Hardy-Weinberg: from allele frequency to genotypes.

• Where does variation come from, and how does speciation occur? Mendel had described inherited 'factors', but Darwin either ignored this or said nothing.
• In the first decade of the twentieth century, Hugo de Vries, working on evening primrose, brought out the idea of mutation: a large difference arising suddenly in a population.
• De Vries held that mutation, not the small heritable variations Darwin described, causes evolution. Mutations are random and directionless; Darwin's variations are small and directional.
• For Darwin evolution was gradual; de Vries thought mutation caused speciation in a single large step, which he called saltation. Later, population genetics clarified the picture.
• Hardy-Weinberg principle: allele frequencies in a population are stable and constant from generation to generation. The gene pool (all the genes and their alleles in a population) stays constant; this is genetic equilibrium.
• The allele frequencies add up to 1. In a diploid, p is the frequency of allele A and q that of allele a, so p + q = 1.
• The chance that A appears on both chromosomes is p × p, so AA individuals have frequency p²; aa has q² and Aa has 2pq.
• So p² + 2pq + q² = 1, the binomial expansion of (p + q)².
• If the measured frequencies differ from these expected values, the difference and its direction show the extent of evolutionary change. A change in allele frequencies, that is a disturbance of Hardy-Weinberg equilibrium, is read as evolution.

Teaching this idea? Put the simulation on your own page or LMS with this code.