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Transcript
Objectives
1. Determine whether evolution occurs by
mechanisms other than selection.
2. Compare and contrast population
outcomes resulting from selection versus
random events.
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 Freeze
Who dies? . . .and therefore, does not reproduce.....
Clicker Q1
A
B
C
D
E
Consequence of Late Spring Freeze?
increase
increase
no change
decrease
decrease
decrease
no change
no change
increase
no change
Survivors
Survivors Reproduce  All Blue Population
Plant Scenario 1: Genetic Drift
• Change in allele frequencies in a
population over time due to random
(stochastic) events in environment
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?
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?
Who dies? . . .and therefore,
does not reproduce
Survivors
Survivors Reproduce
The 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
Small scale changes in allele
frequencies
Variability in population traits
Macroevolution
Large-scale changes
Origin 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
Small scale changes in allele
frequencies
Variability in population traits
Macroevolution
Large-scale changes
Origin of new species
Speciation
• Origination of new species resulting from
reproductive isolation of populations that
have diverged