AP Biology 2007-2008

Chapter 23The Evolution of Populations

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5 Types of SelectionActs to select the individuals that are best adapted for survival and reproduction; resulting in alleles being passed to the next generation in proportions different than frequencies in the present generation

StabilizingDisruptive (Diversifying)DirectionalSexualArtificial

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Stabilizing selection: operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness. (Birth Weights)

Directional selection: An extreme trait makes an organism more fit making one phenotype replace another (Glacier Lilies/pesticides and insects)

Disruptive selection: a process that splits a population into two groups resulting in balanced polymorphism (black-bellied seed cracker finches)

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Effects of Selection Changes in the average trait of a population

DIRECTIONALSELECTION

STABILIZINGSELECTION

DISRUPTIVESELECTION

giraffe neckhorse size human birth weight rock pocket mice

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Natural selection in action (Directional Selection)

Insecticide & drug resistance insecticide didn’t

kill all individuals resistant survivors

reproduce (selective advantage)

resistance is inherited insecticide becomes

less & less effective

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Sexual Selection Acting on reproductive success

attractiveness to potential mate fertility of gametes successful rearing of offspring

Acts in all sexually reproducing species the traits that get you mates

sexual dimorphism (intersexual selection:

Female picks best looking male) influences both morphology &

behavior (intrasexual selection: males

Fight, winner is more desirable to female) can act in opposition to natural selection

Survival doesn’t matterif you don’t reproduce!

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Artificial Selection Humans breeding plants and animals

by seeking individuals with desired traits as a breeding stock animals, plants, etc.

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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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Artificial selection Artificial breeding can use variations in

populations to create vastly different “breeds” & “varieties”

“descendants” of the wolf

“descendants” of wild mustard

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Preserving Variation in a Population Balanced Polymorphism Geographic Variation Sexual Reproduction Outbreeding Diploidy Heterozygote Advantage Frequency-Dependent Selection Evolutionary Neutral Traits

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Balanced Polymorphism The presence of two or more

phenotypically distinct forms of a trait in a single population of a species.

each morph is better adapted in a different area, but

both varieties continue to exist

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Heterozygote Advantage• Heterozygote advantage occurs when

heterozygotes have a higher fitness than do both homozygotes; preserving multiple alleles in a population

• Natural selection will tend to maintain two or more alleles at that locus

• The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

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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

Frequency of sickle cell allele & distribution of malaria

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Frequency-Dependent Selection AKA: minority advantage

decrease of the more common phenotypes, increase of the less common phenotypes

ex: predator-prey relationships allow the less common phenotypes to succeed and reproduce

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Geographic Variation Graded variation in the phenotype of an

organism (cline): variation in appearances due to differences in environments ex. north-south cline

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Sexual Reproduction Variation due to shuffling and

recombination of alleles during meiosis and fertilization independent assortment crossing over random fertilization

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Outbreeding the mating of organisms within one

species that are not closely related; maintaining variation and a strong gene pool

This is one result you get when you Google Image “outbreedin

g”

This is true example

“outbreeding”

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Diploidy Diploidy (2n) maintains genetic

variation in the form of hidden recessive alleles, that could be advantageous when conditions change

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Evolutionary Neutral Traits traits that seem to have no selective

advantage ex.) blood types

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Causes of Evolution of a Population Genetic Drift

Bottleneck effect, flounder effect

Gene Flow Mutations Nonrandom Mating Natural Selection

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Genetic Drift Chance events changing frequency of

traits in a population not adaptation to environmental conditions

not selection founder effect

small group splinters off & starts a new colony bottleneck

some factor (disaster) reduces population to small number & then population recovers & expands again but from a limited gene pool

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Founder effect When 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 population human populations that

started from small group of colonists

example: colonization of New World

albino deer Seneca Army Depot

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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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Gene Flow the movement of alleles into or out of a

population via migration of fertile individuals or gametes between populations

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Mutations changes in genetic material; increasing

diversity either at one loci or several

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Nonrandom Mating Rarely is mating

completely random in a population.

Usually individuals mate with individuals in close proximity.

This promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits

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Forces of evolutionary change Natural selection

traits that improve survival or reproduction will accumulate in the population adaptive change

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Natural Selection Selection acts on any trait that affects

survival or reproduction predation selection physiological selection sexual selection

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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 Hardy Weinberg Equilibrium states that allele and

genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences.

hypothetical: what conditions would cause allele frequencies to not change?

non-evolving populationREMOVE 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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If all five conditions are met then a population is considered to be in equilibrium and NO EVOLUTION is occurring

1) no genetic drift2) no gene flow3) no nonrandom mating4) no mutations5) no natural selection

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Hardy-Weinberg equilibrium Hypothetical, non-evolving population

preserves allele frequencies

Serves as a model (null hypothesis: no relationship between two results) 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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The Hardy-Weinberg Equation enables us to calculate frequencies of

alleles in a population p2 + 2pq + q2 = 1 or p + q = 1 p= dominant allele q= recessive allele

AA Aa

Aa aa

A

A

a

a

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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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Any Questions??Any Questions??Insert Practice Problems

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1. Given a population in Hardy-Weinberg equilibrium with allele frequencies A =0.9 and a = 0.1, determine the frequencies of the three genotypes AA, Aa and aa.

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3. Allele W for white wool is dominant over allele w for black wool. In a sample of 900 sheep, 891 are white and 9 are black. Estimate the allelic frequencies in this sample.

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6. 1 in 1,700 US Caucasian newborns have cystic fibrosis. C for normal is dominant over c for cystic fibrosis.

a) What percentage of the above population has cystic fibrosis (cc)?

b) Calculate the frequencies of the C and c alleles. c) Calculate the frequencies of the normal (CC) and carrier (Cc)

genotypes. d) How many of the 1,700 population members are normal

(CC)? Carriers (Cc)? e) It has been found that a carrier is better able to survive

diseases with severe diarrhea. What would happen to the frequency of the “c” if there was an epidemic of cholera or other type of diarrhea producing disease? Would “c” increase or decrease?

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9. In a tropical forest there is a species of bird that has a variable tail length. Long is incompletely dominant over short. In one population of 2000 birds, 614 have long tails, 973 have medium length tails, and 413 birds have short tails.

a) What is the frequency of each allele in the population? b) Is the population in Hardy Weinberg equilibrium?

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THE END

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Speciation and Reproductive Isolation Species: members in a population who have

the potential to interbreed in nature and produce viable, fertile offspring Reproductive isolation: one group of genes

becomes isolated from one another to begin a separate evolutionary history

Speciation: anything that fragments a population and isolates a small group of individuals Allopatric Sympatric

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Allopatric Speciation: caused by geographic isolation

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Sympatric Speciation: caused by anything besides geographic isolation polyploidy: a cell has two or more complete sets of chromosomes habitat isolation: two organisms live in the same area but rarely

encounter one another behavioral isolation: two species do not mate because of

differences in courtship behavior temporal isolation: populations may mate or flower at different

seasons or different times of day reproductive isolation: closely related species unable to mate

because of a variety of reasons prezygotic barriers postzygotic barriers reproductive isolation

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Patterns of Evolution Divergent Convergent Parallel Coevolution Adaptive Radiation Gradualism Punctuated

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Divergent Evolution Occurs when a population becomes

isolated from the rest of the species, exposed to new selective pressures, and evolves into a new species allopatric speciation sympatric speciation

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Convergent evolution Flight evolved in 3 separate animal groups

evolved similar “solution” to similar “problems” analogous structures

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Convergent evolution Fish: aquatic vertebrates Dolphins: aquatic mammals

similar adaptations to life in the sea

not closely related

Those fins & tails & sleek bodies are

analogous structures!

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Parallel Evolution Convergent evolution in common niches

filling similar ecological roles in similar environments, so similar adaptations were selected

but are not closely related

marsupial mammalsmarsupial mammals

placental mammalsplacental mammals

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Parallel types across continentsNiche Placental Mammals Australian Marsupials

BurrowerMole

Anteater

Mouse

Lemur

Flyingsquirrel

Ocelot

Wolf Tasmanian “wolf”

Tasmanian cat

Sugar glider

Spotted cuscus

Numbat

Marsupial mole

Marsupial mouse

Anteater

Nocturnalinsectivore

Climber

Glider

Stalkingpredator

Chasingpredator

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Coevolution Two or more species reciprocally

affect each other’s evolution predator-prey

disease & host competitive species mutualism

pollinators & flowers

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Adaptive Radiation the emergence of

numerous species from a common ancestor introduced into an environment, filling a niche