Download Chapter 13

The Living World
Fourth Edition
GEORGE B. JOHNSON
13
Evolution and
Natural Selection
PowerPoint® Lectures prepared by Johnny El-Rady
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.1 Evolution: Getting from
There to Here
The word “evolution” refers to how an entity changes
through time
Darwin initially used the phrase “descent with
modification” to explain the concept of evolution
The concept of evolution helps explain the great
paradox of biology:
In life there exists both unity and diversity
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.1 Evolution: Getting from
There to Here
Natural selection, the process that leads to evolution,
occurs in steps
1. Gene variation exists among individuals in a population
2. This variation is often passed to offspring
3. All populations overproduce offspring
4. Individuals with traits that aid survival and reproduction
have a better chance of contributing to the next generation
5. Over time, the population changes such that the traits of
the more successful reproducers are more prevalent
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.1 Evolution: Getting from
There to Here
Macroevolution
Evolutionary change on a grand scale
Encompasses the origins of new species and
major episodes of extinction
Microevolution
Evolutionary change on a small scale
Encompasses the genetic changes that occur
within a species over time
These changes are the result of changes in
gene frequencies
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.1 Evolution: Getting from
There to Here
Jean-Baptiste Lamarck proposed that evolution
occurred by inheritance of acquired characteristics
Individuals passed on to offspring body and
behavior changes acquired during their lives
In Darwin’s theory, by contrast, variation is not
created by experience
It already exists when selection acts on it
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.1 How did long necks evolve in giraffe
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The Rate of Evolution
Different kinds of organisms evolve at different rates
Bacteria evolve much faster than eukaryotes
The rate of evolution also differs within the same
group of species
In punctuated equilibrium, evolution occurs in
spurts
In gradualism, evolution occurs in a gradual,
uniform way
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.2
a) Punctuated equilibrium
b) Gradualism
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.2 The Evidence For Evolution
Evidence for evolution comes from the following
Fossil record
Molecular record
Anatomical record
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fossil Record
Provides the most direct evidence for macroevolution
Fossils are the preserved remains, tracks, or traces
of once-living organisms
They form when organisms become buried in
sediment and calcium in hard surfaces mineralizes
Arraying fossils according to age often provides
evidence of successive evolutionary change
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Large blunt
horns
Small bony
protuberance
Fig. 13.3
Evolution in the
titanotheres
Hoofed mammals
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fossils have been
found linking all
the major groups
The forms linking
mammals to
reptiles are
particularly well
known
Fig. 13.4 Whale
“missing links”
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Molecular Record
New alleles arise by mutations and they come to
predominance through favorable selection
Thus, evolutionary changes involve a continual
accumulation of genetic changes
Distantly-related organisms accumulate a greater
number of evolutionary differences than closelyrelated ones
This divergence is seen among vertebrates in
the 146-amino acid hemoglobin b chain
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The greater the
evolutionary distance
Fig. 13.5 Molecules
reflect evolutionary
divergence
The greater the number
of amino acid differences
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
This same pattern of
divergence is seen
with DNA sequences,
such as that of the
cytochrome c gene
The changes appear
to accumulate at a
constant rate
This phenomenon
is referred to as a
molecular clock
Fig. 13.6
Note: Different proteins evolve at different rates
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Anatomical Record
All vertebrates share a basic set of developmental
instructions
Fig. 13.7
Relict
developmental
forms
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Anatomical Record
Homologous structures
Have different structure and function but are all
derived from the same part of a common ancestor
The same basic bones
are present in each
forelimb
Fig. 13.8
Homology among
vertebrate limbs
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Anatomical Record
Analogous structures
Resemble each other as a result of parallel
evolutionary adaptations to similar environments
They are the result of convergent evolution
Different animals often adapt in similar fashion
when challenged by similar opportunities
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.9 Convergent evolution: many paths to one goal
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.9 Convergent evolution: many paths to one goal
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Anatomical Record
Vestigial organs
Structures that are no longer in use
The human appendix
Apes have a much larger appendix that is
involved in digestion
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.3 Evolution’s Critics
Critics of evolution raise seven principal objections
1.
2.
3.
4.
5.
6.
7.
Evolution is not solidly demonstrated
There are no fossil intermediates
The intelligent design argument
Evolution violates the 2nd law of thermodynamics
Proteins are too improbable
Natural selection does not imply evolution
The irreducible complexity argument
All of these objections are without scientific merit!
