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EVOLUTION OF POPULATIONS
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The Smallest Unit of Evolution

• One misconception is that organisms evolve, in
  the Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but only
  populations evolve
• Genetic variations in populations contribute to
  evolution
• Microevolution is a change in allele frequencies
  in a population over generations

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Concept 23.1 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible

• Two processes, mutation and sexual reproduction,
  produce the variation in gene pools that
  contributes to differences among individuals

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Genetic Variation

• Variation in individual genotype leads to
  variation in individual phenotype
• Not all phenotypic variation is heritable
• Natural selection can only act on variation with
  a genetic component

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Fig. 23-2a
(a)
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Fig. 23-2b
(b)
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Variation Within a Population

• Both discrete and quantitative characters
  contribute to variation within a population
• Discrete characters can be classified on an
  either-or basis
• Quantitative characters vary along a continuum
  within a population

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• Population geneticists measure polymorphisms in a
  population by determining the amount of
  heterozygosity at the gene and molecular levels
• Average heterozygosity measures the average
  percent of loci that are heterozygous in a
  population
• Nucleotide variability is measured by comparing
  the DNA sequences of pairs of individuals

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Variation Between Populations

• Most species exhibit geographic variation,
  differences between gene pools of separate
  populations or population subgroups

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Fig. 23-3
1
2.4
3.14
5.18
6
7.15
13.17
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XX
10.16
9.12
8.11
1
2.19
3.8
4.16
5.14
6.7
9.10
11.12
13.17
15.18
XX
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• Some examples of geographic variation occur as a
  cline, which is a graded change in a trait along
  a geographic axis

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Fig. 23-4
1.0
0.8
0.6
Ldh-B b allele frequency
0.4
0.2
0
46
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42
40
38
36
34
32
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Latitude (N)
Georgia Warm (21C)
Maine Cold (6C)
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Mutation

• Mutations are changes in the nucleotide sequence
  of DNA
• Mutations cause new genes and alleles to arise
• Only mutations in cells that produce gametes can
  be passed to offspring

Animation Genetic Variation from Sexual
Recombination
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Point Mutations

• A point mutation is a change in one base in a gene

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• The effects of point mutations can vary
• Mutations in noncoding regions of DNA are often
  harmless
• Mutations in a gene might not affect protein
  production because of redundancy in the genetic
  code

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• The effects of point mutations can vary
• Mutations that result in a change in protein
  production are often harmful
• Mutations that result in a change in protein
  production can sometimes increase the fit between
  organism and environment

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Mutations That Alter Gene Number or Sequence

• Chromosomal mutations that delete, disrupt, or
  rearrange many loci are typically harmful
• Duplication of large chromosome segments is
  usually harmful
• Duplication of small pieces of DNA is sometimes
  less harmful and increases the genome size
• Duplicated genes can take on new functions by
  further mutation

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Mutation Rates

• Mutation rates are low in animals and plants
• The average is about one mutation in every
  100,000 genes per generation
• Mutations rates are often lower in prokaryotes
  and higher in viruses

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Sexual Reproduction

• Sexual reproduction can shuffle existing alleles
  into new combinations
• In organisms that reproduce sexually,
  recombination of alleles is more important than
  mutation in producing the genetic differences
  that make adaptation possible

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Concept 23.2 The Hardy-Weinberg equation can be
used to test whether a population is evolving

• The first step in testing whether evolution is
  occurring in a population is to clarify what we
  mean by a population

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Gene Pools and Allele Frequencies

• A population is a localized group of individuals
  capable of interbreeding and producing fertile
  offspring
• A gene pool consists of all the alleles for all
  loci in a population
• A locus is fixed if all individuals in a
  population are homozygous for the same allele

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Fig. 23-5
Porcupine herd
MAP AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST TERRITORIES
Porcupine herd range
Fortymile herd range
YUKON
ALASKA
Fortymile herd
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• The frequency of an allele in a population can
  be calculated
• For diploid organisms, the total number of
  alleles at a locus is the total number of
  individuals x 2
• The total number of dominant alleles at a locus
  is 2 alleles for each homozygous dominant
  individual plus 1 allele for each heterozygous
  individual the same logic applies for recessive
  alleles

