If there is only one allele at a locus, its frequency = 1. The population is monomorphic at that locus; the allele is
said to be fixed. The population is said to be polymorphic at a locus, if there are more than one allele at that
The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian
population describe that population’s genetic structure
o Allele frequencies measure the amount of genetic variation in a population; genotype frequencies show
how a population’s genetic variation is distributed among its members
The genetic structure of a population does not change over time if certain conditions exist.
If an allele is not advantageous, its frequency remains constant from generation to generation, its frequency will
not increase even if the allele is dominant
A population of sexually reproducing organisms in which allele and genotype frequencies do not change from
generation to generation is said to be at Hardy–Weinberg equilibrium. Genotype frequencies can be predicted
from allele frequencies.
Five assumptions must be made in order to meet Hardy–Weinberg equilibrium.
o Mating is random.
o Population size is very large. (larger the population, the smaller will be the effect of genetic drift-
random(chance) fluctuations in allele frequencies)
o There is no migration either into or out of the populations.
o There is no mutation. No change to alleles A and a, and no new alleles are added to change the gene
o Natural selection does not affect the survival of particular genotypes. There is no differential survival of
individuals with different genotypes.
If the conditions of the Hardy–Weinberg equilibrium are met, two results follow.
o The frequencies of alleles at a locus will remain constant from generation to generation.
o After one generation of 2andom mat2ng, the genotype frequencies will not change.
The Hardy–Weinberg equation: p + 2pq + q = 1.
Genotype: AA Aa aa
Frequency: p2 2pq q2
Populations in nature never fit the conditions for Hardy-Weinberg equilibrium. Two reasons why this model is
considered important for the study of evolution:
o It is useful in predicting genotype frequencies from allele frequencies; and
o Since the model describes conditions that would result in no evolution, patterns of deviation from the
model help identify specific mechanisms of evolution.
22.2- What Are the Mechanisms of Evolutionary Change?
Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent.
Known evolutionary mechanisms:
Natural selection Mutations Generate Genetic Variation
Origin of genetic variation is mutation; mutation is any change in an organism’s DNA
Most mutations are harmful to their bearers or are neutral, but if environmental conditions
change, previously harmful or neutral alleles may become advantageous
Mutations can restore to populations alleles that other evolutionary processes have removed
Most mutations appear to