Introduction

So far in this course, we've focused on describing the pattern of variation within and among populations. We've talked about inbreeding, which causes genotype frequencies to change, although it leaves allele frequencies the same, and we've talked about how to describe variation among populations. But we haven't yet discussed any evolutionary processes that could lead to a change in allele frequencies within populations.¹

Let's return for a moment to the list of assumptions we developed when we derived the Hardy-Weinberg principle and see what we've done so far.

Assumption #1

Genotype frequencies are the same in males and females, e.g., $x_{11}$ is the frequency of the $A_1A_1$ genotype in both males and females.

Assumption #2

Genotypes mate at random with respect to their genotype at this particular locus.

Assumption #3

Meiosis is fair. More specifically, we assume that there is no segregation distortion, no gamete competition, no differences in the developmental ability of eggs, or the fertilization ability of sperm.

Assumption #4

There is no input of new genetic material, i.e., gametes are produced without mutation, and all offspring are produced from the union of gametes within this population.

Assumption #5

The population is of infinite size so that the actual frequency of matings is equal to their expected frequency and the actual frequency of offspring from each mating is equal to the Mendelian expectations.

Assumption #6

All matings produce the same number of offspring, on average.

Assumption #7

Generations do not overlap.

Assumption #8

There are no differences among genotypes in the probability of survival.

The only assumption we've violated so far is Assumption #2, the random-mating assumption. We're going to spend the next several lectures talking about what happens when you violate Assumptions #3, #6, and #8. When any one of those assumptions is violated we have some form of natural selection going on.²