Grade 11 University Biology – Unit 3 Evolution
Mechanisms of Evolution – Variation Overheads
Section 9.1 Pages 350-357
Mechanisms of Evolution (i.e., How does variation occur?)
 Evolution within a population is microevolution

Genetic Variation – Evolution only occurs where there is a change in
gene frequency in a population. The genetic differences (1) are
heritable and (2) can be passed to the next generation. Variation
within a species results from the variety and combination of alleles
possessed by individuals
There are five sources of genetic variation.
1. Mutations
 Mutation is a random change in the DNA of an individual, and
that a heritable mutation has the potential to impact an entire
gene pool.
 Mutations occur (1) during Crossing Over in meiosis and (2)
from external influences (e.g. radiation)
2. Genetic Drift
 The change in allele frequency in a gene pool due to chance.
 Two Types: (1) Bottleneck Effect which are changes in gene
distribution that result in a rapid decrease in population size
and (2) Founder Effect which is a change in the gene pool that
occurs when a few individuals start a new isolated population
3. Gene Flow
 The movement of alleles from one population to another due
to the migration of individuals. Also called MIGRATION
4. Non-Radom Mating
 Mating among individuals on the basis of mate selection for a
particular genotype or due to inbreeding
5. Natural Selection
 Selective forces (e.g., predation, competition) mean some
individuals are more likely to survive and reproduce. There
are three types of selection.
 Stabilizing Selection – Favours an intermediate
phenotype and acts against extreme variants.
 Directional Selection – Favours the phenotypes at one
extreme over another, resulting in the distribution of
phenotypes shifting toward the extreme.
 Disruptive Selection – Favours the extremes of a range
of phenotypes and eliminates the intermediate
phenotypes.