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Lesson Overvie
17.1 Genes and
Variation
Genes and Variation
Lesson Overview
Genotype and Phenotype in
Evolution
Population – mating group of
organisms of the same species
Gene pool – all genes (and their
alleles) present in a population
Allele frequency - # of times allele
occurs in a population
Changes as population
evolves over time
Natural selection operates on
individuals, but causes a change in
the allele frequency
Genes and Variation
Lesson Overview
For example, this diagram shows
the gene pool for fur color in a
population of wild boar.
Genes and Variation
Lesson Overview
Populations and Gene Pools
Evolution-change in the relative
frequency of alleles in the gene pool
of a population over time.
Natural selection operates on
individuals, but causes a change in
allele frequencies in populations.
Populations evolve not individuals!!
Genes and Variation
Lesson Overview
Sources of genetic variation
Main source of genetic variations in
populations is mutations!!
They occur randomly
Only heritable mutations matter
for evolution
Other sources of variation include:
1. Genetic recombinationcrossing over and independent
assortment in meiosis
2. Lateral gene transfer- (bacteria
only) Bacteria swap plasmids
between members of the same
generation, then pass them to
their offspring
Genes and Variation
Lesson Overview
Natural Selection & Phenotype
An organism’s genotype and
environmental conditions makes up its
phenotype.
Natural selection operates on
variation in organisms’ phenotypes.
Phenotypes that are better suited
for the environment will have higher
fitness
Genes and Variation
Lesson Overview
Single-Gene and Polygenic Traits
Number of phenotypes for a trait
depends on how many genes
control the trait.
Single-gene trait – trait
controlled by one gene.
Ex. Banded or un-banded shell.
NS on these traits leads to
changes in allele and
phenotypic frequencies.
Genes and Variation
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Polygenic trait – trait controlled
by two or more genes.
Ex. Height in humans.
NS on these traits affects
fitness of phenotypes.
Genes and Variation
Lesson Overview
NS on polygenic traits leads to
selection in populations
When NS on polygenic traits affects the
fitness of phenotypes, it leads to
selection:
1. Directional selection
2. Stabilizing selection
3. Disruptive selection
Genes and Variation
Lesson Overview
Types of selection in
populations
Directional selection – organisms
at one end of The curve have a
higher fitness than those in the
middle or at the other end
Stabilizing selection – organisms
in the Center have highest fitness
Disruptive selection – organisms
at the ends of curve have highest
fitness
Genes and Variation
Lesson Overview
NS is not the only source of change in allele
frequencies
Genetic drift – change in allele frequency that occurs
in small populations due to random chance
*Genetic bottleneck – change in allele frequency
following a dramatic reduction in population size
*Founder effect – change in allele frequency following
migration of a small subgroup out of the population
to start a new population