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Blue edged slides taken from slide shows by: Kim Foglia http://www.explorebiology.com

Chapter 23- Evolution of populations

Chapter 24-

Origin of species

AP Biology 2007-2008

Populations evolve

Individuals are selected

REZNICK & ENGLER- (1980’s)Guppy experiments

Changes in populations happened within 11 years

Age and size at sexual maturity change depending on predators

IMAGE FROM Campbell and Reece AP BIOLOGY

Small killifish eat juvenile guppies Large pike-cichlids eat adult guppies

Guppies in populations with pike-cichlid predatorsbegin reproducing at a younger age and are smallerat maturity than guppies in populations preyed on bykillifish

IMAGE FROM Campbell and Reece AP BIOLOGY

Moving guppies to pools with different predators changes size and age of maturity in population

Changes are heritable

IMAGE FROM Campbell and Reece AP BIOLOGY

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Changes in populationsPesticidemolecule

Insect cellmembrane

Target site

Resistanttarget site

Insecticide resistance

Target site

Decreased number of target sites

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Fitness

� Survival & Reproductive success

� individuals with one phenotype leave more surviving offspring

Body size & egg laying in water striders

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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

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Mean beak depth of parents (mm)

Medium ground finch8

8 9 10 11

9

10

11

1977 1980 1982 1984

Dry year

Dry year

Dry year

Wet year

Beak d

ep

thB

eak d

ep

th o

fo

ffsp

rin

g (m

m)

Where does Variation come from?� Mutation

� random changes to DNA

� errors in mitosis & meiosis

� environmental damage

� Sex

� 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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5 Agents of evolutionary changeMutation Gene Flow

Genetic Drift Selection

Non-random mating

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1. Mutation & Variation

� Mutation creates variation

� new mutations are constantly appearing

� Mutation changes DNA sequence

� changes amino acid sequence?

� changes protein?

� changes structure?

� changes function?

� changes in protein may change phenotype &

therefore change fitness

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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 mixingacross 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?Are we moving towards a blended world? AP Biology

3. Non-random mating

� Sexual selection

SEXUAL SELECTIONFavors traits with no advantage for survivalother than fact that males/females prefer them

Leads to pronounced differences between sexes =SEXUAL DIMORPHISM

http://informalfotos.com/Fauna/Male%20Peacock%20displaying.JPGhttp://www.distinctivecruises.com/AfricanSafaris/MaleFemaleLion.jpghttp://espanol.wunderground.com/data/wximagenew/i/icmoore/1013.jpg

KIN SELECTIONNatural selection that favors altruistic behavior by enhancing reproductive success of relatives

Bird that calls to warn others is in danger of being eaten,but does it anyway.

Image from: http://www.kittens-lair.net/store/en/articles/sylvester4.jpg

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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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Founder effect

� When a new population is started by only a few individuals

� some rare alleles may be at high

frequency; others may be missing

� skew the gene pool of

new population

� human populations that started from small group of colonists

� example:colonization of New World

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Bottleneck effect

� When large population is drastically reduced by a disaster

� famine, 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% diversity

� as if all cheetahs are

identical twins

� 2 bottlenecks

� 10,000 years ago

� Ice Age

� last 100 years

� poaching & loss of habitat

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Conservation issues

� Bottlenecking is an important concept in conservation

biology of endangered species

� loss of alleles from gene pool

� reduces variation

� reduces adaptability

Breeding programs must consciously outcrossBreeding programs must consciously outcross

Peregrine Falcon

Golden Lion Tamarin AP Biology

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

POLYGENIC traits are controlled by two or more genes.

A bell shaped curve is typical of polygenic traits

Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

DIRECTIONAL SELECTIONGraph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

Individuals at one end of the curve have higher fitness than individuals in middle or at other end.

Graph shifts as some individuals fail to survive at one end and succeed and reproduce at other

Low mortality, high fitness

High mortality, low fitness

KEY

Food becomes scarce.

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Beak size varies in a population

Birds with bigger beaks can feed more easily on harder, thicker shelled seeds.

A food shortage causes small and medium size seeds to run low.

Birds with bigger beaks would be selected for and increase in numbers in population.

EXAMPLE OF DIRECTIONAL SELECTION

http://www.animalbehavior.org/ABS/Stars/ONI/Podos_-_finch_graphic.jpg

STABILIZING SELECTIONGraph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

Individuals in center of the curve have higher fitness than individuals at either end

Graph stays in same place but narrows as more organisms in middle are produced.

Key

Brightness of

Feather Color

Selection against both

extremes keep curve narrow and in same

place.

Section 16-2

Low mortality, high fitness

High mortality, low fitness

Stabilizing SelectionMale birds use their plumage to attract mates.

Male birds with less brilliant and showy plumage are less likely to attract a mate

Male birds with showy plumage are more likely to attract a mate.

Male birds withshowier, brightlycolored plumagealso attractpredators, and are less likely to live long enough tofind a mate.

The mostfit is malebird in the middle--showy, but not too showy.

STABILIZING SELECTION

Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

Human babies born with low birth weight are less likely to survive.

Babies born too large have difficultybeing born.

Average size babies are selected for.

EXAMPLE OF STABILIZING SELECTION

DISRUPTIVE SELECTION

Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

Individuals at extremes of the curve

have higher fitness than individuals in middle.

Can cause graph to split into two. Selection creates two DIFFERENT PHENOTYPES

A bird population lives in area where climate change causes medium size seeds become scarce while large and small seeds are still plentiful.

Birds with bigger or smaller beaks would have greater fitness and thepopulation may split into TWO GROUPS. One that eats small seeds and one that eats large seeds.

EXAMPLE OF DISRUPTIVE SELECTION

http://www.animalbehavior.org/ABS/Stars/ONI/Podos_-_finch_graphic.jpg

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WHAT IS A SPECIES?

• Defined by Ernst Mayr• Population whose members can interbreed

AND produce viable, fertile offspring

TWO DISTINCT SPECIES Body and coloration are similar . . . but their songs and other behaviors are different enoughto prevent interbreeding

In the wild lions and tigers don’t interbreedbut in zoos… can get hybrids

LIGER - Male lion X female tigerTIGON- Male tiger X female lion

Largest cat in world. . . but male ligers are sterile

http://www.readthesmiths.com/articles/Images/Humor/Liger.jpghttp://laweekly.blogs.com/joshuah_bearman/images/tigon.jpg AP Biology

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Blue Footed Booby dance

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Hybrid

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(DIVERGENT EVOLUTION)

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CURRENT DEBATENOT over whether evolution happens

… overwhelming evidence here!

BUT . . .Does speciation happen gradually OR rapidly in response to environmental change?

GRADUALISM

Change happens slowly

“Baby steps” over longPeriods of time

CHARLES DARWINCHARLES LYELL

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PUNCTUATED EQUILIBRIUM

Niles Eldredge- Curator American Museum of Natural History

Rate of speciation is NOT CONSTANT

Rapid change when 1st split from parent population

Unchanging for long periods

Stephen Jay Gould (1941-2002) Paleontologist/evolutionary biologist

TIME

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