2. Evolutionary Forces
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Transcript of 2. Evolutionary Forces
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2007-2008
Evolutionary ForcesWhat changes populations?
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Evolution is…A change in gene frequency within a
population’s gene poolCan occur via
Natural SelectionVarious types of selection pressures
Genetic DriftRandom occurrences
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Forces of evolutionary change
Natural selectiontraits that improve survival
or reproduction will accumulate in the populationadaptive change
Includes predation, physiological, sexual selection & coevolution
Genetic driftfrequency of traits can change
in a population due to chance eventsrandom changeIncludes founder & bottleneck effects
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Natural SelectionSelection acts on any trait that affects survival
or reproductionpredation selectionphysiological selectionsexual selection
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Predation Selection
Predation selectionSelects for traits that affect survival act on both predator & prey
behaviorscamouflage & mimicryspeeddefenses (physical & chemical)
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Physiological SelectionActing on body functions
disease resistancephysiology efficiency (using oxygen, food,
water)biochemical versatilityprotection from injury
HOT STUFF!Some fish had thevariation of producinganti-freeze protein
5.5 mya5.5 myaThe Antarctic Ocean The Antarctic Ocean freezes overfreezes over
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Sexual Selection
Acting on reproductive successattractiveness to potential matefertility of gametessuccessful rearing of offspring
Survival doesn’t matterif you don’t reproduce!Organisms want their genes prevail and live on!
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The lion’s mane…
Females are attracted to males with larger, dark manes
Correlation with higher testosterone levelsbetter nutrition & healthmore muscle & aggressionbetter sperm count / fertility longer life
But imposes a cost to maleHOT! Is it worth it??
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CoevolutionTwo or more species reciprocally
affect each other’s evolutionpredator-prey
disease & hostcompetitive speciesmutualism
pollinators & flowers
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Effects of SelectionChanges in the average trait of a population
DIRECTIONALSELECTION
STABILIZINGSELECTION
DISRUPTIVESELECTION
giraffe neckhorse size human birth weight rock pocket mice
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Genetic Drift Chance events changing frequency of traits in a
populationnot adaptation to environmental conditions
not selectionfounder effect
small group splinters off & starts a new colonybottleneck
some factor (disaster) reduces population to small number & then population recovers & expands again but from a limited gene pool
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Founder effectWhen a new population is started
by only a small group of individuals just by chance some rare alleles may
be at high frequency; others may be missing
skew the gene pool of new populationhuman populations that
started from small group of colonists
example: colonization of New World
albino deer Seneca Army Depot
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Distribution of blood typesDistribution of the O type blood allele in native
populations of the world reflects original settlement
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Bottleneck effectWhen large population is drastically reduced by a
disasterfamine, natural disaster, loss of habitat… loss of variation by chance event
alleles lost from gene pool not due to fitness
narrows the gene pool
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Cheetahs All cheetahs share a small number of alleles
less than 1% diversityas if all cheetahs are
identical twins
2 bottlenecks10,000 years ago
Ice Age last 100 years
poaching & loss of habitat
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Any Questions??