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Essentials of the Living World
Second Edition
George B. Johnson
Jonathan B. Losos
Chapter 15
Evolution and Natural Selection
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
15.1 Evolution: Getting from There
to Here
• The idea of evolution by means of natural
selection has played a central role in the
science of biology
 proposed by Charles Darwin in 1859 with the
publication of On the Origin of Species
 may be summarized, as Darwin did, as
“descent with modification”
• all species arise from other, preexisting species
15.1 Evolution: Getting from There
to Here

Macroevolution is evolutionary change of a grand scale
•

for example, changes that result in the creation of new species
Microevolution is evolutionary change at the level of a
population
•
these are changes that occur within a species that make that
species different from its immediate ancestor
•
adaptation results from microevolutionary changes that increase
the likelihood of survival and reproduction of particular genetic
traits in a population
15.1 Evolution: Getting from There
to Here
• Darwin did not invent the idea of evolution
• Prior to Darwin there was no consensus among
biologists about the mechanism causing
evolution
• A predecessor to Darwin, Jean-Baptiste
Lamarck proposed that evolution occurred by
the inheritance of acquired characteristics
15.1 Evolution: Getting from There
to Here
• According to Lamarck, individuals passed
on to offspring body and behavior changes
acquired during their lives
 for example, giraffes evolved long necks
because ancestral giraffes tended to stretch
their necks and this neck extension was
passed on to subsequent generations
Figure 15.1(a) How did long necks
evolve in giraffes?
15.1 Evolution: Getting from There
to Here
• According to Darwin, the variation is not
created by experience but already exists
when selection acts on it
 populations of ancestral giraffes contained
variation in neck length
 individuals who were able to feed higher up
on the trees had more food and so were able
to survive and reproduce better than their
shorter-necked relatives
Figure 15.1 (b) How did long necks
evolve in giraffes?
15.1 Evolution: Getting from There
to Here
• There are two views concerning the rate of
evolutionary change
 gradualism states that evolutionary change occurs
extremely slowly
• such change would be nearly imperceptible from generation
to generation, but would accumulate over the course of
millions of years
 punctuated equilibrium states that species
experience long periods of little or no evolutionary
change (termed stasis), interrupted by bursts of
evolutionary change
Figure 15.2 Two views of the pace
of macroevolution
15.2 The Evidence for Evolution
• There are many lines of evidence
supporting Darwin’s theory of evolution
 fossil record comprises the most direct
evidence of macroevolution
 fossils are the preserved remains, tracks, or
traces of once-living organisms
• they are created when organisms become buried
in sediment
• by dating the rocks in which the fossils occur, one
can get an accurate idea of how old the fossils are
15.2 The Evidence for Evolution
• Fossils in rock represent a history of
evolutionary change
 fossils are treated as samples of data and are
dated independently of what the samples are
like
 successive changes through time are a data
statement
 thus, the statement that macroevolution has
occurred is a factual observation
Figure 15.3 Testing the theory of
evolution with fossil titanotheres
15.2 The Evidence for Evolution
• The anatomical record also reflects evolutionary
history
 for example, all vertebrate embryos share a basic set
of developmental instructions
Figure 15.4 Embryos show our early evolutionary history
15.2 The Evidence for Evolution
• Homologous structures are derived from the
same body part present in an ancestor
 for example, the same bones might be put to different
uses in related species
• Analagous structures are similar-looking
structures in unrelated lineages
 these are the result of parallel evolutionary
adaptations to similar environments
• this form of evolutionary change is referred to as convergent
evolution
Homologous versus Analagous
Structures
Figure 15.5 Homology among
vertebrate limbs
Figure 15.6 Convergent evolution:
many paths to one goal
15.2 The Evidence for Evolution
• Traces of our evolutionary past are also
evident at the molecular level
 organisms that are more distantly related
should have accumulated a greater number of
evolutionary differences than two species that
are more closely related
 the same pattern of divergence can be seen
at the protein level
Figure 15.7 Molecules reflect
evolutionary divergence
15.2 The Evidence for Evolution
• Evolutionary changes appear to
accumulate at a constant rate
 this permits changes in an individual gene,
compared over a broad array of organisms, to
be dated from the time of divergence
 this dating is referred to as a molecular clock
 for example, changes that have accumulated
in the cytochrome c gene
Figure 15.8 The molecular clock of
cytochrome c
15.3 Evolution’s Critics
• The theory of evolution by natural selection is
the subject of often-bitter public controversy
 the controversy began soon after the publication of
The Origin of Species but, by the turn of the twentieth
century, evolution was generally accepted by the
world’s scientific community
 more recent criticism has come from the following
sources
•
•
•
•
the Fundamentalist Movement
the Scientific Creationist Movement
Local Action
Intelligent Design
15.3 Evolution’s Critics
•
Critics have raised a variety of objections to Darwin’s theory of evolution by natural
selection

“Evolution is just a theory.”

