1. Hardy-Weinberg
The Hardy-Weinberg theorem (p2+2pq+q2 = 1) describes gene frequencies in a stable population that are well adapted to the environment. It assumes the following:
Extremely large populations
No gene flow between populations
Random mating
No natural selection
No mutation
As you can see the chance for a these conditions leading to a stable population is entirely improbable. Any departure in these conditions leads to evolution.
Why it doesn’t work

2. Application of Hardy Weinberg
An investigator has determined by inspection that 16% of a human population has a recessive trait (tt). Complete all the genotype and allele frequencies for this population, assuming that it is in Hardy-Weinberg equilibrium.
p2+2pq+q2 = 1 & p + q = 1
Since the trait is expressed in the homozygous form (tt) .16 = q2
q: t2 = 0.16, √t2 = √16 = 0.4 so p + q = 1, p = 1, p = 0.6
p2(TT)= .62 = 0.36
2pq (Tt) = 2(0.6)(0.4) = 0.48

3. Agents of Evolution
Genetic Drift - deviations from expected (Hardy-Weinberg) frequencies
bottle-neck effect
caused by a disaster suddenly altering the environmental pressures causing the genes of a few survivors to dominate
founder effect
when a few individuals move and start a new population allowing their genes to dominate
gene flow
changes to a gene pool as individuals move out and into a breeding population
tends to reduce differences between populations

4. Stabilization of a gene pool
Directional selection
selection toward a phenotype that is best adapted to the environment
most common type of selection
Disruptive selection
occurs when environmental factors favor individuals on both extremes of the phenotypic range
often to facilitate different food sources

5. Stabilization of a gene pool
Stabilizing selection
selects against the extremes of phenotypes
humans
Balancing Selection
occurs when nature allows 2 distinctly different polymorphisms to exist in relatively equal frequencies (balanced polymorphism)
leads to heterozygous advantage and frequency dependent selection
heterozygous advantage - heterozygous individuals exhibit less genetic disorders
frequency dependent selection - frequency of the dominant phenotype becomes less fit over time (prey species)