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Evolution of Populations
• Natural selection acts on
individuals,
• but …………………… evolve
• Populations evolve
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Microevolution
– Is change in the genetic makeup of a
population from generation to generation
Figure 23.2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Modern Synthesis
• Population genetics
– Is the study of how populations change
genetically over time
• The modern synthesis
– Integrates Mendelian genetics with the
Darwinian theory of evolution by natural
selection
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gene Pools and Allele Frequencies
• A population
– Is a localized group of individuals that are capable of
interbreeding and producing fertile offspring
MAP
AREA
•
Fairbanks
Fortymile
herd range
•
Whitehorse
Figure 23.3
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The gene pool
– Is the total aggregate of genes in a population
at any one time
– Consists of all gene loci in all individuals of the
population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem
– Describes a population that is not evolving
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hardy-Weinberg Equilibrium
• Hardy-Weinberg equilibrium
– Describes a population in which random
mating occurs
– Describes a population where allele
frequencies do not change
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• If p and q represent the relative frequencies of
the only two possible alleles in a population at
a particular locus, then
– p2 + 2pq + q2 = 1
– And p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents
the frequency of the heterozygous genotype
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem
– Describes a hypothetical population
• In real populations
– Allele and genotype frequencies do change
over time
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The five conditions for non-evolving
populations are rarely met in nature
– Extremely large population size
– No gene flow
– No mutations
– Random mating
– No natural selection
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Two processes, mutation and sexual
recombination
– Produce the variation in gene pools that
contributes to differences among individuals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Three major factors alter allele
frequencies and bring about most
evolutionary change ???
• Natural selection
• Genetic drift
• Gene flow
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Natural Selection
• Differential success in reproduction
– Results in certain alleles being passed to the
next generation in greater proportions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genetic Drift
• Statistically, the smaller a sample
– The greater the chance of deviation from a
predicted result
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Genetic drift
– Describes how allele frequencies can fluctuate
unpredictably from one generation to the next
– Tends to reduce genetic variation
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect
• In the bottleneck effect
– A sudden change in the environment may
drastically reduce the size of a population
– The gene pool may no longer be reflective of
the original population’s gene pool
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Original
population
Figure 23.8 A
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Bottlenecking
event
Surviving
population
Gene Flow
• Gene flow
– Causes a population to gain or lose alleles
– Results from the movement of fertile
individuals or gametes
– Tends to reduce differences between
populations over time
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Some examples of geographic variation occur
as a cline, which is a graded change in a trait
along a geographic axis
Heights of yarrow plants grown in common garden
EXPERIMENT
Researchers observed that the average size
Mean height (cm)
of yarrow plants (Achillea) growing on the slopes of the Sierra
Nevada mountains gradually decreases with increasing
elevation. To eliminate the effect of environmental differences
at different elevations, researchers collected seeds
from various altitudes and planted them in a common
garden. They then measured the heights of the
resulting plants.
Atitude (m)
RESULTS The average plant sizes in the common
garden were inversely correlated with the altitudes at
which the seeds were collected, although the height
differences were less than in the plants’ natural
environments.
CONCLUSION The lesser but still measurable clinal variation
in yarrow plants grown at a common elevation demonstrates the
role of genetic as well as environmental differences.
Figure 23.11
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sierra Nevada
Range
Great Basin
Plateau
Seed collection sites
Evolutionary Fitness
• The phrases “struggle for existence” and
“survival of the fittest”
– Can be misleading
• Reproductive success
– Is generally more subtle and depends on many
factors
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3 Modes of Selection
• Directional selection
– Favors individuals at one end of the
phenotypic range
• Disruptive selection
– Favors individuals at both extremes of the
phenotypic range
• Stabilizing selection
– Favors intermediate variants and acts against
extreme phenotypes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The three modes of selection
Original population
Original
population
Phenotypes (fur color)
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.
(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.
Fig 23.12 A–C
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(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.
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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%
Sexual Selection
• Sexual selection
– Is natural selection for mating success
– Can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings