Chapter23

Chapter 23

The Evolution of Populations

PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece

Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Overview: The Smallest Unit of Evolution

• One common misconception about evolution is
that individual organisms evolve, in the
Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but
populations evolve


• Genetic variations in populations
– Contribute to evolution

Figure 23.1

• Concept 23.1: Population genetics provides a
foundation for studying evolution
• Microevolution
– Is change in the genetic makeup of a
population from generation to generation

Figure 23.2

The Modern Synthesis
• Population genetics
– Is the study of how populations change
genetically over time
– Reconciled Darwin’s and Mendel’s ideas


• The modern synthesis
– Integrates Mendelian genetics with the
Darwinian theory of evolution by natural
selection
– Focuses on populations as units of evolution


Gene Pools and Allele Frequencies
• A population
– Is a localized group of individuals that are capable of
interbreeding and producing fertile offspring

CANADA
ALASKA
MAP
AREA

Beaufort Sea

Porcupine
herd range
N
TE OR
RR TH
IT W E
O S
RI T
ES

•
Fairbanks

Fortymile
ALASKA

•
YUKON

herd range
Whitehorse

Figure 23.3

• 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


The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem
– Describes a population that is not evolving

– States that the frequencies of alleles and
genotypes in a population’s gene pool remain
constant from generation to generation
provided that only Mendelian segregation and
recombination of alleles are at work


• Mendelian inheritance
– Preserves genetic variation in a population
Generation
1
CRCR CWCW
genotype genotype
Plants mate

Generation
2
All CRCW
(all pink flowers)

50% CR 50% CW
gametes gametes
Come together at random

Generation
3
25% CRCR 50% CRCW 25% CWCW

50% CR 50% CW
gametes gametes
Come together at random

Generation
4
25% CRCR 50% CRCW 25% CWCW

Alleles segregate, and subsequent
generations also have three types
Figure 23.4 of flowers in the same proportions


Preservation of Allele Frequencies
• In a given population where gametes contribute
to the next generation randomly, allele
frequencies will not change


Hardy-Weinberg Equilibrium
• Hardy-Weinberg equilibrium
– Describes a population in which random
mating occurs
– Describes a population where allele
frequencies do not change


• A population in Hardy-Weinberg equilibrium Gametes for each generation are drawn at random from
the gene pool of the previous generation:

80% CR (p = 0.8) 20% CW (q = 0.2)

Sperm
CR CW
(80%) (20%)

p2 pq

(80%)
CR
Eggs
p2
64% 16%
CRCR CRCW
(20%)

qp 16% 4%
CW

CRCW CWCW

q2
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium:

64% CRCR, 32% CRCW, and 4% CWCW

Gametes of the next generation:
64% CR from 16% CR from 80% CR = 0.8 = p
+ CRCW homozygotes
=
CRCR homozygotes
4% CW from 16% CW from 20% CW = 0.2 = q
+ CRCW heterozygotes
=
CWCW homozygotes
With random mating, these gametes will result in the same
mix of plants in the next generation:

Figure 23.5 64% CRCR, 32% CRCW and 4% CWCW plants


• 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


Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem
– Describes a hypothetical population

• In real populations
– Allele and genotype frequencies do change
over time


• 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


Population Genetics and Human Health
• We can use the Hardy-Weinberg equation
– To estimate the percentage of the human
population carrying the allele for an inherited
disease


• Concept 23.2: Mutation and sexual
recombination produce the variation that
makes evolution possible
• Two processes, mutation and sexual
recombination
– Produce the variation in gene pools that
contributes to differences among individuals


Mutation
• Mutations
– Are changes in the nucleotide sequence of DNA

– Cause new genes and alleles to arise

Figure 23.6

Point Mutations
• A point mutation
– Is a change in one base in a gene

– Can have a significant impact on phenotype

– Is usually harmless, but may have an adaptive
impact


Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that affect many loci
– Are almost certain to be harmful

– May be neutral and even beneficial


• Gene duplication
– Duplicates chromosome segments


Mutation Rates
• Mutation rates
– Tend to be low in animals and plants

– Average about one mutation in every 100,000
genes per generation
– Are more rapid in microorganisms


Sexual Recombination
• In sexually reproducing populations, sexual
recombination
– Is far more important than mutation in
producing the genetic differences that make
adaptation possible


• Concept 23.3: Natural selection, genetic drift,
and gene flow can alter a population’s genetic
composition
• Three major factors alter allele frequencies and
bring about most evolutionary change
– Natural selection

– Genetic drift

– Gene flow


Natural Selection
• Differential success in reproduction
– Results in certain alleles being passed to the
next generation in greater proportions


