populations are the units of evolution
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Populations are the units of evolution. Figure 13.6. Why do organisms change?. What is evolving? gene pool, microevolution Five agents of evolution 3. Types of natural selection. - PowerPoint PPT PresentationTRANSCRIPT
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Populations are the units of evolution
Figure 13.6
1. What is evolving? gene pool, microevolution
2. Five agents of evolution
3. Types of natural selection
Why do organisms change?
• Evolution happens when populations of organisms with inherited variations are exposed to environmental factors that favor the reproductive success of some individuals over others
Figure 1.6C
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Populations are the units of evolution
• A population is a group of interbreeding individuals
• A species is a group of populations whose individuals can interbreed and produce fertile offspring
Figure 13.6
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• gene pool = total collection of genes in a population at any one time
• Microevolution is a change in the relative frequencies of alleles in a gene pool
What is evolving?
Five agents of microevolution
1. Mutation changes alleles
2. Genetic drift = random changes in allele frequency
Bottleneck Founder effect
LARGE POPULATION = 10,000 SMALL POPULATION = 10
allele frequency =1,000
10,000 = 10% allele frequency =110 = 10%
50% of population survives,including 450 allele carriers
50% of population survives,with no allele carrier amongthem
allele frequency =450
5,000 = 9% allele frequency =05 = 0%
little change in allele frequency(no alleles lost)
dramatic change in allele frequency(potential to lose one allele)
Genetic drift - effects of population size:
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Founder effect
Figure 13.11B, C
Bottleneck effect
Population size is critical in preserving species.
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3. Gene flow can change a gene pool due to the movement of genes into or out of a population
ex. Migration 4. Nonrandom mating within a population
5. Natural selection leads to differential reproductive success
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• Male and female lions
Figure 13.20x
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• Natural selection
- results in the accumulation of traits that adapt a population to its environment
- the only agent of evolution that results in adaptation.
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What are sources of genetic variation?
• Mutation can create new alleles, new genes.
• Sex - Recombination of genes in sexual reproduction
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• Natural selection tends to reduce variability in populations. Mechanisms which counteract:
– The diploid condition preserves variation by “hiding” recessive alleles (Bb)
– Balanced polymorphism (2+ phenotypes stable in population) may result from:
1. heterozygote advantage Aa > aa and AA
2. frequency-dependent selection
3. variation of environment for a population
Why doesn’t natural selection eliminate all genetic variation in populations?
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• Many populations exhibit polymorphism and geographic variation
Figure 13.13
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• Neutral; no apparent advantage or disadvantage
– Example: human fingerprints
Not all genetic variation may be subject to natural selection
Figure 13.16
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• Low genetic variability may reduce their capacity to survive as humans continue to alter the environment
– cheetah populations
Endangered species often have reduced variation
Figure 13.17
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• the contribution it makes to the gene pool of the next generation relative to the contribution made by other individuals
• Production of fertile offspring is the only score that counts in natural selection
What is an organism’s evolutionary fitness?
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There are three general outcomes of natural selection
Figure 13.19
Freq
uenc
y of
indi
vidu
als
Originalpopulation
Phenotypes (fur color)
Originalpopulation
Evolvedpopulation
Stabilizing selection Directional selection Diversifying selection
beak depth
1976
1978Averagebeak depth,1978
Averagebeak depth,
1976
Beak depth (mm)Shift of average beakdepth during drought
5 6 7 8 9 10 11 12 13 140
20
40
60
80Nu
mbe
r of i
ndiv
idua
ls
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• The excessive use of antibiotics is leading to the evolution of antibiotic-resistant bacteria
– Example: Mycobacterium tuberculosis
Figure 13.22
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• This is due to:
– historical constraints
– adaptive compromises
– chance events
– availability of variations
Natural selection cannot fashion perfect organisms
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• appearance alone does not always define a species
Figure 14.1A
– Example: eastern and western meadowlarks
What is a species?
What is a species?
• Naturally interbreeding populations- potentially interbreeding- reproductively isolated from other species
What about asexually reproducing organisms?Extinct species?Shy species?
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• When geographically isolated, species evolution may occur– gene pool then changes to cause
reproductive isolation
= allopatric speciation
When does speciation occur?MECHANISMS OF SPECIATION
Figure 14.3
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• A ring species may illustrate the process of speciation
Figure 14.1C
OREGONPOPULATION
1
2
COASTALPOPULATIONS
Yellow-eyed
Monterey 3
SierraNevada
Yellow-blotched
Gap in ring Large-
blotched
INLANDPOPULATIONS
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Reproductive barriers between species
• Habitat - different locations
• Timing - mating, flowering
• Behavioral - mating rituals, no attraction
• Mechanical - structural differences
• Gametic - fail to unite
• Hybrid weak or infertile
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• Courtship ritual in blue-footed boobies is an example of behavioral isolation
• Many plant species have flower structures that are adapted to specific pollinators
– mechanical isolationFigure 14.2A, B
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• Hybrid sterility is one type of postzygotic barrier
– A horse and a donkey may produce a hybrid offspring, a mule
– Mules are sterile
Figure 14.2C
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Sympatric speciation
• No geographical isolation
• Mutation creates reproductive isolation
• Polyploidization
• Hybridization
Mediumground finch
Cactusground finch
Smalltree finch
Largeground finch
Smallground finch
Large cactusground finch
Sharp-beakedground finch
Vegetarianfinch
Seedeaters
Ground finches
Cactus flowereaters
Budeaters
Tree finches
Insecteaters
Mediumtree finch
Largetree finch
Mangrovefinch
Woodpeckerfinch
Greenwarbler finch
Graywarbler finch
Warbler finches
Common ancestor fromSouth America mainland
When does speciation occur?
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• Specialists - Galapagos finches
• Generalists - horseshoe crabs, cockroaches
• New environments
- ecological niche
When does speciation occur?
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• Adaptive radiation on an island chain
- specialization for different niches
Figure 14.4B
Species Afrom mainland
1
A
2B
B
3BC 4
C
C5
BC
D
C D
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• Continental drift is the slow, steady movement of Earth’s crustal plates on the hot mantle
Continental drift has played a major role in macroevolution
Figure 15.3A
PacificPlate
NorthAmerican
Plate
NazcaPlate
SouthAmerican
Plate
AfricanPlate
EurasianPlate
Splitdeveloping
Indo-AustralianPlate
Edge of one plate being pushed over edge of neighboring plate (zones of violent geologic events)
Antarctic Plate
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• influenced the distribution of organisms
– Continental mergers triggered extinctions
– Separation of continents caused the isolation and diversification of organisms
Figure 15.3B
Mill
ions
of y
ears
ago
EurasiaCEN
OZO
ICM
ESO
ZOIC
PALE
OZO
IC
North America
AfricaIndiaSouth
America
AntarcticaAustralia
Laurasia
Gondwana
Pangaea
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Speciation - how much change is needed?
• Gradual vs. jerky
• Evidence:
– Fossil record
– Genetic differences between species
– Homeotic genes
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• homeotic genes control body development
• Single mutation can result in major differences in body structure
Figure 11.14
Mouse chromosomes
Mouse embryo (12 days)
Adult mouse
Fly chromosomes
Fruit fly embryo (10 hours)
Adult fruit fly