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• By convention, if there are 2 alleles at a locus,
  p and q are used to represent their frequencies
• The frequency of all alleles in a population will
  add up to 1
• For example, p q 1

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The Hardy-Weinberg Principle

• The Hardy-Weinberg principle describes a
  population that is not evolving
• If a population does not meet the criteria of the
  Hardy-Weinberg principle, it can be concluded
  that the population is evolving

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Hardy-Weinberg Equilibrium

• The Hardy-Weinberg principle states that
  frequencies of alleles and genotypes in a
  population remain constant from generation to
  generation
• In a given population where gametes contribute to
  the next generation randomly, allele frequencies
  will not change
• Mendelian inheritance preserves genetic variation
  in a population

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Fig. 23-6
Alleles in the population
Frequencies of alleles
Gametes produced
p frequency of
Each egg
Each sperm
CR allele 0.8
q frequency of
80 chance
80 chance
20 chance
20 chance
CW allele 0.2
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• Hardy-Weinberg equilibrium describes the constant
  frequency of alleles in such a gene pool
• 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
• where p2 and q2 represent the frequencies of the
  homozygous genotypes and 2pq represents the
  frequency of the heterozygous genotype

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Fig. 23-7-1
80 CR ( p 0.8)
20 CW (q 0.2)
Sperm
CW (20)
CR (80)
CR (80)
Eggs
16 ( pq) CRCW
64 ( p2) CRCR
4 (q2) CW CW
16 (qp) CRCW
CW (20)
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Fig. 23-7-2
64 CRCR, 32 CRCW, and 4 CWCW
Gametes of this generation
64 CR       16 CR       80 CR 0.8 p
4 CW         16 CW      20 CW 0.2 q
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Fig. 23-7-3
64 CRCR, 32 CRCW, and 4 CWCW
Gametes of this generation
64 CR       16 CR       80 CR 0.8 p
4 CW         16 CW      20 CW 0.2 q
Genotypes in the next generation
64 CRCR, 32 CRCW, and 4 CWCW plants
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Fig. 23-7-4
80 CR ( p 0.8)
20 CW (q 0.2)
Sperm
CR (80)
CW (20)
CR (80)
Eggs
16 ( pq) CR CW
64 ( p2) CR CR
4 (q2) CW CW
16 (qp) CR CW
CW (20)
64 CR CR, 32 CR CW, and 4 CW CW
Gametes of this generation
64 CR         16 CR       80 CR  0.8 p
4 CW          16 CW      20 CW 0.2 q
Genotypes in the next generation
64 CR CR, 32 CR CW, and 4 CW CW plants
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Conditions for Hardy-Weinberg Equilibrium

• The Hardy-Weinberg theorem describes a
  hypothetical population
• In real populations, allele and genotype
  frequencies do change over time

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• The five conditions for nonevolving populations
  are rarely met in nature
• No mutations
• Random mating
• No natural selection
• Extremely large population size
• No gene flow

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• Natural populations can evolve at some loci,
  while being in Hardy-Weinberg equilibrium at
  other loci

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Applying the Hardy-Weinberg Principle

• We can assume the locus that causes
  phenylketonuria (PKU) is in Hardy-Weinberg
  equilibrium given that
• The PKU gene mutation rate is low
• Mate selection is random with respect to whether
  or not an individual is a carrier for the PKU
  allele

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• Natural selection can only act on rare homozygous
  individuals who do not follow dietary
  restrictions
• The population is large
• Migration has no effect as many other populations
  have similar allele frequencies

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• The occurrence of PKU is 1 per 10,000 births
• q2 0.0001
• q 0.01
• The frequency of normal alleles is
• p 1 q 1 0.01 0.99
• The frequency of carriers is
• 2pq 2 x 0.99 x 0.01 0.0198
• or approximately 2 of the U.S. population

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Concept 23.3 Natural selection, genetic drift,
and gene flow can alter allele frequencies in a
population

• Three major factors alter allele frequencies and
  bring about most evolutionary change
• Natural selection
• Genetic drift
• Gene flow

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Natural Selection

• Differential success in reproduction results in
  certain alleles being passed to the next
  generation in greater proportions

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• Genetic drift occurs when changes in gene
  frequencies from one generation to another occur
  because of chance events (sampling errors) that
  occur when populations are finite in size.
• The smaller the sample, the greater the chance of
  deviation from an idealized result.
• Genetic drift at small population sizes often
  occurs as a result of two situations
• bottleneck effect
• founder effect.

