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Evolution of Populations
Chapter 23
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Macroevolution
Evolution on a large scale Changes in plants & animals Where new forms replace old Major episodes of extinction
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Microevolution
Changes within a population Changes in allele frequencies Leads to adaptation of an
organism
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Variation
Gene variation Driving force behind evolution New genes & alleles can arise by
mutation or gene duplication Sexual reproduction
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Genetic Variation from Sexual Recombination
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Population genetics
Study of the properties of genes in populations
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Population
Group of individuals Same species Interbreed Fertile offspring
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Population
Contains a great deal of variation Variation-raw material for
evolution
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Gene pool
All the alleles Of all individuals within a
population
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Hardy-Weinberg Principle
Determines if population is evolving
Frequencies of alleles in population Used for baseline of genes in a
population
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Hardy-Weinberg
Equilibrium When proportions of genotypes
remain the same Generation to generation
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Hardy-Weinberg
Original proportions of genotypes in a population remain constant if
1. Large population 2. Random mating 3. No mutations 4. No gene flow 5. No natural selection
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Hardy-Weinberg
P+q=1 alleles
p=dominant q=recessive
p2 + 2pq + q2 = 1 genotypes
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Hardy-Weinberg
84 black 16 white (100 total)
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Hardy-Weinberg
p2 + 2pq + q2 = 1 P + q=1
q2 = .16 q = .4 p = .6 p2 = .36 2pq = .48
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Hardy-Weinberg
If the dominant allele is 30% of the gene pool
What is % dominant phenotype % recessive phenotype % hybrid
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Hardy-Weinberg
Factors that affect evolutionary change
1. Mutations 2. Nonrandom mating 3. Gene flow 4. Genetic drift 5. Natural selection
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Mutation
Occurs at a low rate Not a strong influence on
evolutionary change
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Nonrandom mating
Individuals with one genotype mate with another at a greater rate
Not a strong influence on allele frequency
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Gene flow
Movement of alleles from one population to another
Populations exchange genetic information
Example New animal comes into population Mates & survives
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Gene flow
Bees and pollen Seeds Reduces genetic differences
between populations
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Gene flow
Insecticide resistant alleles Mosquito West Nile & Malaria Spreading the allele
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Gene flow
Advantage when a beneficial mutation enters a population
Select for the allele Disadvantage when an inferior
allele enters the population Select against the allele
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Genetic drift
Change in allele frequency due to chance alone
Small populations
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Genetic drift
Only a few possible alleles are present
Example: Red, blue, yellow seeds If blue & yellow are isolated from
red Eventually the population will only
have blue or yellow and no red
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Genetic drift
May see a rise in harmful alleles Lose alleles
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Fig. 23-8-3
Generation 1
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
p (frequency of CR) = 0.7q (frequency of CW
) = 0.3
Generation 2
CR CWCR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
p = 0.5q = 0.5
Generation 3p = 1.0q = 0.0
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR CR CR
CR CR
CR CR CR CR
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Genetic drift
1. Founders effects Few individuals leave a population New isolated population Few alleles present Island populations Amish (polydactyly)
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..\..\..\Desktop\polydactyl.jpg
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Genetic drift
2. Bottleneck Occurs when a few surviving
individuals have only a few genes Loss of genetic variability Occurs when a natural event
happens– Flood, drought, disease etc.
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Fig. 23-9
Originalpopulation
Bottleneckingevent
Survivingpopulation
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Genetic drift
Northern elephant seal California Reduced to few seals in a
population due to hunting Has rebounded in numbers Organisms with limited genetic
variation
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Fig. 23-10a
Rangeof greaterprairiechicken
Pre-bottleneck(Illinois, 1820)
Post-bottleneck(Illinois, 1993)
(a)
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Selection
Natural selection the process that causes evolutionary change
Adaptive evolution
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Selection
Natural selection to happen & cause evolutionary change
1. Must have variation in individuals among population
Enables choice of traits that are better able to survive
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Selection
2. Variation causes different number of offspring surviving
3. Variation must be genetically inherited
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Selection
Individuals with a certain phenotype
Leave more surviving offspring than other phenotypes
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Relative fitness
Reproductive success Number of surviving offspring left
for the next generation Green vs brown frogs Green leave 4 offspring Brown leave 2.5 offspring More green mating eventually lose
the brown phenotype
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Relative fitness
1. Survival (how long) 2. Mating success 3. Number of offspring Examples: larger organisms mate
more Larger fish or frogs leave more
offspring
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Forms of selection
1. Disruptive selection 2. Directional selection 3. Stabilizing selection
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Forms of selection
1. Disruptive selection Eliminates intermediate type Favors extremes Example: African-bellied seed cracker finch Large beak Large seeds Small beak Small seeds
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Original population
(b) Disruptive selection
Phenotypes (fur color)
Fre
qu
enc
y o
f in
div
idu
als
Evolved population
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Forms of selection
2. Directional selection Favors one extreme
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Original population
(a) Directional selection
Phenotypes (fur color)
Fre
qu
enc
y o
f in
div
idu
als
Original population
Evolved population
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Forms of selection
3. Stabilizing selection Eliminates both extremes Example: birth weight of newborns Small & large newborns can be
harmful Increased death rate Intermediate BW best survival
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Original population
(c) Stabilizing selection
Phenotypes (fur color)
Fre
qu
enc
y o
f in
div
idu
als
Evolved population
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Selection
Environment imposes conditions Determines selection Cause evolutionary change.
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Selection
1. Selection to avoid predators Adaptation that decreases the
chance of being captured
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Selection
2. Selection to match climatic condition Enzyme alleles Vary depending on geographic location Fish enzyme for LDH Coverts pyruvate to lactate Works better in colder weather Fish swim faster
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Selection
3. Selection for pesticide resistance
Housefly developed a resistant target receptor
Do not absorb the insecticide Rats have developed resistance to
Warfarin (blood thinner)
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Sexual selection
Sexual dimorphism: Differences in secondary sexual
characteristics Intrasexual selection: Selection between same sex Competing for mates Male fighting
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Sexual selection
Intersexual selection: Selection of mate Females choosing male mate “good genes”
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Fig. 23-15
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Fig. 23-19
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Maintaining variation
1. Frequency-dependent selection 2. Oscillating selection 3. Heterozgote advantage
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Frequency-dependent selection Fitness of a phenotype depends on
frequency within population Negative frequency-dependent
selection Rare phenotypes favored Predator preys on the more common
phenotype Allowing less common phenotype to
thrive
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Frequency-dependent selection Positive frequency-dependent
selection Predator feeds on rare phenotype Favoring common phenotype
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Oscillating selection
When one phenotype is favored at one time
Another phenotype is favored at a different time
Birds beak size and drought
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Heterozygote advantage
Favored genotype has both alleles Example: sickle cell anemia Heterozygous for disease does
better against malaria
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The Sickle-Cell Allele
Events at the Molecular Level
Sickle-cell alleleon chromosome
Template strand
Effects on IndividualOrganisms
Consequences for Cells
Fiber
An adeninereplaces a thymine.Wild-type
allele
Sickle-cellhemoglobin
Low-oxygenconditions
Sickled redblood cell
Normal redblood cell
Normal hemoglobin(does not aggregate
into fibers)
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The Sickle-Cell Allele
Evolution in Populations
KeyFrequencies ofthe sickle-cell allele
Distribution of malariacaused by Plasmodium falciparum(a parasitic unicellular eukaryote)
3.0–6.0% 6.0–9.0% 9.0–12.0%12.0–15.0% 15.0%