1. Determine whether evolution occurs by mechanisms other than selection.

2. Compare and contrast population outcomes resulting from selection versus random events.

Objectives

Can evolution occur by mechanisms other than selection?

1. Genetic drift;2. Gene flow.

Campbell, Chapter 23, pp.475-485

Plant Scenario 1: Initial Population

Late Spring Freeze

Late Spring FreezeWho dies? . . .and therefore, does not reproduce.....

Consequence of Late Spring Freeze?

A increase decreaseB increase no changeC no change no changeD decrease increaseE decrease no change

Clicker Q1

Survivors

Survivors Reproduce All Blue Population

• Change in allele frequencies in a population over time due to random (stochastic) events in environment

Plant Scenario 1: Genetic Drift

Q: What if blue was better able to survive the effects of a spring freeze?

Plant Scenario 2: Initial Population

Q: What if blue was better able to survive the effects of a spring freeze?

LATE SPRING FREEZE

Q: What if blue was better able to survive the effects of a spring freeze? Who dies? . . .and therefore, does not reproduce...

LATE SPRING FREEZE

Q: What if blue was better able to survive the effects of a spring freeze?

Late Spring Freeze

Who dies? . . .and therefore, does not reproduce

Survivors

Survivors ReproduceThe outcome is: All Blue Population.

Same end result as Scenario 1. Is the mechanism the same?

Plant Scenario 3: Initial Population

A squirrel drops a seed from a fruit it found across the road.

Immigration: “new” alleles are introduced into the population by interbreeding

What happens over time. . .

Gene Flow

• Physical movement of alleles into or out of a population by immigration/emigration.

Summary: Mechanisms of Evolution

• Selection (natural, sexual, artificial)• Genetic Drift (random events in

environment)• Gene Flow (immigration/emigration)

Scales of Evolution

Microevolution Macroevolution

Small scale changes in allele frequencies

Variability in population traits

Large-scale changesOrigin of new species

What is a species?

• In your group, write down a definition of species.

What is a species?

• Biological Species Concept: –Population of organisms capable of

interbreeding AND producing viable and fertile offspring

• Can you think of any exceptions to this definition?

Campbell, Chapter 24, pp. 487-492

Could speciation happen here?

A road is built that prevents pollen exchange.

Resulting subpopulations can’t interbreed due to barrier

Divergence

Divergence

• Accumulation of genetic change over time (due to drift, flow, different selective agents, random mutations)

If

can no longer interbreed

with

then the 2 subpopulations have achieved reproductive isolation new species!

Reminder: Scales of Evolution

Microevolution Macroevolution

Small scale changes in allele frequencies

Variability in population traits

Large-scale changesOrigin of new species

Speciation

• Origination of new species resulting from reproductive isolation of populations that have diverged