Other Factors Can Affect Genetic Variation In A Population

• Natural Selection drives the evolution (change in genetic material) of a population

• Other factors can cause changes in the genetic material (gene pool) of a population– Mutation– Migration– Assortative (nonrandom)

mating– Genetic Drift

Genetic Drift

• Change In Allele Frequency Caused By Random Chance– Random chance =

natural disasters, disease, etc.

– Has a greater affect on small populations

– Often causes reduction in alleles present in the gene pool.

The Bottleneck Affect

• A direct result of genetic drift– As a result of some

random event, several alleles are lost from a population.

3 General Types of Natural Selection

1. Stabilizing Selection

2. Directional Selection

3. Disruptive Selection

1. Nature selects the average form of a trait (human baby size)

2. Nature selects one extreme form of a trait (e.g. only the very long tongue in anteaters, not the short tongue)

3. Nature selects both the extreme forms of a trait (e.g. very bright shell and very dull shell)

The 3 Types of Natural Selection

Sexual Selection: A Special Type of Directional Selection

• Individuals of one sex (usually males) of a species are more likely to get picked as partners by the other sex if their characteristics are bolder, brighter, louder or stronger than other members of the same sex.

Of Course, Being More Noticeable Does Have Its Disadvantages . . .

Before We Go On, An Important Question: What is a species?

• Members of a species are morphologically (physically) similar, and can successfully interbreed (produce fertile offspring).

How Do New Species Form?

• Isolation• Isolating Mechanisms

– Prezygotic Isolation– Postzygotic Isolation

Geographic Isolation

• Physical separation of members of a population (by formation of a canyon, mountain, river, etc.)– Leads to different

adaptations on each side of barrier, and eventually new species

Geographic Isolation = New Species: An Example

Reproductive Isolation• Species isolated

because they reproduce in different seasons or times of day

Behavioral Isolation• Species Differ In

Their Mating Rituals (e.g. different bird songs, mating colors, dances, pheromones, etc.)

Ecological Isolation• Ecological Isolation

– Species inhabit the same area, but different habitats, so they don’t encounter each other

Mechanical

• Mechanical Isolation– Body structure

prevents mating – In layman’s terms - the

parts don’t fit– Famous quote in AP

Bio class at PHS• “You mean it’s like

trying to park a cadillac in an outhouse?”

Reproductive Isolation

• Some barriers to successful breeding arise after fertilization and are called postzygotic isolating mechanisms– Sterile offspring (e.g.

Horse + Donkey = Mule)

How quickly do new species form?

• 2 hypotheses– Gradualism

• Many transitional forms between common ancestor and modern organisms

– Punctuated Equilibrium• Sudden shifts in form from

an ancestor to newer organism, so not many transitional forms between common ancestor and modern organisms

Patterns of Evolution

• Convergent Evolution

• Divergent Evolution– Adaptive Radiation– Artificial Selection

• Coevolution

Convergent Evolution

• Organisms appear similar, but aren’t closely related

• Why?– Similar environment

selects for similar traits

Divergent Evolution• Related populations

or species become less and less alike

A Second Type Of Divergent Evolution

• Artificial Selection– Humans, rather than

nature, select traits

Coevolution

• Two or more species in close association w/one another change together– Predator/Prey– Parasite/Host– Plant/Pollinator

Finally – THE END!