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.4 Genetic Change Within
Populations: The Hardy-Weinberg Rule
Population genetics is the study of the properties of
genes in a population
Genetic variation in populations puzzled scientists
Dominant alleles were believed to drive recessive alleles
out of populations
In 1908, G. Hardy and W. Weinberg pointed out that
in large populations with random mating, allele
frequencies remain constant
Dominant alleles do not, in fact, replace recessive ones
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
A population that is in Hardy-Weinberg equilibrium
is not evolving
Hardy and Weinberg came to their conclusion by analyzing
allele frequencies in successive generations
Frequency =
Number of individuals
falling within a category
Total number of individuals
being considered
If a population of 100 cats has 84 black and 16 white
Then the frequencies of black and white
phenotypes are 0.84 and 0.16, respectively
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
B allele  Black color
b allele  White color
By convention
The more common allele (B) is designated p
The less common allele (b) is designated q
p+q=1
The Hardy-Weinberg equilibrium can be written as
an equation
Individuals homozygous
(p + q)2 = p2 + 2pq + q2
for allele b
Individuals homozygous
for allele B
Individuals heterozygous
for alleles B and b
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The equation allows calculation of allele frequencies
Frequency of white (bb) cats = 16/100 = 0.16
=> q2 = 0.16
=> q = √ 0.16 = 0.4
p + q =1
=> p = 1 – q = 1 – 0.4 = 0.6
What about genotype frequencies?
Frequency of the homozygous dominant genotype is
36 out of 100 cats are black (BB)
p2 = (0.6)2 = 0.36
Frequency of the heterozygous genotype is
48 out of 100 cats are black (Bb)
2pq = 2(0.6)(0.4) = 0.48
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.10
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Hardy-Weinberg Assumptions
The Hardy-Weinberg equation is true only if the
following five assumptions are met
1.
2.
3.
4.
5.
Large population size
Random mating
No mutation
No migration
No natural selection
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.5 Why Allele Frequencies Change
Five evolutionary forces can significantly alter the
allele frequencies of a population
1.
2.
3.
4.
5.
Mutation
Migration
Genetic drift
Nonrandom mating
Selection
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Mutation
Table 13.1
Errors in DNA
replication
The ultimate source of
new variation
Mutation rates are too
low to significantly
alter allele frequencies
on their own
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Migration
Table 13.1
Movement of
individuals from one
population to another
Immigration: movement
into a population
Emigration: movement
out of a population
A very potent agent of
change
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Genetic Drift
Random loss of alleles
Table 13.1
More likely to occur in
smaller population
Founder effect
Small group of individuals
establishes a population
in a new location
Bottleneck effect
A sudden decrease in
population size to natural
forces
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Nonrandom Mating
Mating that occurs more
or less frequently than
expected by chance
Table 13.1
Inbreeding
Mating with relatives
Increases homozygosity
Outbreeding
Mating with non-relatives
Increases heterozygosity
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Selection
Some individuals leave
behind more offspring
than others
Table 13.1
Artificial selection
Breeder selects for
desired characteristics
Natural selection
Environment selects for
adapted characteristics
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.6 Forms of Selection
Selection is a statistical concept
One cannot predict the fate of any single individual
But it is possible to predict which kind of individual will tend
to become more common in a population
Three types of natural selection have been identified
Stabilizing selection
Acts to eliminate both extreme phenotypes
Disruptive selection
Acts to eliminate intermediate phenotypes
Directional selection
Acts to eliminate a single extreme phenotype
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.12 Three kinds of natural selection
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Stabilizing Selection
Increase in the
frequency of the
intermediate phenotype
In humans, infants
with intermediate
weight at birth have
the highest survival
rate
In chicken, eggs of
intermediate weight
have the highest
hatching success
Fig. 13.13
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Disruptive Selection
In the African seedcracker finch, largeand small-beaked
birds predominate
Can open tough shells
of large seeds
Fig. 13.14
Intermediate-beaked
birds are at a
disadvantage
Unable to open large
seeds
Too clumsy to open
small seeds
More adept at
handling small seeds
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Directional Selection
Drosophila flies that
flew toward light
were eliminated
from the population
The remaining flies
were mated and the
experiment repeated
for 20 generations
Fig. 13.15
Phototropic flies are far less
frequent in the population
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.7 Sickle-Cell Anemia
Sickle-cell anemia is a hereditary disease affecting
hemoglobin molecules in the blood
It was first detected on December 31st, 1904
Fig. 13.16
Sickled
RBCs
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The sickle-cell mutation changes the 6th amino acid in the bhemoglobin chain from glutamic acid to valine
This causes hemoglobin molecules to clump
The result is sickled red blood cells
In normal RBCs, the hemoglobin chains do not clump
Fig. 13.17a
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Sickle-cell homozygosity leads to a reduced life span
Heterozygosity produces enough hemoglobin to
keep RBCs healthy
The disease originated in Central Africa
It affects 1 in 500 African Americans
But it is almost unknown in other racial groups
Why is the defective allele still around?
