Download Natural selection

Populations are the units of evolution
Figure 13.6
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1. What is evolving?
gene pool, microevolution
2. Four agents of evolution
3. Types of natural selection
• 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
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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What is evolving?
• 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
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Four agents of microevolution
1. Mutation changes alleles
2. Genetic drift = random changes in allele
frequency
Bottleneck
Founder effect
Genetic drift - effects of population size:
LARGE POPULATION = 10,000
1,000
allele frequency = 10,000 = 10%
50% of population survives,
including 450 allele carriers
450
allele frequency = 5,000 = 9%
little change in allele frequency
(no alleles lost)
SMALL POPULATION = 10
allele frequency =
1
10
= 10%
50% of population survives,
with no allele carrier among
them
allele frequency =
0
5
= 0%
dramatic change in allele frequency
(potential to lose one allele)
Bottleneck effect
Founder effect
Figure 13.11B, C
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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. Natural selection leads to differential
reproductive success
Nonrandom mating changes genotype frequency
but not allele frequency.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Natural selection
- results in the accumulation of traits that adapt
a population to its environment
- the only agent of evolution that results in
adaptation.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
What are sources of genetic variation?
• Recombination of genes in sexual reproduction
• Mutation can create new alleles, new genes.
- homeobox genes can create major changes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
What is an organism’s evolutionary fitness?
• An individual’s Darwinian fitness is the
contribution it makes to the gene pool of the
next generation relative to the contribution
made by other individuals; i.e., number of
progeny
• Production of fertile offspring is the only score
that counts in natural selection
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There are three general outcomes of natural
selection
Frequency of
individuals
Original
population
Phenotypes (fur color)
Original
population
Evolved
population
Stabilizing selection
Directional selection
Diversifying selection
Figure 13.19
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80
beak depth
1976
Number of individuals
60
40
Average
beak depth,
1976
20
Average
beak depth,
1978
1978
0
5
6
7
8
9
10
11
12
13
Beak depth (mm)
Shift of average beak
depth during drought
14
20
70
Infant
deaths
60
Infant
births
15
50
Percent
of infant
deaths
Percent of
births in
10
population
40
30
20
5
10
0
2
3
4
5
6
7
8
Birth weight in pounds
9
10
11
0
Why doesn’t natural selection eliminate all genetic
variation in populations?
• 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
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• Many populations exhibit polymorphism and
geographic variation
Figure 13.13
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Not all genetic variation may be subject to natural
selection
• Some variations may be neutral, providing no
apparent advantage or disadvantage
– Example: human fingerprint patterns
Figure 13.16
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Endangered species often have reduced variation
• Low genetic variability may reduce their
capacity to survive as humans continue to alter
the environment
– cheetah populations have extreme genetic
uniformity
Figure 13.17
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Why do male and female animals differ in appearance?
• Sexual selection leads to the evolution of
secondary sexual characteristics
• Sexual selection may produce sexual
dimorphism
Figure 13.20A, B
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Natural selection cannot fashion perfect organisms
• This is due to:
– historical constraints
– adaptive compromises
– chance events
– availability of variations
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The excessive use of antibiotics is leading to the
evolution of antibiotic-resistant bacteria
– Example:
Mycobacterium
tuberculosis
Figure 13.22
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What is a species?
• appearance alone does not always define a
species
– Example: eastern and western meadowlarks
Figure 14.1A
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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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MECHANISMS OF SPECIATION
When does speciation occur?
• When geographically isolated, species evolution
may occur
– gene pool then changes to cause reproductive
isolation
= allopatric speciation
Figure 14.3
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• A ring species may illustrate the process of
speciation
1
OREGON
POPULATION
Sierra
Nevada
COASTAL
POPULATIONS
Yelloweyed
Yellowblotched
2
Gap in
ring
Monterey
INLAND
POPULATIONS
Largeblotched
3
Figure 14.1C
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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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• 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
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When does speciation occur?
• Specialists - Galapagos finches
• Generalists - horseshoe crabs, cockroaches
• New environments
- ecological niche
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• Adaptive radiation on an island chain
- specialization for different niches
1
A
Species A
from mainland
2
B
B
3
B
C
B
4
C
C D
C
C
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D
5
Figure 14.4B
Cactus
ground finch
Medium
ground finch
Large
ground finch
Small
Large cactus
ground finch ground finch
Small
tree finch
Vegetarian
finch
Medium
tree finch
Large
tree finch
Woodpecker
finch
Mangrove
finch
Green
Gray
warbler finch warbler finch
Sharp-beaked
ground finch
Seed
eaters
Cactus flower
eaters
Ground finches
Bud
eaters
Insect
eaters
Tree finches
Warbler finches
Common ancestor from
South America mainland
Figure 15.9
Continental drift has played a major role in
macroevolution
• Continental drift is the slow, steady movement
of Earth’s crustal plates on the hot mantle
Eurasian
Plate
North
American
Plate
African
Plate
Pacific
Plate
Nazca
Plate
South
American
Plate
Split
developing
Indo-Australian
Plate
Antarctic Plate
Edge of one plate being pushed over edge of
neighboring plate (zones of violent geologic events)
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Figure 15.3A
CENOZOIC
• influenced the distribution
of organisms
Eurasia
Africa
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MESOZOIC
Antarctica
PALEOZOIC
– Separation of
continents caused the
isolation and
diversification of
organisms
Millions of years ago
– Continental mergers
triggered extinctions
India
South
America
Laurasia
Figure 15.3B
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
Fly chromosomes
Mouse chromosomes
Fruit fly embryo (10 hours)
Mouse embryo (12 days)
Adult fruit fly
Adult mouse
Figure 11.14
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings