Transcript How natural selection affects variation

BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 13
How Populations Evolve
Modules 13.13 – 13.22
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
VARIATION AND NATURAL SELECTION
13.13 Variation is extensive in most populations
• Phenotypic variation may be environmental or
genetic in origin
– But only genetic changes result in evolutionary
adaptation
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• Many populations exhibit polymorphism and
geographic variation
Figure 13.13
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13.14 Connection: Mutation and sexual
recombination generate variation
A1
Parents
A1
A2
A3
MEIOSIS
A1
A2
A3
Gametes
FERTILIZATION
Offspring,
with new
combinations
of alleles
A1
A2
A1
A3
and
Figure 13.14
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13.15 Overview: How natural selection affects
variation
• Natural selection tends to reduce variability in
populations
– The diploid condition preserves variation by
“hiding” recessive alleles
– Balanced polymorphism may result from the
heterozygote advantage
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13.16 Not all genetic variation may be subject to
natural selection
• Some variations may be neutral, providing no
apparent advantage or disadvantage
– Example: human fingerprints
Figure 13.16
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13.17 Connection: Endangered species often have
reduced variation
• Low genetic variability may reduce the capacity
of endangered species to survive as humans
continue to alter the environment
– Studies have shown that cheetah populations
exhibit extreme genetic uniformity
– Thus they may have a
reduced capacity to
adapt to environmental
challenges
Figure 13.17
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13.18 The perpetuation of genes defines
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
• Production of fertile offspring is the only score
that counts in natural selection
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13.19 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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13.20 Sexual selection may produce sexual
dimorphism
• Sexual selection leads to the evolution of
secondary sexual characteristics
– These may give individuals an advantage in
mating
Figure 13.20A, B
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13.21 Natural selection cannot fashion perfect
organisms
• This is due to:
– historical constraints
– adaptive compromises
– chance events
– availability of variations
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13.22 Connection: The evolution of antibiotic
resistance in bacteria is a serious public
health concern
• The excessive use of antibiotics is leading to the
evolution of antibiotic-resistant bacteria
– Example:
Mycobacterium
tuberculosis
Figure 13.22
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings