more about assumptions will be covered on the next slide.
The coin example is mentioned as foreshadowing the approach: demonstrating how probabilities of outcomes align with predictions under random processes.
Practical framing: HW equilibrium is a baseline for understanding whether evolutionary forces are acting on a population.

Hardy-Weinberg equilibrium describes a population that is not evolving.
In HW equilibrium, allele frequencies remain constant from generation to generation.
The only processes that influence allele frequencies in the life cycle, under HW assumptions, are meiosis and random fertilization.
When equilibrium holds, genotype frequencies are predictable from allele frequencies.

Let p be the frequency of one allele and q be the frequency of the other allele at a locus.
The two allele frequencies satisfy the relation: p + q = 1.
Under HW equilibrium, the predicted genotype frequencies are: p^2,\; 2pq,\; q^2 for the three possible genotypes.
The sum of genotype frequencies equals 1: p^2 + 2pq + q^2 = 1.
Relationship note: If p changes, q changes in the opposite direction so that their sum remains 1; thus, as one increases, the other decreases (but they always sum to 1).

Equilibrium condition: Observed genotype frequencies match the predicted HW frequencies (under the current p and q).
Evolution condition: Observed genotype frequencies differ from the predicted HW frequencies.
The key diagnostic is the comparison between predicted vs actual genotype frequencies; a mismatch indicates that some evolutionary force is acting, or that assumptions of random mating or large population size are violated.
The slide emphasizes that the only thing that should influence allele frequencies (in the life cycle under HW assumptions) is meiosis and random fertilization; violations imply outside evolutionary forces.

Allele frequency relationship: p + q = 1.
Genotype frequency relationship under HW: p^2,\; 2pq,\; q^2 for the genotypes corresponding to the two alleles.
Sum identity for genotype frequencies: p^2 + 2pq + q^2 = 1.
Intuition about changing allele frequencies: If p changes over time, q = 1 - p changes correspondingly, ensuring that equilibrium relationships hold when the population is not evolving.

The instructor plans to dive into the assumptions on the next slide and mentions class work.
Typical HW assumptions (contextual framing, not all spelled out in the transcript):
- No mutation
- No migration (no gene flow)
- No natural selection
- Very large population size (no genetic drift)
- Random mating
These assumptions are explored further in class/work in upcoming sessions (lab tomorrow or Wednesday, as noted).

HW equilibrium provides a baseline null model for evolution in a population; deviations indicate the action of evolutionary forces.
It helps in understanding how allele frequencies translate into genotype frequencies and why certain patterns (p^2, 2pq, q^2) arise under random mating.
The approach mirrors the coin-flip analogy: random processes lead to predictable proportions if the population meets HW assumptions.

The practical takeaway is to test whether a real population is evolving by comparing observed genotype frequencies to HW-predicted frequencies.
If discrepancies exist, researchers infer that outside forces (as described by HW assumptions) are at play, and further investigation into which forces (selection, drift, migration, mutation, non-random mating) is warranted.

Hardy-Weinberg equilibrium describes a non-evolving population under random mating with no external evolutionary forces.
Allele frequencies remain constant across generations: p{t+1} = pt and q{t+1} = qt, with p + q = 1.
Genotype frequencies in equilibrium are p^2,\; 2pq,\; q^2, and sum to 1: p^2 + 2pq + q^2 = 1.
Observed vs predicted genotype frequencies determine whether the population is evolving.
The material sets up for deeper discussion of assumptions in upcoming lectures and labs, and reinforces the concept that meiosis and random fertilization are the core processes driving the genotype proportions under HW conditions.