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Variation & Genetic Drift
Unit 6: Evolution
Chapter 16-1 & 2
Learning goals
 1. Explain what a gene pool and
relative frequency are. In evolution
what happens to the relative frequency?
 2. Explain why variation in a gene poll
is important and what the two sources
of variation are?
 3. Describe genetic drift and the three
causes of genetic drift.
Gene Pools & Variation
 Gene Pool = all the genes (all alleles) in a
population
 Population: group of organisms of the same
species that can successfully reproduce
 Relative Frequency: the percent of a
specific type of allele present in a
population
 Evolution is a change in the relative frequency
of alleles in a population
 Variation of genes within a species’
gene pool increases the chance that at
least some members will survive and
reproduce when environmental
conditions change
Sources of Variation
 Two Main sources of genetic variation:
 Mutations
 Sexual Reproduction
1. Mutations: changes in the DNA
sequence can lead to new or different
traits.
 Spontaneous mutations are always occurring so we
can never completely breed them out
 2. Sexual Reproduction: during meiosis many
combinations of alleles can be passed on due
to independent assortment and crossing over.
Natural Selection on Traits
 Natural selection never acts directly on
genes, it “selects” who survives based on
the best phenotype of a trait
 If an individual dies without reproducing, it
does not contribute its alleles to the
population’s gene pool
 If an individual survives and produces many
offspring, its alleles stay in the gene pool and
may increase in frequency.
Genetic Drift
 Genetic Drift: the random change in
the number of alleles for certain
traits in a population
 The shift in alleles changes the
appearance of the population.
 A change in alleles can happen by
chance.
Genetic Drift change in alleles in a gene pool due to chance
 Three causes of genetic drift
 1. Gene Flow: Individuals move into
(immigration) or out of (emigration) the
population (they bring or take their
genes with them)
 Ex: In the 1800s & 1900s, a large number of
people emigrated from Europe and
immigrated to America
 2. Founder Effect: a small group gets
isolated and separated from the others
Descendants
Population A
Population B
 3. Bottleneck Effect: a natural disaster
occurs and only a few survive
Learning goals
 1. Explain what a gene pool and
relative frequency are. In evolution
what happens to the relative frequency?
 2. Explain why variation in a gene poll
is important and what the two sources
of variation are?
 3. Describe genetic drift and the three
causes of genetic drift.