“No one ever saw a fin on the way to becoming a leg.”

“The organs of living creatures are too complex for a random process to have produced.”

“A jumble of soda cans doesn’t by itself jump neatly into a stack—things become more
disorganized due to random events, not more organized.”

“Hemoglobin has 141 amino acids. The probability that the first one would be leucine is 1/20,
and that all 141 would be the ones they are by chance is (1/20)141, an impossibly rare event.”

“No scientist has come up with an experiment where fish evolve into frogs and leap away
from predators.”

“Because the peptide bond does not form spontaneously in water, amino acids could never
have spontaneously linked together to form proteins.”
15.3 Evolution’s Critics
• The previous quotations attack evolution on the following
grounds, which have been refuted by biologists
 evolution is not solidly demonstrated
 there are no fossil intermediates
 the intelligent design argument
 evolution violates the second law of thermodynamics
 natural selection does not imply evolution
 life could not have evolved in water
15.3 Evolution’s Critics
• The irreducible-complexity fallacy refers to
claims by proponents of intelligent design that
the molecular machinery of the cell is irreducibly
complex
• Yet natural selection acts on the systems and
not the parts so that, at every stage of evolution,
the system is functioning
 for example, the mammalian blood clotting system
has evolved in stages from much simpler systems
Figure 15.10 How blood clotting
evolved
15.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
• Population genetics is the study of the
properties of genes in populations
• Gene pool is the sum of all of the genes in
a population, including all alleles in all
individuals
15.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
• Variation within populations puzzled many scientists
 why don’t dominant alleles drive recessive alleles out of
populations?
• G.H. Hardy and W. Weinberg, in 1908, studied allele
frequencies in a gene pool
 in a large population in which there is random mating, and in the
absence of forces that change allele frequencies, the original
genotype proportions remain constant from generation to
generation
 because the proportions do not change, the genotypes are said
to be in Hardy-Weinberg equilibrium
 If the allele frequencies are not changing, the population is not
evolving
15.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
• Hardy and Weinberg arrived at their
conclusion by analyzing the frequencies of
alleles in successive generations
 frequency is the proportion of individuals with
a certain characteristic compared to an entire
population
 knowing the frequency of the phenotype, one
can calculate the frequency of the genotypes
and alleles in the population
15.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
• By convention, the frequency of the more
common of the two alleles is designated
by the letter p and that of the less common
allele by the letter q
• Because there are only two alleles, the
sum of p and q must always equal 1
15.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
• In algebraic terms, the Hardy-Weinberg
equilibrium is written as an equation
p2
+
2pq
+
q
=
1
Figure 15.11 Hardy-Weinberg
equilibrium
15.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
•
The Hardy-Weinberg equilibrium only works if the
following five assumptions are met
1.
The size of the population is very large or effectively infinite.
2.
Individuals can mate with one another at random.
3.
There is no mutation.
4.
There is no immigration or emigration.
5.
All alleles are replaced equally from generation to generation.
15.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
• Most human populations are large and
randomly mating with respect to most
traits and thus are similar to an ideal
population envisioned by Hardy and
Weinberg
 for example, the frequency of heterozygote
carriers for recessive genetic disorders can be
estimated using the Hardy-Weinberg
equilibrium
15.5 Agents of Evolution
•
Five factors can alter the proportions of homozygotes
and heterozygotes enough to produce significant
deviations from Hardy-Weinberg predictions
1. mutation
1. migration
2. genetic drift
3. nonrandom mating
4. selection
15.5 Agents of Evolution
• Mutation is a change in a nucleotide
sequence in DNA
 mutation rates are generally too low to
significantly alter Hardy-Weinberg proportions
 mutations must affect the DNA of the germ
cells or the mutation will not be passed on to
offspring
 however, no matter how rare, mutation is the
ultimate source of variation in a population
15.5 Agents of Evolution
• Migration is the movement of individuals
between populations
 the movement of individuals can be a
powerful force upsetting the genetic stability
of natural populations
• the magnitude of the effects of migration is based
on two factors
– the proportion of migrants in the population
– the difference in allele frequencies between the migrants
and the original population
15.5 Agents of Evolution
• Genetic drift describes random changes
in allele frequencies
 in small populations, the frequencies of
particular alleles may be changed drastically
by chance alone
 in extreme cases, individual alleles of a given
gene may be
• all represented in few individuals
• accidentally lost if individuals fail to reproduce or
die
15.5 Agents of Evolution
•
A series of small populations that are isolated from one another may
come to differ strongly as the result of genetic drift
 founder effect occurs when one of a few individuals migrate and
become the founders of a new, isolated population at some distance
from their place of origin
• the alleles that they carry will become a significant fraction of the new
population’s genetic endowment
 bottleneck effect occurs when a population is drastically reduced in
size
• the surviving individuals constitute a random genetic sample of the original
population
15.5 Agents of Evolution
• Nonrandom mating occurs when
individuals with certain genotypes mate
with one another either more or less
commonly than would be expected by
chance
 sexual selection is choosing a mate based
on, often, physical characteristics
 nonrandom mating alters genotype
frequencies but not allele frequencies
15.5 Agents of Evolution
• Selection, according to Darwin, occurs if
some individuals leave behind more
progeny than others
 the likelihood that they will do so is affected by
their individual characteristics
• in artificial selection, a breeder selects for the
desired characteristics
• in natural selection, conditions in nature
determine which kinds of individuals in a
population are the most fit
Table 15.1 Agents of Evolution
15.5 Agents of Evolution
• Stabilizing selection is a form of
selection in which both extremes form an
array of phenotypes are eliminated
 the result is an increase in the frequency of
the already common intermediate phenotype
 for example, human birthweight is under
stabilizing selection
Figure 15.14 (a) Forms of selection
found in nature
15.5 Agents of Evolution
• Disruptive selection is a form of selection
in which the two extremes in an array of
phenotypes become more common in the
population
 selection acts to eliminate the intermediate
phenotypes
 for example, beak size in African blackbellied
seedcracker finches is under disruptive
selection because the available seeds are
only large or small
Figure 15.14 (b) Forms of selection
found in nature
15.5 Agents of Evolution
• Directional selection is a form of
selection that occurs when selection acts
to eliminate one extreme from an array of
phenotypes
 for example, the enzyme lactate
dehydrogenase has a cold-adapted form that
is more common in northern latitudes
Figure 15.14 (c) Forms of selection
found in nature
Figure 15.13 Three kinds of natural
selection
15.6 Sickle-Cell Anemia
• Sickle-cell anemia is a hereditary disease
affecting hemoglobin molecules in the
blood
 the disorder results from a single nucleotide
change in the gene encoding β-hemoglobin
• this causes the sixth amino acid in the chain to
change from glutamic acid (very polar) to valine
(nonpolar)
• as a result, the hemoglobin molecules clump
together and deform the red blood cell into “sickleshape”
Figure 15.16 Why the sickle-cell
mutation causes hemoglobin to clump
15.6 Sickle-Cell Anemia
• Persons homozygous for the sickle-cell genetic
mutation frequently have a reduced lifespan
 the sickled form of hemoglobin does not carry oxygen
atoms
 the red blood cells that are sickled do not flow
smoothly through capillaries
• Heterozygous individuals make enough function
hemoglobin to keep their red blood cells healthy
15.6 Sickle-Cell Anemia
• The frequency of sickle-cell allele is about 0.12
in Central Africa
 one in 100 people is homozygous for the defective
allele and develops the fatal disorder
 sickle-cell anemia strikes roughly two African
Americans out of every thousand
• If natural selection drives evolution, why has
natural selection not acted against the defective
allele in Africa and eliminated it from the
population here?
15.6 Sickle-Cell Anemia
• The defective allele has not been
eliminated from Central Africa because
people who are heterozygous are much
less susceptible to malaria
 the payoff in survival of heterozygotes makes
up for the price in death of homozygotes
• this is called heterozygote advantage
• stabilizing selection occurs because malarial
resistance counterbalances lethal anemia
Figure 15.17 How stabilizing selection
maintains sickle-cell anemia
20% of individuals are heterozygous and survive malaria
1% of individuals are homozygous and die of sickle cell anemia
15.7 Selection on Color in Guppies
• Guppies are colorful fish that a popular for
aquariums
 on the island of Trinidad, guppies are found in two
very different stream environments
• in pools above waterfalls, the guppies are found along with
the killifish, a seldom predator of guppies
• in pools below waterfalls, the guppies are found in pools
along with the pike cichlid, a voracious predator of guppies
• guppies can move between pools by swimming upstream
during floods
15.7 Selection on Color in Guppies
• Guppy populations above and below
waterfalls exhibit many differences
 guppies in high-predation pools are not as
colorful as guppies in low-predation pools
 guppies in high-predation pools tend to
reproduce at an earlier age and attain
relatively smaller adult body sizes
 these differences suggest the function of
natural selection
Figure 15.18 The evolution of
protective coloration in guppies
15.7 Selection on Color in Guppies
• John Endler conducted experiments on guppies to determine
whether predation risk was really the driving selective force in this
system
 in a controlled laboratory setting, he created artificial pool environments
in which he placed guppies in one of three conditions:
•
•
•
with no predator present
with killifish present (low predation risk)
with cichlid present (high predation risk)
 after 10 guppy generations, he found that the guppies from no or low
predation risk pools were both larger and more colorful than the guppies
from the high predation risk pool
 he later found the same results in field experiments
Figure 15.19 Evolutionary change
in spot number
15.8 The Biological Species
Concept
• Speciation is the macroevolutionary
process of forming new species from preexisting species
 it involves successive change
• first, local populations become increasingly
specialized
• then, if they become different enough, natural
selection may act to keep them that way
15.8 The Biological Species
Concept
• Ernst Mayr coined the biological species
concept, which defines species as
“groups of actually or potentially interbreeding
natural populations which are reproductively
isolated from other such groups”
• Populations whose members do not mater with
each other and cannot produce fertile offspring
are said to be reproductively isolated and,
thus, members of different species
15.8 The Biological Species
Concept
• Barriers called reproductive isolating
mechanisms cause reproductive isolation
by preventing genetic exchange between
species
 prezygotic isolating mechanisms
• prevent the formation of zygotes
 postzygotic isolating mechanisms
• prevent the proper functioning of zygotes once
they have formed
15.8 The Biological Species
Concept
• There are six different prezygotic
reproductive isolating mechanisms