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• in a small wildflower population with a stable
  size of only ten plants, genetic drift can
  completely eliminate some alleles.

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• The bottleneck effect occurs when the numbers of
  individuals in a larger population are
  drastically reduced by a disaster.
• By chance, some alleles may be overrepresented
  and others underrepresented among the survivors.
• Some alleles may be eliminated altogether.
• Genetic drift will continue to impact the gene
  pool until the population is large enough to
  minimize the impact of sampling errors.

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• Bottlenecking is an important concept in
  conservation biology of endangered species.
• Populations that have suffered bottleneck
  incidents have lost at least some alleles from
  the gene pool.
• This reduces individual variation and
  adaptability.
• The genetic variation in the three small
  surviving wild populations of cheetahs is similar
  to highly inbred lab mice

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• The founder effect occurs when a new population
  is started by only a few individuals that do not
  represent the gene pool of the larger source
  population.
• At an extreme, a population could be started by
  single pregnant female or single seed with only a
  tiny fraction of the genetic variation of the
  source population.
• Founder effects have been demonstrated in human
  populations that started from a small group of
  colonists.

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• Natural selection is clearly a violation of the
  conditions necessary for the Hardy-Weinberg
  equilibrium.
• Selection results in some alleles being passed
  along to the next generation in numbers
  disproportionate to their frequencies in the
  present generation.

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Natural selection accumulates and maintains
favorable genotypes in a population.

• In our wildflower example, if herbivorous insects
  are more likely to locate and eat white flowers
  than red flowers, then plants with red flowers
  (either RR or Rr) are more likely to leave
  offspring than those with white flowers (rr).
• If pollinators were more attracted by red flowers
  than white flowers, red flowers would also be
  more likely to leave more offspring.
• Either mechanism, differential survival or
  differential reproduction, will increase the
  frequency of the R allele in the population and
  decrease that of the r allele.

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• Gene flow is genetic exchange due to migration
  of fertile individuals or gametes between
  populations.
• if a nearby wildflower population consisted
  entirely of white flowers, its pollen (r alleles
  only) could be carried into our target
  population.
• This would increase the frequency of r alleles in
  the target population in the next generation.

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• Gene flow tends to reduce differences between
  populations.
• If extensive enough, gene flow can amalgamate
  neighboring populations into a single population
  with a common genetic structure.
• The migration of people throughout the world is
  transferring alleles between populations that
  were once isolated, increasing gene flow.

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Genetic Variation is the Substrate for Natural
Selection
1. Genetic variation occurs within and between
populations 2. Mutation and sexual recombination
generate genetic variation 3. Diploidy and
balanced polymorphisms preserve variation
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Effects of Genetic Drift A Summary

• Genetic drift is significant in small populations
• Genetic drift causes allele frequencies to change
  at random
• Genetic drift can lead to a loss of genetic
  variation within populations
• Genetic drift can cause harmful alleles to become
  fixed

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Concept 23.4 Natural selection is the only
mechanism that consistently causes adaptive
evolution

• Only natural selection consistently results in
  adaptive evolution

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A Closer Look at Natural Selection

• Natural selection brings about adaptive evolution
  by acting on an organisms phenotype

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Relative Fitness

• The phrases struggle for existence and
  survival of the fittest are misleading as they
  imply direct competition among individuals
• Reproductive success is generally more subtle and
  depends on many factors

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• Relative fitness is the contribution an
  individual makes to the gene pool of the next
  generation, relative to the contributions of
  other individuals
• Selection favors certain genotypes by acting on
  the phenotypes of certain organisms

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Directional, Disruptive, and Stabilizing Selection

• Three 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

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• Directional selection is most common during
  periods of environmental change or when members
  of a population migrate to a new habitat with
  different environmental conditions.
• Directional selection shifts the frequency curve
  for a phenotypic character in one direction by
  favoring what had been rare individuals.