Genetic Drift
• Statistically, the smaller a sample
– The greater the chance of deviation from a
predicted result


• Genetic drift
– Describes how allele frequencies can fluctuate
unpredictably from one generation to the next
– Tends to reduce genetic variation

CRCR CRCR CW CW CRCR CRCR

CRCW Only 5 of CRCW Only 2 of CRCR CRCR
10 plants 10 plants
leave leave
CWCW CRCR offspring CW CW offspring CRCR CRCR
CRCR

CRCW CRCW CRCR CRCR

CRCR CRCW CW CW CRCR CRCR

CRCR CRCW CRCW CRCW CRCR CRCR

Generation 1 Generation 2 Generation 3
p (frequency of CR) = 0.7 p = 0.5 p = 1.0
q (frequency of CW) = 0.3 q = 0.5 q = 0.0

Figure 23.7

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 Original Bottlenecking Surviving
population and gold marbles are absent. population event population

Figure 23.8 A

• Understanding the bottleneck effect
– Can increase understanding of how human
activity affects other species

(b) Similarly, bottlenecking a population
of organisms tends to reduce genetic
variation, as in these northern
elephant seals in California that were
once hunted nearly to extinction.

Figure 23.8 B


The Founder Effect
• The founder effect
– Occurs when a few individuals become
isolated from a larger population
– Can affect allele frequencies in a 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


• Concept 23.4: Natural selection is the primary
mechanism of adaptive evolution
• Natural selection
– Accumulates and maintains favorable
genotypes in a population


Genetic Variation
• Genetic variation
– Occurs in individuals in populations of all
species
– Is not always heritable

(a) Map butterflies that (b) Map butterflies that
emerge in spring: emerge in late summer:
orange and brown black and white
Figure 23.9 A, B

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


• Polymorphism

• Phenotypic polymorphism
– Describes a population in which two or more
distinct morphs for a character are each
represented in high enough frequencies to be
readily noticeable

• Genetic polymorphisms
– Are the heritable components of characters
that occur along a continuum in a population


• Measuring Genetic Variation

• Population geneticists
– Measure the number of polymorphisms in a
population by determining the amount of
heterozygosity at the gene level and the
molecular level

• Average heterozygosity
– Measures the average percent of loci that are
heterozygous in a population


Variation Between Populations
• Most species exhibit geographic variation
– Differences between gene pools of separate
populations or population subgroups

1 2.4 3.14 5.18 6 7.15

8.11 9.12 10.16 13.17 19 XX

1 2.19 3.8 4.16 5.14 6.7

9.10 11.12 13.17 15.18 XX
Figure 23.10

• 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
of yarrow plants (Achillea) growing on the slopes of the Sierra
Nevada mountains gradually decreases with increasing

Mean height (cm)
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.

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.
Atitude (m)

Sierra Nevada Great Basin
Range Plateau

CONCLUSION The lesser but still measurable clinal variation
in yarrow plants grown at a common elevation demonstrates the Seed collection sites
role of genetic as well as environmental differences.

Figure 23.11

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


Directional, Disruptive, and Stabilizing 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
– Favors intermediate variants and acts against
extreme phenotypes

• The three modes of selection

Frequency of individuals
Original population

Original Evolved Phenotypes (fur color)
population population

(a) Directional selection shifts the overall (b) Disruptive selection favors variants (c) Stabilizing selection removes
makeup of the population by favoring at both ends of the distribution. These extreme variants from the population
variants at one extreme of the mice have colonized a patchy habitat and preserves intermediate types. If
distribution. In this case, darker mice are made up of light and dark rocks, with the the environment consists of rocks of
favored because they live among dark result that mice of an intermediate color are an intermediate color, both light and
rocks and a darker fur color conceals them at a disadvantage. dark mice will be selected against.
from predators.

Fig 23.12 A–C

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


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

Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of 5.0–7.5%
malaria caused by 7.5–10.0%
Plasmodium falciparum 10.0–12.5%
(a protozoan)
>12.5%
Figure 23.13

• Frequency-Dependent Selection

• In frequency-dependent selection
– The fitness of any morph declines if it becomes
too common in the population


• An example of frequency-dependent selection
On pecking a moth image
the blue jay receives a
food reward. If the bird
Parental population sample
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).

Experimental group sample
0.06

Phenotypic diversity
0.05

0.04

Frequency-
0.03
independent control

0.02
0 20 40 60 80 100
Generation number
Plain background Patterned background
Figure 23.14

Neutral Variation
• 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


• 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
Asexual reproduction Sexual reproduction

Female Generation 1
Female

Generation 2

Male

Generation 3

Generation 4

Figure 23.16

• 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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