Determine whether evolution occurs by mechanisms other than selection.

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Objectives

1. Determine whether evolution occurs by mechanisms
   other than selection.
2. Compare and contrast population outcomes
   resulting from selection versus random events.

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Can evolution occur by mechanisms other than
selection?

• Genetic drift
• Gene flow.
• Campbell, Chapter 23, pp.475-485

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Plant Scenario 1 Initial Population
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Late Spring Freeze
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Late Spring Freeze
Who dies? . . .and therefore, does not
reproduce.....
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Consequence of Late Spring Freeze?
Clicker Q1
A increase decrease
B increase no change
C no change no change
D decrease increase
E decrease no change
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Survivors
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Survivors Reproduce ? All Blue Population

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Plant Scenario 1 Genetic Drift

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

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Plant Scenario 2 Initial Population
Q What if blue was better able to survive the
effects of a spring freeze?
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LATE SPRING FREEZE
Q What if blue was better able to survive the
effects of a spring freeze?
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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...
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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
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Survivors
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Survivors Reproduce
The outcome is All Blue Population. Same end
result as Scenario 1. Is the mechanism the same?
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Plant Scenario 3 Initial Population
A squirrel drops a seed from a fruit it found
across the road.
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Immigration new alleles are introduced
into the population by interbreeding
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What happens over time. . .
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Gene Flow

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

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Summary Mechanisms of Evolution

• Selection (natural, sexual, artificial)
• Genetic Drift (random events in environment)
• Gene Flow (immigration/emigration)

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Scales of Evolution

• Microevolution Macroevolution

Small scale changes in allele frequencies
Variability in population traits
Large-scale changes Origin of new species
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What is a species?

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

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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

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Could speciation happen here?
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A road is built that prevents pollen exchange.
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Resulting subpopulations cant interbreed due to
barrier
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Divergence
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Divergence

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

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can no longer interbreed with
If
then the 2 subpopulations have achieved
reproductive isolation ? new species!
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Reminder Scales of Evolution

• Microevolution Macroevolution

Small scale changes in allele frequencies
Variability in population traits
Large-scale changes Origin of new species
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Speciation

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