People who are heterozygous for the sickle-cell
allele have less susceptibility to malaria
This is an example of heterozygote advantage
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.18
Stabilizing selection is thus acting on the sickle-cell
allele
It occurs because malarial resistance
counterbalances lethal anemia
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.8 Selection on Color in Guppies
Poecilia reticulata (guppy) is a popular aquarium fish
In nature, it is found in small streams in NE South
America and in mountainous streams in Trinidad
Due to dispersal barriers, guppies can be found in
two different pool environments
Below waterfalls, where risk of predation is high
Above waterfalls, where risk of predation is low
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.19 The evolution of protective coloration in guppies
A voracious
predator of guppies
Rarely preys
on guppies
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
In low-predation pools,
males
Display gaudy colors
and spots
Reproduce at a late age
Attain larger adult sizes
In high-predation pools,
males
Exhibit drab coloration
Reproduce younger
Attain relatively small
adult sizes
In the absence
of predators,
larger more
colorful fish
may produce
more offspring
In the presence
of predators,
smaller and less
colorful fish are
likely favored by
selection
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The evolution of these differences in guppies was
experimentally tested in laboratory greenhouses
Fig. 13.20
Indistinguishable from
low-predation controls
Same results were obtained
in field experiments
Smaller and
drab in color
Thus, natural selection can lead to rapid evolutionary change
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.9 The Biological Species Concept
Speciation is the species-forming process
It involves progressive change
1. Local populations become increasingly specialized
2. Natural selection acts to keep them different enough
Ernst Mayr coined the biological species concept
“Species are groups of actually or potentially interbreeding
natural populations, which are reproductively isolated from
other such groups”
Reproductively isolated populations
Populations whose members do not mate with each other
or who cannot produce fertile offspring
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.10 Isolating Mechanisms
Reproductive isolating mechanisms are the barriers
that prevent genetic exchange between species
Prezygotic isolating mechanisms
Prevent the formation of zygotes
Postzygotic isolating mechanisms
Prevent the proper functioning of zygotes after
they have formed
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Prezygotic Isolating Mechanisms
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.21 Lions and tigers are ecologically isolated
Live in forest
Live in open
grassland
Tiglon
Hybridization has been successful in captivity
But it does not occur in the wild
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Prezygotic Isolating Mechanisms
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Prezygotic Isolating Mechanisms
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Postzygotic Isolating Mechanisms
Examples:
Hybridization between sheep and goats produces
embryos that die in the earliest embryonic stages
Leopard frogs of the United States are a group of similar
species, and NOT a single species as was long assumed
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.22 Postzygotic isolation in leopard frogs
Rana pipiens
Rana blairi
Rana sphenocephala
Rana berlandieri
Hybrids between them produced defective embryos in the lab
Their mating calls also differ substantially
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.11 Working with the
Biological Species Concept
Speciation is a two-part process
1. Identical populations must diverge
2. Reproductive isolation must evolve to maintain these
differences
Speciation occurs much more readily in the
absence of gene flow
This much more likely in geographically isolated
populations
Populations can become geographically isolated
for several reasons
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 13.23
New colonization
Barriers to
movement
Extinction of
intermediate populations
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Allopatric speciation
The differentiation of geographically isolated populations
into distinct species
Sympatric speciation
The differentiation of populations within a common
geographical area into distinct species
Instantaneous sympatric speciation may occur through
polyploidy
More than two sets of chromosomes
Autopolyploidy  All chromosomes from one species
Allopolyploidy  Chromosomes derived from two
species, via hybridization
Much more common in plants than animals
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Problems with the Biological Species Concept
The biological species concept has been criticized
for several reasons
1. The extent to which all species are truly are
reproductively isolated
It is becoming increasingly evident that hybridization is
not that uncommon in plants and animals
2. It can be difficult to apply the concept to populations
that do not occur together in nature
It is not possible to observe whether they would
interbreed naturally
3. The concept is more limited than its name would imply
Many organisms are asexual and reproduce without
mating
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Problems with the Biological Species Concept
For these reasons, other concepts have been
proposed to define a species
However, none has universal applicability
Indeed, because of the diversity of organisms, it may
be that there is no single definition of a species
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.12 The Pace of Evolution
The standard view since Darwin was that
evolutionary change occurred extremely slowly
Imperceptible changes accumulate such that, over
thousands or millions of years, major changes could occur
This is termed gradualism
In 1972, Niles Eldredge and Stephen Jay Gould
proposed the Punctuated Equilibrium hypothesis
Evolutionary change occurs in bursts separated by long
periods of little or no evolutionary change (termed stasis)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
13.12 The Pace of Evolution
Examples of both gradualism and punctuated
equilibrium exist
So speciation clearly occurs in different ways
However, the idea that speciation is necessarily
linked to phenotypic change has not been supported
Speciation can occur without phenotypic change
Phenotypic change can occur within a species in
the absence of speciation
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display