AP Biology 2006-2007

Ch. 22 - Evolutionby

Natural Selection

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This is not just a process of the past…

It is all around us today

Artificial selection

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Selective breeding

the raw genetic material (variation) is hidden there

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Selective breeding

Hidden variation can be exposed through selection!

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In historical contextOther people’s ideas paved the

path for Darwin’s thinking

competition:struggle for survivalpopulation growth

exceeds food supply

land masses change over immeasurable time

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A Reluctant Revolutionary Returned to England in 1836

wrote papers describing his collections & observations

long treatise on barnacles draft of his theory of

species formation in 1844 instructed his wife to

publish this essay upon his death

reluctant to publish but didn’t want ideas to die with him

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And then came the letter….

Alfred Russel Wallace a young naturalist working in the East Indies, had written a short paper with a new idea. He asked Darwin to evaluate his ideas and pass it along for publication.

Alfred Russel Wallace a young naturalist working in the East Indies, had written a short paper with a new idea. He asked Darwin to evaluate his ideas and pass it along for publication.

Then, in 1858, Darwin received a letter Then, in 1858, Darwin received a letter that changed everything…that changed everything…

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The time was ripe for the idea!

Your words have come true

with a vengeance… I never saw a more striking

coincidence…so all my originality, whatever it may amount to,

will be smashed.

To Lyell—

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Voyage: 1831-1836

November 24, 1859, Darwin published

“On the Origin of Species by Means of Natural Selection”

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Essence of Darwin’s ideas Natural selection

variation exists in populations over-production of offspring

more offspring than the environment can support competition

for food, mates, nesting sites, escape predators differential survival

successful traits = adaptations differential reproduction

adaptations become more adaptations become more common in populationcommon in population

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LaMarckian vs. Darwinian view

LaMarck in reaching higher

vegetation giraffes stretch their necks & transmits the acquired longer neck to offspring

Darwin giraffes born with longer

necks survive better & leave more offspring who inherit their long necks

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Take out the sheets from

Yesterday – complete!

AP Biology 2006-2007

Slide & Image Storage

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Living creatures havechanged Earth’s environment,

making other life possible

Life has changed over time & in turn has changed the Earth

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Evolution as Change Over Time

Evolution!

Evolution!

Evolution!

idea acceptedidea acceptedbefore Darwinbefore Darwin

Evolution!

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GalapagosRecent volcanic origin most of animal species on the Galápagos live nowhere else in world, but they resemble species living on South American mainland.

500 miles west of mainland

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AP Biology 2006-2007

"Nothing in biology makes sense except in the light of evolution."

-- Theodosius DobzhanskyMarch 1973

Geneticist, Columbia University

(1900-1975)

Coherent explanation of observations

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(5)(5) Over long periods of time, and given a steady input of Over long periods of time, and given a steady input of new variation into a population, these processes lead to new variation into a population, these processes lead to the the emergence of new speciesemergence of new species

Essence of Darwin’s ideas(1)(1) VariationVariation exists in natural populations exists in natural populations

(2)(2) Many Many more offspringmore offspring are born each season are born each season than can possibly survive to maturitythan can possibly survive to maturity

(3)(3) As a result, there is a As a result, there is a struggle for existencestruggle for existence

- - competitioncompetition

(4)(4) Characteristics beneficialCharacteristics beneficial in the struggle in the struggle for existence will tend to become more for existence will tend to become more common in the population, changing the common in the population, changing the average characteristics of the populationaverage characteristics of the population

- - adaptationsadaptations

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Stick your neck out…Ask Questions!

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The Birds… Galápagos birds

22 of the 29 species of birds on the Galapagos are endemic found only on these islands

collected specimens of all One particular group…

at first, he paid little attention to a series of small birds

some were woodpecker-like, some warbler-like, & some finch-like

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Darwin’s finches

Large-seed eater? Small-seed eater?

Warbler? Leaf-browser?

Finch? Sparrow?

Warbler? Wren?

Darwin was amazed to find out they were all finches 14 species but only one

species on South American mainland 500 miles away

all the birds had to originally come from mainland species

How didone species

of finches becomeso many different

ones now?

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MeasuringEvolution of Populations

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5 Agents of evolutionary changeMutation Gene Flow

Genetic Drift Selection

Non-random mating

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Populations & gene pools Concepts

a population is a localized group of interbreeding individuals

gene pool is collection of alleles in the population remember difference between alleles & genes!

allele frequency is how common is that allele in the population how many A vs. a in whole population

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Evolution of populations Evolution = change in allele frequencies

in a population hypothetical: what conditions would

cause allele frequencies to not change? non-evolving population

REMOVE all agents of evolutionary change

1. very large population size (no genetic drift)

2. no migration (no gene flow in or out)

3. no mutation (no genetic change)

4. random mating (no sexual selection)

5. no natural selection (everyone is equally fit)

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Hardy-Weinberg equilibrium Hypothetical, non-evolving population

preserves allele frequencies

Serves as a model (null hypothesis) natural populations rarely in H-W equilibrium useful model to measure if forces are acting

on a population measuring evolutionary change

W. Weinbergphysician

G.H. Hardymathematician

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Hardy-Weinberg theorem Counting Alleles

assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q

frequencies must add to 1 (100%), so:

p + q = 1

bbBbBB

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Hardy-Weinberg theorem Counting Individuals

frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq

frequencies of all individuals must add to 1 (100%), so:

p2 + 2pq + q2 = 1

bbBbBB

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H-W formulas Alleles: p + q = 1

Individuals: p2 + 2pq + q2 = 1

bbBbBB

BB

B b

Bb bb

AP BiologyWhat are the genotype frequencies?What are the genotype frequencies?

Using Hardy-Weinberg equation

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

population: 100 cats84 black, 16 whiteHow many of each genotype?

population: 100 cats84 black, 16 whiteHow many of each genotype?

bbBbBB

p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16

Must assume population is in H-W equilibrium!

Must assume population is in H-W equilibrium!

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Using Hardy-Weinberg equation

bbBbBB

p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16

Assuming H-W equilibriumAssuming H-W equilibrium

Sampled data Sampled data bbBbBB

p2=.74p2=.74 2pq=.102pq=.10 q2=.16q2=.16

How do you explain the data? How do you explain the data?

p2=.20p2=.20 2pq=.642pq=.64 q2=.16q2=.16

How do you explain the data? How do you explain the data?

Null hypothesis Null hypothesis

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Application of H-W principle Sickle cell anemia

inherit a mutation in gene coding for hemoglobin oxygen-carrying blood protein recessive allele = HsHs

normal allele = Hb

low oxygen levels causes RBC to sickle breakdown of RBC clogging small blood vessels damage to organs

often lethal

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Sickle cell frequency High frequency of heterozygotes

1 in 5 in Central Africans = HbHs

unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs

usually die before reproductive age

Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?

Suggests some selective advantage of being heterozygous…Suggests some selective advantage of being heterozygous…

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Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

1

2

3

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Heterozygote Advantage In tropical Africa, where malaria is common:

homozygous dominant (normal) die or reduced reproduction from malaria: HbHb

homozygous recessive die or reduced reproduction from sickle cell anemia: HsHs

heterozygote carriers are relatively free of both: HbHs

survive & reproduce more, more common in population

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele &

distribution of malaria