Mechanisms of Evolution

Changes in allele frequency in a population

Four main processes

• Mutation
• Gene flow
• Genetic drift
• Natural selection

Movement of alleles into or out of a population

Tends to lessen genetic differences between populations

Changes in allele frequency due to random chance

Much larger effect in small populations

Population Bottleneck

Drastic reduction in population size

New population may not have same gene pool as previous generations

Founder Effect

Few members of a population leave

Establish a new population

Patterns of Natural Selection

Original Population: Normal distribution of variations

Sexual Selection

Competition for mates can lead to evolution of traits that do not directly effect survival

Leads to sexual dimorphism

Hardy-Weinberg Equilibrium

A population in Hardy-Weinberg equilibrium has constant allele frequencies over generations

Conditions for Hardy-Weinberg

1. Random mating

2. Large population

3. No natural selection

4. No mutations

5. No gene flow

Within a population of butterflies, the color brown (B) is dominant over the color white (b). If 40% of all butterflies are white, calculate the following:

a.The percentage of butterflies in the population that are heterozygous.

b.The frequency of homozygous dominant individuals.

Macroevolution

What is a species?

Reproductive Defintion

When might this definition not work?

Other definitions:

• Morphological
• Ecological
• Palentological

Reproductive Isolation

Isolating mechanisms may occur before or after fertilization

Mechanisms that prevent mating attempts

Mechanisms that prevent cross-breeding after a mating attempt

Speciation

Two factors must be present for speciation to occur

• Isolation
• Genetic divergence

Allopatric: Geographical separation of populations

Sympatric: No geographic separation

Mechanisms of Evolution

Microevolution

Evolution of separate gene pools

Changes above the species level

Gene Flow

Genetic Drift

There are multiple definitions of a species

Species can be hard to recognize

Postzygotic Blocks

Prezygotic Blocks

Immigration/Emmigration

Individuals able to mate and produce fertile offspring

Cordilleran flycatcher

Pacific-slope flycatcher

European (German) male

African (Masai) woman

Same species

Different species

Geographic isolation

Gametic incompatability

Species are separated by a geographic barrier and so rarely if ever meet

Sperm from one species may not be able to penetrate eggs of a different species

Ecological isolation

Hybrid inviability

Species may live in same area, but are in separate, unique ecological niches

Hybrid offspring fail to survive to maturity

Temporal isolation

Hybrid infertility

Species mate at different times of year

Hybrid offspring are sterile

Bishop Pine

Monterey Pine

Behavioral isolation

Disruptive Selection

Directional Selection

Stabilizing Selection

Specific courtship/mating rituals must be followed in order for mating to occur

Selection against both extremes

Favors the average, most common form

Selection against one extreme

Favors the opposite extreme

Selection against average form

Favors the two extremes

May lead to divergence

Mechanical isolation

Birds of Paradise

Sexual organs are not compatible for mating

Male giraffes "necking"

Peacock and peahen

Stag beetles

Original population

New population

Apple maggot fly

Rate of Speciation

Example Hardy-Weinberg problem