How Populations Evolve
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Transcript of How Populations Evolve
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How Populations Evolve
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Charles Darwin : Theory of Evolution by Natural Selection
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Evolution – Change over time of heritable characteristics
• Selection is a major driving force for evolution• Members of a population vary in their inherited traits• All species are capable of producing more offspring
than the environment can support• Individuals whose inherited traits give them a higher
probability of reproducing in a given environment leave more offspring
• Unequal reproduction leads to accumulation of favorable traits in a population over generations (adaptation to the environment)
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Artificial Selection
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Natural Selection
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Fossil Record – Evidence for Evolution
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Tiktaalik – “fishapod”
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Archaeopteryx dino to bird transition
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Whale evolution
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Multiple Lines of Scientific evidence support evolution
• Biogeography – closely related species live in close proximity. Especially apparent on islands.
• Comparative Anatomy – closely related species share common anatomical features (homology).
• Molecular Biology – genetic code is shared across all life. Closely related species have more similar genetic sequences.
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Homology across mammal forearms
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Vestigial Structures
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Phylogeny – evolutionary tree
Tetrapodlimbs
Amnion
Lungfishes
Amphibians
Mammals
Lizardsand snakes
Crocodiles
Ostriches
Hawks andother birds
Feathers
TetrapodsA
mniotes
Birds
1
2
3
4
5
6
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Evolution of populations
• A population is a group of interbreeding individuals of the same species that live in the same region.– Have variation of traits within the population due
to mutation and sexual reproduction.– Populations evolve by changes in allele
frequencies.
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Mutations create new alleles, very rarely are these new mutations beneficial
Sexual reproduction results in reshuffling of allele combinations
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Hardy-Weinberg Equilibrium• A population whose allele frequencies do not
change (evolve) is in HWE
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Hardy-Weinberg Equations
p + q = 1
p2 + 2pq + q2 = 1
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Assumptions of HWE
• Very large population size• No gene flow between populations (no
migration)• No mutations• Random mating between individuals• No selection
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Genetic Drift• Random change in allele frequency due to
sampling effect– Stronger in smaller populations
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Natural Selection• Selection results in individuals that are
adapted to the environment to reproduce at a higher rate than those that are not.
• Over generations the frequency of alleles that are adaptive increase, while those that are maladaptive decrease.
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Sexual Selection leads to differences between sexes
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Balancing Selection maintains variation
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Natural Selection does not produce perfect organisms
• Selection can only act on existing variation• Evolution is limited by historical constraints
(what was adapted in the past)• Adaptations are often compromises• Drift, selection, and environment interact to
produce allele frequencies