geographical isolation
ecological isolation
temporal isolation
behavioral isolation
mechanical isolation
prevention of gamete fusion
15.8 The Biological Species
Concept
• Geographical isolation occurs simply in
cases when species exist in different
areas and are not able to interbreed
• Ecological isolation results from two
species who occur in the same area but
utilize different portions of the environment
and are unlikely to hybridize
Figure 15.20 Lions and tigers are
ecologically isolated
15.8 The Biological Species
Concept
• Temporal isolation results from two species
having different reproductive periods, or
breeding seasons, that preclude hybridization
• Behavioral isolation refers to the often
elaborate courtship and mating rituals of some
groups of animals, which tend to keep these
species distinct in nature even if they inhabit the
same places
15.8 The Biological Species
Concept
• Mechanical isolation results from
structural differences that prevent mating
between related species of animals and
plants
• Prevention of gamete fusion blocks the
union of gametes even following
successful mating
15.8 The Biological Species
Concept
• If hybrid matings do occur, and zygotes are
produced, many postzygotic factors may prevent
those zygotes from developing into normal
individuals
 in hybrids, the genetic complements of two species
may be so different that they cannot function together
normally in embryonic development
 even if hybrids survive the embryo stage, they may
not develop normally
 finally, many hybrids are sterile
Figure 15.21 Postzygotic isolation
in leopard frogs.
Table 15.2 Isolating Mechanisms
15.10 Working with the Biological
Species Concept
• Speciation is a two-part process
 first, initially identical populations must diverge
 second, reproductive isolation must evolve to
maintain these differences
• There are two mechanisms for speciation
 allopatric speciation
• geographically isolated populations become new species
due to their evolving reproductive isolation
 sympatric speciation
• one species splits into two at a single locality
15.10 Working with the Biological
Species Concept
• Speciation is much more likely in
geographically isolated populations
 for example, allopatric speciation can explain
how isolated populations of kingfishers in New
Guinea are strikingly different from each other
and from the mainland population
Figure 15.22 Populations can become
geographically isolated for a variety of reasons
15.10 Working with the Biological
Species Concept
• Instantaneous sympatric speciation occurs when
an individual is reproductively isolated from all
other members of its species through
polyploidy, a process common in plants
 polyploidy can arise in two ways
• autopolyploidy involves all sets of chromosomes come from
the same individual
• allopolyploidy arises when two different species hybridize
and the resulting offspring have a mixture of chromosomes
 the polyploids can either self-reproduce or breed with
other polyploids
15.10 Working with the Biological
Species Concept
• There are some problems associated with the
biological species concept
 many recognized species of plants and some animals
can still hybridize
• these observations suggest that reproductive isolation may
not be the only force maintaining species integrity
 nothing is known about extinct species’ reproduction
 the concept is irrelevant to the many kinds of
organisms who do not reproduce sexually
Inquiry & Analysis
• At what latitude do fish
populations exhibit the
greatest variability in
allele a frequency?
• Where along this
latitudinal gradient in the
frequency of allele a
would you expect to find
the highest frequency of
heterozygous
individuals?
Pie chart and graph on effect of
latitude on allele frequency