Transcript Agents of Evolutionary Change

Evolution of Populations
Raven Ch. 21
AP Biology
2007-2008
Doonesbury - Sunday February 8, 2004
Populations evolve
 Natural selection acts on individuals

Variation results in:
 differential survival
 “survival of the fittest”
 differential reproductive success
 who bears more offspring
 Populations evolve
genetic makeup of
population changes
over time
 favorable traits
(greater fitness)
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
Mummichog
Individuals
DON’Tare
evolve…
Individuals
survive
orselected
don’t survive…
Populations
evolve
Individuals
reproduce
or don’t…
AP Biology
2007-2008
Variation & natural selection
 Variation is the raw material for natural
selection

there have to be differences within population

some individuals must be more fit than
others
 Fitness- Survival & Reproductive success

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individuals with one phenotype leave more
surviving offspring
Where does Variation come from?

random changes to DNA
 errors in mitosis & meiosis
Wet year
Beak depth
 Mutation
Dry year
Dry year
 environmental damage

mixing of alleles
 recombination of alleles
 new arrangements in every offspring
 new combinations = new phenotypes

spreads variation
 offspring inherit traits from parent
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1980
1982
1984
11
Beak depth of
offspring (mm)
 Sex
1977
Dry year
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
Changes in populations
Bent Grass on toxic mine site
Pocket Mice in desert lava flows
Pesticide
molecule
Target site
Resistant
target site
Insect
cell site
Target
membrane
Insecticide resistance
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Decreased number of target sites
5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
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Non-random mating
Selection
1. Mutation & Variation
 Mutation creates variation
Mutation rates in individuals are low (1 in
100,000 division)
 But are constantly appearing in populations

 Mutation changes DNA sequence.

As a result, may change:
 Protein amino acid sequence
 may change protein structure?
 may change protein function?
 changes in protein may
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change phenotype &
therefore change fitness
2. Gene Flow
 Movement of individuals &
alleles in & out of populations
seed & pollen distribution by
wind & insect
 migration of animals

 sub-populations may have
different allele frequencies
 causes genetic mixing
across regions
 reduce differences
between populations
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Human evolution today
 Gene flow in human
populations is
increasing today

transferring alleles
between populations
Are we moving towards a blended world?
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3. Non-random mating
 Sexual selection
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4. Genetic drift
 Effect of chance events

founder effect
 small group splinters off & starts a new colony

bottleneck
 some factor (disaster) reduces population to
small number & then population recovers &
expands again
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5. Natural selection
 Differential survival & reproduction due
to changing environmental conditions
 climate change
 food source availability
 predators, parasites, diseases
 toxins

combinations of alleles
that provide “fitness”
increase in the population
 adaptive evolutionary change
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5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
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Non-random mating
Selection
Any Questions??
AP Biology
2005-2006