The Mechanisms of Evolution
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Transcript of The Mechanisms of Evolution
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The Mechanisms of Evolution
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Life’s History: Seen in Fossils & Relics
• Broad patterns– Change through time
• communities change• habitats change
– Each of Earth’s biotas replaced a similar, but distinct biota
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Modern Life: Seen by Direct Observation
• Modern life exhibits characteristic patterns– species are variable– in the short term
• species are stable• environments are stable
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Biological Evolution
• encompasses the changes in Earth’s biotas– detected in remnants of the changes found in
the modern biota
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The Contribution of Charles Darwin• Darwin proposed a mechanism by which
evolution may have occurred– based on observations in South America
• SA flora & fauna differed from European• temperate SA forms resembled tropical SA forms more than temperate European forms
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The Contribution of Charles Darwin• Darwin proposed a mechanism by which
evolution may have occurred– based on observations in South America,
especially the Galápagos Islands• ~600 miles west of Ecuador• each with dramatically different conditions and communities
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Darwin’s map
Darwin’s Travels
Figure 23.1
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The Contribution of Charles Darwin• Darwin observed stable, variable populations
– each possessed adaptations (n) to its environment
• Darwin imagined the different island populations came from a founding population– populations underwent adaptation (v) and
now thrive under different conditions
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The Contribution of Charles Darwin• In 1859 Darwin proposed a mechanism by
which adaptation may have occurred– species change over time (are not
immutable)– some changes enable species to more
effectively inhabit their environments– adaptive changes occur by natural selection
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The Concept of Natural Selection• artificial selection of domesticated species
mimics natural selection– artificial selection
• breeders retain desirable individuals and remove undesirable individuals
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Artificial Selection - PracticalFigure 23.4
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Artificial Selection - HobbyFigure 23.2
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Artificial Selection - ExperimentalFigure 23.5
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The Concept of Natural Selection• artificial selection of domesticated species
mimics natural selection– natural selection occurs
• when some individuals produce more offspring than other individuals
• because more individuals are produced than the environment can support–those best-suited to the conditions
survive & reproduce, others don’t
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The Concept of Natural Selection
• natural selection is a conservative process– in a stable environment, “average”
individuals will survive and reproduce– in a changing environment changes, those
best-suited to new conditions will survive and reproduce
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The Contribution of Charles Darwin• Darwin did not know the mechanisms of
heredity
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Evolution: change in the genetic composition of a population over time
• evolution is population-based• phenotypic variation in a population is due to
genotypic differences in individuals• evolution results from differential success of
individuals with different heritable phenotypes
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Evolution: change in the genetic composition of a population over time
• at one genetic locus– an individual has two alleles– a population may have many alleles– the sum of all alleles for all loci in a
population is its gene pool
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a population’s gene pool
for the X locus:
Figure 23.3
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a population’s gene pool
for the X locus:
Figure 23.3allele
frequenciesX1 = 0.2X2 = 0.5X3 = 0.3
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a population’s gene pool for the X locus:Figure 23.3
genotypefrequenciesX1X1 = 0.1 X1X2 = 0.1 X1X3 = 0.1 X2X2 = 0.3 X2X3 = 0.3 X3X3 = 0.1
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Evolution: change in the genetic composition of a population over time
• a population’s genetic structure– allele frequencies – genotype frequencies
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Evolution: change in the genetic composition of a population over time
• a genotype’s or phenotype’s relative contribution to the next generation = fitness– depends on the survival and reproductive
success of individuals with it
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the mathematics of population genetics• for a population with only two alleles, A & a,
at a locus– the frequency of allele A is p
and– the frequency of a is q = 1- p
• allele frequencies can be calculated from genotype frequenciesp = (2NAA + NAa)/2Nand q = (2Naa + NAa)/2N
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the mathematics of population genetics• equal allele frequencies do not imply equal
genotype frequencie• Figure 23.6
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the mathematics of population genetics• undisrupted, a population’s genetic structure
remains the same over time
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undisrupted, a
population’s genetic structure
remains the same over time
Figure 23.7p = 0.55
q = 0.45 q = 0.45
p = 0.55
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Hardy and Weinberg did the math
• a population in Hardy-Weinberg equilibrium– has allele frequencies p & q– has genotype frequencies p2, q2 and 2pqand – succeeding generations will have the same
genetic structure
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Hardy and Weinberg did the math
• Hardy-Weinberg equilibrium requires– random mating– a large population size– no migration– negligible mutation– stabilizing natural selection
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Hardy-Weinberg agents of evolution• changes in a population’s genetic structure
occur because of agents of evolution– mutation
• spontaneous, random changes • usually detrimental or neutral• may be pre-adaptive• natural rates are very low• rates of accumulation vary
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Hardy-Weinberg agents of evolution• changes in a population’s genetic structure
occur because of agents of evolution– gene flow
• migration incorporates new alleles or changes allele frequencies
• migration is typical among populations of the same species
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bottlenecks shrink populations abruptlyFigure 23.8
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Prairie Chicken -millions to <hundred
Figure 23.9
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• changes in a population’s genetic structure occur because of agents of evolution– random genetic drift:
• chance events that alter allele frequencies–most likely in small populations–bottlenecks shrink populations abruptly–the founder effect occurs when a small sub-population is displaced
Hardy-Weinberg agents of evolution
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founder effect occurs when a
smallsub-population
isdisplaced
Figure 23.10
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Hardy-Weinberg agents of evolution• changes in a population’s genetic structure
occur because of agents of evolution– random genetic drift:
• chance events that alter allele frequencies–bottlenecks & the founder effect
produce low allelic variation compared to the parent population
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Hardy-Weinberg agents of evolution• changes in a population’s genetic structure
occur because of agents of evolution– assortative mating
• one genotype prefers another genotype• results in changed genotype frequencies
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Assortative Mating in PrimulaFigure 23.11
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Hardy and Weinberg did the math• a population in Hardy-Weinberg equilibrium
– has allele frequencies p & q– has genotype frequencies p2, q2 and 2pqand – succeeding generations with have the same
genetic structure
IF…
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Hardy and Weinberg did the math• Hardy-Weinberg equilibrium requires
– random mating– a large population size– no migration– negligible mutation– stabilizing natural selection
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Hardy-Weinberg agents of evolution• changes in a population’s genetic structure
occur because of agents of evolution– natural selection
• enhanced reproductive success by individuals with particular genotypes
• may lead to a change in allele frequency• leads to adaptation (v.)
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Natural Selection May Have Different Effects Under Different Circumstances
Figure 23.12
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Figure 23.13
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See page 460
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Figure 23.14
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two food sources that differ significantly in
hardness
produce a bimodal distribution
in
beak sizesFigure 23.15