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• Diversifying selection occurs when environmental
  conditions favor individuals at both extremes of
  the phenotypic range over intermediate
  phenotypes.

Fig. 23.12
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• Diversifying selection can result in
  balanced polymorphism.
• Two distinct bill types are present in
  black-bellied seedcrackers in which larger-billed
  birds are more efficient when feeding on hard
  seeds and smaller-billed birds are more efficient
  when feeding on soft seeds.

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• Stabilizing selection favors intermediate
  variants and acts against extreme phenotypes.
• Stabilizing selection reduces variation and
  maintains the predominant phenotypes.
• Human birth weight is subject to stabilizing
  selection.
• Babies much larger or smaller than 3-4 kg have
  higher infant mortality.

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The Key Role of Natural Selection in Adaptive
Evolution

• Natural selection increases the frequencies of
  alleles that enhance survival and reproduction
• Adaptive evolution occurs as the match between an
  organism and its environment increases

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Fig. 23-14a
Color-changing ability in cuttlefish
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Fig. 23-14b
Movable bones
(b) Movable jaw bones in snakes
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• Because the environment can change, adaptive
  evolution is a continuous process
• Genetic drift and gene flow do not consistently
  lead to adaptive evolution as they can increase
  or decrease the match between an organism and its
  environment

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Sexual Selection

• Sexual selection is natural selection for mating
  success
• It can result in sexual dimorphism, marked
  differences between the sexes in secondary sexual
  characteristics

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Sexual selection may lead to pronounced
secondary differences between the sexes

• Males and females of a species differ not only in
  their reproductive organs, but often also in
  secondary sexual characteristics that are not
  directly associated with reproduction.
• These differences, termed sexual dimorphism, may
  include size differences, coloration differences,
  enlarged or exaggerated features, or other
  adornments.
• Males are usually the larger and showier sex, at
  least among vertebrates.

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Fig. 23-15
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• Sexual dimorphism is a product of sexual
  selection
• Intrasexual selection is direct competition
  among individuals of one sex (usually males) for
  mates of the opposite sex.
• Competition may take the form of direct physical
  battles between individuals.
• The stronger individuals gain status.
• More commonly ritualized displays discourage
  lesser competitors and determine dominance.

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• Intersexual selection or mate choice occurs when
  members of one sex (usually females) are choosy
  in selecting among individuals of the other sex.
• Males with the most masculine features are the
  most attractive to females.
• Interestingly, these features may not be adaptive
  in other ways and expose these individuals to
  extra risks.

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• The underlying bases of female choice is
  probably not aesthetic.
• Recent research is investigating the hypothesis
  that females use these sexual advertisements to
  measure the general health of a male.
• Individuals with infections or other problems are
  likely to have a relatively dull, disheveled
  plumage.
• These individuals are unlikely to win many
  females.
• For the female that chooses a healthy mate, even
  if his inclination is just a pre-wired response
  to visual signals, the benefit is a greater
  probability of having healthy offspring.

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• How do female preferences evolve?
• The good genes hypothesis suggests that if a
  trait is related to male health, both the male
  trait and female preference for that trait should
  be selected for

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The Preservation of Genetic Variation

• Various mechanisms help to preserve genetic
  variation in a population

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Diploidy

• Diploidy maintains genetic variation in the form
  of hidden recessive alleles

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Balancing Selection

• Balancing selection occurs when natural selection
  maintains stable frequencies of two or more
  phenotypic forms in a population

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Heterozygote Advantage

• Heterozygote advantage occurs when heterozygotes
  have a higher fitness than do both 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

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Fig. 23-17
Frequencies of the sickle-cell allele
02.5
2.55.0
5.07.5
Distribution of malaria caused by Plasmodium
falciparum (a parasitic unicellular eukaryote)
7.510.0
10.012.5
gt12.5
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Why Natural Selection Cannot Fashion Perfect
Organisms

• Selection can act only on existing variations
• Evolution is limited by historical constraints
• Adaptations are often compromises
• Chance, natural selection, and the environment
  interact