Evolution vs. NonEvolution

Evolution Defined
 A change in allele frequencies in a population
 Therefore: If there is no change in allele frequency in a population, we assume that no evolution has occurred.

Hardy and Weinberg defined five conditions (see the next lecture) under which no evolution would occur.
 If these five conditions are met, no evolution occurs, and we have a Hardy–Weinberg Equilibrium.
 It is called an equilibrium because the allele frequency does not change: it is in a state of equilibrium.
 We determine allele frequencies using the Hardy–Weinberg equations.

These equations require two assumptions about the population:
 There are only two alleles for a trait (e.g. A and a).
 One allele is dominant (A) and one allele is recessive (a).
Hardy–Weinberg Equations
General
 p represents the frequency of the dominant allele (in this example, A).
 q represents the frequency of the recessive allele (in this example, a).
 Both variables range from 0 (none of the population has that allele) to 1 (100 % of the population has that allele).
 Both equations below will always be true if the two assumptions mentioned above are met.
Allele Frequency Equation: p + q = 1
 p is the frequency of the A allele.
 q is the frequency of the a allele.
Genotype Frequency Equation: p^{2} + 2pq + q^{2} = 1
 p^{2} is the frequency of the AA genotype.
 2pq is the frequency of the Aa genotype.
 q^{2} is the frequency of the aa genotype.
Practice Problem
(See page 431 in Campbell's Biology, 5th Edition for a discussion of this problem.)
Parental Population
Second Generation
Result
 No change in allele frequency
 Thus, no evolution
 Therefore, all five Hardy–Weinberg equilibrium conditions must have been met!