## Evolution vs. Non-Evolution

• 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: p2 + 2pq + q2 = 1

• p2 is the frequency of the AA genotype.
• 2pq is the frequency of the Aa genotype.
• q2 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 Figure 23.3(a), page 431, Campbell's Biology, 5th Edition

### Second Generation Figure 23.3(b), page 431, Campbell's Biology, 5th Edition

### Result

• No change in allele frequency
• Thus, no evolution
• Therefore, all five Hardy–Weinberg equilibrium conditions must have been met!