Transcript Natural Selection (22) The Evolution of Populations (23)

Natural Selection (22)
The Evolution of Populations (23)
Natural Selection: A Summary
• Natural selection is the mechanism which directs
the process of descent with modification
• Natural selection says:
– Individuals with certain heritable characteristics
survive and reproduce at a higher rate than other
individuals
– Natural selection increases the adaptation of
organisms to their environment over time
– If an environment changes over time, natural
selection may result in adaptation to these new
conditions and may give rise to new species
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Darwin saw organisms adapted to their environment
(b) Insect-eater
(a) Cactus-eater
(c) Seed-eater
Evolution is supported by an overwhelming
amount of scientific evidence
• Evidence continues to fill the gaps identified by
Darwin in The Origin of Species
1. Direct observations
2. Homology
3. Embryology
4. Vestigial Organs
5. Fossil record
6. Biogeography
7. Biochemistry (later chapters)
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Evidence 1: Direct Observations of
Evolutionary Change
• Two examples provide evidence for
natural selection: natural selection in
response to introduced plant species, and
the evolution of drug-resistant bacteria
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Can a change in a population’s food source result in evolution by natural selection?
RESULTS
Beak
10
On native species,
southern Florida
8
Number of individuals
6
4
2
0
Museum-specimen average
10
On introduced species,
central Florida
8
6
4
2
0
6
7
8
9
Beak length (mm)
10
11
Figure 22.14
MRSA plasmid DNA
2,750,000
1
250,000 base pairs
2,500,000
Chromosome map
of S. aureus clone USA300
500,000
Key to adaptations
2,250,000
Methicillin resistance
Ability to colonize hosts
750,000
Increased disease severity
2,000,000
Increased gene exchange
(within species) and
toxin production
1,750,000
1,500,000
1,250,000
1,000,000
Evidence type 2: Homology
• Homology is similarity resulting from common
ancestry
• Homologous structures are anatomical
resemblances that represent variations on a
structural theme present in a common ancestor
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Figure 22.15
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Cat
Whale
Bat
Figure 22.16
Evidence 3: Comparative embryology reveals
anatomical homologies not visible in adult organisms
Pharyngeal
pouches
Post-anal
tail
Chick embryo (LM)
Human embryo
Comparing physical structure might bring
erroneous conclusions regarding
relationships
• Convergent evolution is the evolution of
similar, or analogous, features in distantly
related groups
• Analogous traits arise when groups
independently adapt to similar
environments in similar ways
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Evidence 4: Vestigial structures are remnants
of features that served important functions in
the organism’s ancestors.
– Human vestigial structures-ex. coccyx
– Examples of homologies at the molecular level
are genes shared among organisms inherited
from a common ancestor
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Evidence 5: The Fossil Record
• The fossil record provides evidence of the
extinction of species, the origin of new
groups, and changes within groups over
time
• Fossils can document important transitions
– For example, the transition from land to sea in
the ancestors of cetaceans
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Evidence 6. Biogeography
• Biogeography, the geographic distribution of
species, provides evidence of evolution
• Earth’s continents were formerly united in a
single large continent called Pangaea, but
have since separated by continental drift
• An understanding of continent movement
and modern distribution of species allows us
to predict when and where different groups
evolved
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(23) Evolution of Populations-Microevolution
• Natural selection acts on individuals, but only
populations evolve.
• Consider, for example, a population of medium
ground finches on Daphne Major Island
– During a drought, large-beaked birds were more
likely to crack large seeds and survive
– The finch population evolved by natural selection
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Figure 23.1
Average beak depth (mm)
Figure 23.2
10
9
8
0
1978
1976
(similar to the (after
prior 3 years) drought)
• Microevolution is a change in allele frequencies in a
population over generations
• Three mechanisms cause allele frequency change:
– Natural selection-individuals with beneficial
heritable traits survive to reproduce
– Genetic drift-chance events that alter gene
frequencies
– Gene flow-transfer of alleles between populations
• Only natural selection causes adaptive evolution
© 2011 Pearson Education, Inc.
We have seen evidence for evolution now
how do populations (not individuals)
actually evolve?
1.
2.
3.
4.
5.
Mutations
Sexual reproduction
Natural selection
Genetic drift
Need gene flow