Natural Selection
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Natural selection
Speciation
Population Genetics
Evidence of evolution
Macroevolution
Gradualism
Punctuated Equilibrium
 Evolution
 Evolution
that
occurs gradually
over a long
period of time.
that
occurs very
quickly after a
long period of
stability
Reproductive isolating
mechanisms
Prezygotic mechanism
Postzygotic mechanism
 Factors

that
prevent
individuals from
mating
Factors that allow an
individual to
reproduce but the
offspring are infertile.
 Or Zygote forms but
does not survive
birth.
Isolating Mechanisms

Geographic Isolation:
 Species
occur in different areas,
and are often separated by
terrestrial and aquatic barriers
Isolating Mechanisms

Temporal Isolation:
 Individuals
do not mate because
they are reproductively active
and different times.
 Different times of day, different
seasons
Isolating Mechanisms

Ecological isolation:
 Individuals
only mate within their
preferred habitat.
Isolating Mechanisms

Behavioral isolation:
 Individuals
of different species
may meet, but one does not
recognize the sexual cues that
may be given.
Isolating Mechanisms

Mechanical isolation:
 Copulation
may be attempted but
transfer of sperm does not take
place.
Isolating Mechanisms

Gametic incompatibility:
 Sperm
transfer takes place, but
the egg is not fertilized.
Artificial Selection
3 Types of Natural Selection:
Directional Selection

Selects for one of the extreme phenotypes
- directional selection occurs in
response to a change in the environment
that gives a competitive advantage to a
particular phenotype
Examples of Directional
Selection
Bacteria developing resistance to antibiotics
The beak sizes of ground finches on Daphne
Island
3 Types of Natural Selection:
Stabilizing Selection
Selects for the average phenotype and
against the extreme phenotypes
 Occurs when the environment is stable for
long periods
 Example birth weight of babies.

Stabilizing Selection
3 Types of Natural Selection:
Disruptive Selection
Selects against the average and selects
both extremes.
 Example: African Seed Crackers

African Seed Crackers
Birds feed on two
types of seeds one
large, one small.
Birds with average
size bills can’t eat
either type efficiently
And so aren’t common
in the population
Genetic Drift: Another Force
for Change

Genetic Drift = Changes in a population’s
gene pool that happen by chance
- its effect is most profound in small
populations
- it reduces genetic variation in these
populations
- this could reduce a population’s
ability to survive environmental
change
Genetic Drift Continued
An organism can be well suited to its
environment can be removed by chance
 The result: Organisms
that aren’t as well
adapted can survive to
reproduce by chance
- this shifts the gene pool
of the population

Genetic Drift

Once an allele is removed from a
population, it is unlikely to return
Genetic Drift –
The Bottle-neck effect

Something happens to catastrophically
reduce a population’s size for at least
one generation

Dramatically changes allele frequencies
in a small population
Bottle necked species

Elephant seals hunted to near
extinction – 20 individuals
- which 20 survived? The best
adapted or the least desireable?
Was their survival random luck?
-now number 30,000 but all individuals are
descended from the 20 survivors
-little genetic diversity this population has a reduced
ability to survive an environmental challenge
Founder Effect
Occurs when a small population migrates to a
new area or becomes isolated
 Limited gene pool – over time some generally
rare traits become more common
- Huntington’s disease in South Africa
- Amish communities
 FE most commonly recognized in genetic
disorders – occurs in other traits as well

Genetic Drift as a force for
change

In small isolated populations - genetic
drift could allow otherwise rare alleles
to become common
- this could cause the isolated
population to diverge from the main
population and over time become a
new species