2. evolutionary forces

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??