Natural Selection VideosNatural selectionSpeciationPopulation GeneticsEvidence of evolution

MacroevolutionGradualismEvolution that

occurs gradually over a long period of time.

Punctuated EquilibriumEvolution that

occurs very quickly after a long period of stability

Reproductive isolating mechanismsPrezygotic mechanismFactors that

prevent individuals from mating

Postzygotic mechanism 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 antibioticsThe 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 CrackersBirds feed on twotypes of seeds one large, one small. Birds with average size bills can’t eat either type efficientlyAnd so aren’t commonin 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