Post on 20-Jun-2015
2007-2008
Evolutionary ForcesWhat changes populations?
Evolution is…A change in gene frequency within a
population’s gene poolCan occur via
Natural SelectionVarious types of selection pressures
Genetic DriftRandom occurrences
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
Natural SelectionSelection acts on any trait that affects survival
or reproductionpredation selectionphysiological selectionsexual selection
Predation Selection
Predation selectionSelects for traits that affect survival act on both predator & prey
behaviorscamouflage & mimicryspeeddefenses (physical & chemical)
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
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!
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??
CoevolutionTwo or more species reciprocally
affect each other’s evolutionpredator-prey
disease & hostcompetitive speciesmutualism
pollinators & flowers
Effects of SelectionChanges in the average trait of a population
DIRECTIONALSELECTION
STABILIZINGSELECTION
DISRUPTIVESELECTION
giraffe neckhorse size human birth weight rock pocket mice
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
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
Distribution of blood typesDistribution of the O type blood allele in native
populations of the world reflects original settlement
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
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
Any Questions??