Download Mechanisms of Evolution Key Concepts

Mechanisms of Evolution
Microevolution
Population Genetics
Key Concepts
• 23.1: Population genetics provides a foundation
for studying evolution
• 23.2: Mutation and sexual recombination
produce the variation that makes evolution
possible
• 23.3: Natural selection, genetic drift, and gene
flow can alter a population’s genetic composition
• 23.4: Natural selection is the primary
mechanism of adaptive evolution
• Concept 23.4: Natural selection is the
primary mechanism of adaptive
evolution
• Natural selection
– Accumulates and maintains favorable
genotypes in a population
– Is the only deviation from the Hardy-Weinberg
that leads to adaptation (of the population to
the environment)
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A Closer Look at Natural
Selection
• From the range of variations available in a
population
– Natural selection increases the frequencies of
certain genotypes, fitting organisms to their
environment over generations
Evolutionary Fitness
• The phrases “struggle for existence” and
“survival of the fittest”
– Are commonly used to describe natural
selection
– Can be misleading
• Reproductive success
– Is generally more subtle and depends on many
factors
• Fitness
– Is the contribution an individual makes to the
gene pool of the next generation, relative to the
contributions of other individuals
• Relative fitness
– Is the contribution of a genotype to the next
generation as compared to the contributions of
alternative genotypes for the same locus
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3 Modes of Selection
• Selection
– Favors certain genotypes by acting on the
phenotypes of certain organisms
• Three modes of selection are
– Directional
– Disruptive
– Stabilizing
• Directional selection
– Favors individuals at one end of the phenotypic
range
• Disruptive selection
– Favors individuals at both extremes of the
phenotypic range
• Stabilizing selection
Frequency of individuals
– Favors intermediate variants and acts against
extreme phenotypes
Original
population
Evolved
population
(a) Directional selection shifts the overall
makeup of the population by favoring
variants at one extreme of the
distribution. In this case, darker mice are
favored because they live among dark
rocks and a darker fur color conceals them
from predators.
Original population
Phenotypes (fur color)
(b) Disruptive selection favors variants
at both ends of the distribution. These
mice have colonized a patchy habitat
made up of light and dark rocks, with the
result that mice of an intermediate color are
at a disadvantage.
(c) Stabilizing selection removes
extreme variants from the population
and preserves intermediate types. If
the environment consists of rocks of
an intermediate color, both light and
dark mice will be selected against.
Fig 23.12 A–C
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The Preservation of
Genetic Variation
• Various mechanisms help to preserve
genetic variation in a population
Diploidy
• Diploidy
– Maintains genetic variation in the form of
hidden recessive alleles
Balancing Selection
• Balancing selection
– Occurs when natural selection maintains
stable frequencies of two or more phenotypic
forms in a population
– Leads to a state called balanced
polymorphism
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Heterozygote Advantage
• Some individuals who are heterozygous at a
particular locus
– Have greater fitness than homozygotes
• Natural selection
– Will tend to maintain two or more alleles at that
locus
• The sickle-cell allele
– Causes mutations in hemoglobin but also
confers malaria resistance
– Exemplifies the heterozygote advantage
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
Figure 23.13
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent
Selection
• In frequency-dependent selection
– The fitness of any morph declines if it
becomes too common in the population
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Example of frequencydependent selection
On pecking a moth image
the blue jay receives a
food reward. If the bird
does not detect a moth
on either screen, it pecks
the green circle to continue
to a new set of images (a
new feeding opportunity).
Parental population sample
Phenotypic diversity
Experimental group sample
0.06
0.05
0.04
Frequencyindependent control
0.03
0.02
0
Plain background
Patterned background
20
60
40
80
Generation number
100
Figure 23.14
Neutral Variation
• Is genetic variation that appears to confer
no selective advantage
Sexual Selection
• Sexual selection
– Is natural selection for mating success
– Can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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Intrasexual selection
• Is a direct competition among individuals
of one sex for mates of the opposite sex
Intersexual selection
• Occurs when individuals of one sex
(usually females) are choosy in selecting
their mates from individuals of the other
sex
• May depend on the showiness of the
male’s appearance
Figure 23.15
The Evolutionary Enigma of
Sexual Reproduction
• Sexual reproduction
– Produces fewer reproductive offspring than asexual
reproduction, a so-called reproductive handicap
Sexual reproduction
Asexual reproduction
Generation 1
Female
Female
Generation 2
Male
Generation 3
Generation 4
Figure 23.16
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• If sexual reproduction is a handicap, why
has it persisted?
– It produces genetic variation that may aid in
disease resistance
Why Natural Selection Cannot
Fashion Perfect Organisms
• Evolution is limited by historical constraints
• Adaptations are often compromises
• Chance and natural selection interact
• Selection can only edit existing variations
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