Modes of Natural Selection

Ch. 17: Evolution of
• Natural Selection acts upon an organism’s
phenotype, not its genotype
• Individuals with favorable phenotypes
pass on more copies of their genes to the
next generation
Evolution is defined as a
change in a population’s
gene pool over time.
A population is a group of individuals of
the same species.
A population’s gene pool consists of all the
genes/alleles that are present in a population.
Genotypic and phenotypic “frequencies”
What is the frequency of the green phenotype?
What is the frequency of the brown phenotype?
What is the frequency of the green allele?
What is the frequency of the brown allele?
Sources of genetic variation: mutations
Mutations may increase or decrease one’s fitness.
Most mutations are neutral. In order to matter in
evolution, they must occur in gametes.
Sources of genetic variation: crossing
over during meiosis
Crossing over does not create new alleles, just new
combinations of alleles in individuals.
Some traits are controlled by many
genes (polygenic)
Some traits are controlled by a single
Five Major Causes of Evolution
1. Natural Selection
2. Genetic Drift
3. Migration
4. Mutations
5. Non-Random Mating
1. Natural Selection
Directional Selection—occurs when
one extreme phenotype is favored
Stabilizing Selection—occurs when
the average phenotype is favored
Disruptive Selection—occurs when
both extreme phenotypes are
What pattern of selection has occurred?
What pattern of selection has occurred?
What pattern of selection has occurred?
2. Genetic Drift
changes in a population gene
pool due to chance events.
More likely to occur in a small population.
Example 1: Bottleneck Effect
Bottleneck effect: a few individuals
survive a catastrophic event. The new
population has dramatically different gene
Example 2: Founder Effect
When a new population is established by a
very small number of individuals from a
larger population.
Mass Extinctions and Genetic Drift
have shaped the history of life on Earth
3. Migration
Immigration or emigration between
populations changes allele
4. Mutations
5. Non-random mating
every individual does not have
an equal chance of
An example of NONrandom mating is assortive
Sexual Selection (Bird of
Paradise) (Lyre Bird) (Bower Birds