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Page 1: Chapter  23

2004-2005 AP Biology

Chapter 23.

Evolution of Populations

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2004-2005AP Biology

Populations evolve Natural selection acts on individuals

differential survival “survival of the fittest”

differential reproductive success who bears more offspring

Populations evolve genetic makeup of

population changes over time

favorable traits (greater fitness) become more common Bent Grass on

toxic mine site

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Individuals DON’T evolve!!!

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Mutation & Variation Mutation creates variation

new mutations are constantly appearing

Mutation changes DNA sequence changes amino acid sequence? changes protein?

change structure? change function?

changes in protein may change phenotype & therefore change fitness

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Sex & Variation Sex spreads variation

one ancestor can have many descendants

sex causes recombination offspring have new combinations

of traits = new phenotypes

Sexual reproduction recombines alleles into new arrangements in every offspring

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Variation impacts natural selection Natural selection requires a source of

variation within the population there have to be differences some individuals must be more fit than

others

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Changes in populations Evolution of populations is really

measuring changes in allele frequency all the genes & alleles in a population =

gene pool Factors that alter allele frequencies

in a population natural selection genetic drift

founder effect bottleneck effect

gene flow

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Natural selection Natural selection adapts a population to

its environment a changing environment

climate change food source availability new predators or diseases

combinations of alleles that provide “fitness” increase in the population

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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: white people colonizing New World

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Distribution of blood types Distribution of the O type blood allele in native

populations of the world reflects original settlement

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Distribution of blood types Distribution of the B type blood allele in native

populations of the world reflects original migration

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Out of AfricaLikely migration paths of humans out of AfricaLikely migration paths of humans out of Africa

Many patterns of human traits reflect this migrationMany patterns of human traits reflect this migration

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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

alleles lost from gene pool 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 ability to

adapt at risk populations

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Gene flow Population spread over large area

migrations = individuals move from one area to another

sub-populations may have different allele frequencies

Migrations cause genetic mixing across regions = gene flow new alleles are moving

into gene pool 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?

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Any Questions??

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Chapter 23.

MeasuringEvolution of Populations

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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 would it be like if

allele frequencies didn’t change? non-evolving population

1. very large population size (no genetic drift)

2. no migration (movement in or out)

3. no mutation (no genetic change)

4. random mating (no sexual selection)

5. no natural selection (no selection)

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Hardy-Weinberg equilibrium Hypothetical, non-evolving population

preserves allele frequencies

Serves as a model 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 Alleles

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

frequencies must add to 100%, so:

p + q = 1

bbBbBB

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Hardy-Weinberg theorem 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 100%, so:

p2 + 2pq + q2 = 1

bbBbBB

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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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How do allele frequencies change?

Think of all the factors that would keep a population out of H-W equilibrium!

Think of all the factors that would keep a population out of H-W equilibrium!

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Real world application of H-W Frequency of allele in human population

Example: What % of human population carries allele for

PKU (phenylketonuria) Should you screen prospective parents?

~ 1 in 10,000 babies born in the US is born with PKU results in mental retardation, if untreated

disease is caused by a recessive allele

PKU = homozygous recessive (aa)

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H-W & PKU disease frequency of homozygous recessive individuals

q2 (aa) = 1 in 10,000 = 0.0001 frequency of recessive allele (q):

q = √0.0001 = 0.010.01 frequency of dominant allele (p):

p (A) = 1 – 0.01 = 0.99 frequency of carriers, heterozygotes:

2pq = 2 x (0.99 x 0.01) = 0.0198 = ~2% ~2% of the US population carries the PKU allele

300,000,000 x .02 = 6,000,000 people

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Any Questions??


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