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22
The Mechanisms of Evolution
22 The Mechanisms of Evolution
• 22.1 What Facts Form the Base of Our Understanding of
Evolution?
• 22.2 What Are the Mechanisms of Evolutionary Change?
• 22.3 What Evolutionary Mechanisms Result in
Adaptation?
• 22.4 How Is Genetic Variation Maintained
within Populations?
• 22.5 What Are the Constraints on Evolution?
• 22.6 How Have Humans Influenced Evolution?
22.1 What Facts Form the Base of Our Understanding of
Evolution?
The young Charles Darwin was
passionately interested in geology and
natural science.
In 1831, he was recommended for a
position on the H.M.S. Beagle, for a 5year survey voyage around the world.
Figure 22.1 Darwin and the Voyage of the Beagle (Part 1)
Figure 22.1 Darwin and the Voyage of the Beagle (Part 2)
Figure 22.1 Darwin and the Voyage of the Beagle (Part 3)
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin often went ashore to study rocks
and collect specimens, and make
observations about the natural world.
In the Galapagos Islands he observed
that species were similar to, but not the
same as, species on the mainland of
South America. He also realized that
species varied from island to island.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin postulated that species had
reached the islands from the mainland,
but then had undergone different
changes on different islands.
Part of the puzzle was determining what
could be a mechanism for such
changes.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
These observations, and many others,
led Darwin to propose an explanatory
theory for evolutionary change based on
two propositions:
• Species change over time.
• The process that produces the change
is natural selection.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin continued to amass evidence to
support his ideas until 1858, when he
received a letter from another naturalist,
Alfred Russel Wallace.
Wallace proposed a theory of natural
selection almost identical to Darwin’s.
A paper with the work of both men was
presented in 1858 to the Linnean
Society of London.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin published his book, The Origin of
Species in 1859.
The book provided exhaustive evidence
from many different fields to support
evolution and natural selection.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin and Wallace were both influenced
by economist Thomas Malthus, who
published An Essay on the Principle of
Population in 1838.
Populations of all species have the
potential for rapid increase.
But this does not occur in nature, so
death rate must also be high.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin observed that, though offspring
tended to resemble their parents, they
are not identical.
He suggested that slight variations
among individuals affect the chances of
surviving and producing offspring:
natural selection.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Natural selection:
Differential contribution of offspring to the
next generation by various genetic
types belonging to the same population.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin had observed variation and
artificial selection of certain desirable
traits in plants and animals by breeders.
Darwin himself bred pigeons.
Figure 22.2 Many Types of Pigeons Have Been Produced by Artificial Selection
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Individuals do not evolve. Populations do.
Population: A group of individuals of the
same species that live and interbreed in
a particular geographic area.
Members of a population become
adapted to the environment in which
they live.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Adaptations: The processes by which
useful characteristics evolve; and the
characteristics themselves.
An organism is considered to be adapted
to a particular environment when it can
be demonstrated that a slightly different
organism survives and reproduces less
well in that environment.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
For a population to evolve, its members
must possess heritable genetic variation.
The phenotype is the physical expression
of an organism’s genes.
Features of a phenotype are the
characters (e.g., eye color), specific form
of a character is a trait (e.g., blue).
22.1 What Facts Form the Base of Our Understanding of
Evolution?
A heritable trait is at least partly
determined by genes.
Genetic makeup of an organism is the
genotype.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Population genetics has three main
goals:
• Explain the origin and maintenance of
genetic variation
• Explain patterns and organization of
genetic variation
• Understand mechanisms that cause
changes in allele frequencies
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Different forms of a gene are called
alleles.
The gene pool is the sum of all copies of
all alleles at all loci in a population.
Figure 22.3 A Gene Pool
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Populations have genetic variation for
many characters.
Artificial selection for different characters
in a single species of wild mustard
produced many crop plants.
Figure 22.4 Many Vegetables from One Species (Part 1)
Figure 22.4 Many Vegetables from One Species (Part 2)
22.1 What Facts Form the Base of Our Understanding of
Evolution?
In laboratory experiments with
Drosophila, researchers selected for
high or low numbers of body bristles
from an initial population with
intermediate numbers.
After 35 generations, numbers for both
high-bristle and low-bristle fell outside
the range of the original population.
Figure 22.5 Artificial Selection Reveals Genetic Variation
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Locally interbreeding groups are called
Mendelian populations.
Allele frequencies, or their proportion in
the gene pool, are estimated by
counting alleles in a sample of
individuals.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Allele frequency:
number of copies of the allele in the population
p
sum of alleles in the population
If a locus has two alleles, A and a, there
could be three genotypes: AA, Aa, and
aa. The population is polymorphic at
that locus.
Calculate genotype & allele frequencies:
• 100 monsters = AA
• 75 monsters = Aa
• 2 monsters = aa
Figure 22.6 Calculating Allele Frequencies
22.1 What Facts Form the Base of Our Understanding of
Evolution?
If p is the frequency of allele A, and
q is the frequency of allele a,
p+q=1
q=1–p
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Genotype frequencies may not be the
same as allele frequencies.
Frequencies of different alleles at each
locus and the frequencies of genotypes
in a Mendelian population make up the
genetic structure of the population.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
If certain conditions are met, the genetic
structure of a population
does not change over time.
If an allele is not advantageous, its
frequency remains constant.
The Hardy-Weinberg equilibrium
describes a model situation in which
allele frequencies do not change.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Conditions that must be met:
• Mating is random.
• Population size is infinite. Large populations
aren’t affected by genetic drift.
• No gene flow—no migration into or out of the
population.
• No mutation.
• Natural selection does not affect survival of
any genotypes.
22.1 What Facts Form the Base of Our Understanding of
Evolution?
If these conditions hold:
Allele frequencies remain constant;
after one generation, genotype
frequencies occur in these proportions:
Genotype
AA
Aa
aa
Frequency p2
2pq
q2
Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 1)
Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 2)
22.1 What Facts Form the Base of Our Understanding of
Evolution?
For generation 1, probability of two A
alleles coming together is:
p  p  p 2  (0.55) 2  0.3025
Probability of two a alleles:
q  q  q  (0.45)  0.2025
2
2
22.1 What Facts Form the Base of Our Understanding of
Evolution?
There are two ways of producing a
heterozygote:
p  q or q  p , or 2 pq
The Hardy-Weinberg equation:
p  2 pq  q  1
2
2
22.1 What Facts Form the Base of Our Understanding of
Evolution?
Populations in nature never fit the
conditions for Hardy-Weinberg
equilibrium.
But, it is useful in predicting genotype
frequencies from allele frequencies; and,
because the model describes conditions
that would result in no evolution, patterns
of deviation from the model help identify
specific mechanisms of evolution.
22.2 What Are the Mechanisms of Evolutionary Change?
Hardy-Weinberg equilibrium is a null
hypothesis that assumes evolutionary
forces are absent.
Known evolutionary mechanisms:
• Mutation
• Gene flow
• Genetic drift
• Nonrandom mating
• Natural selection
H-W Practice!
IB Questions
22.2 What Are the Mechanisms of Evolutionary Change?
Mutation is the origin of genetic
variation.
Mutation is any change in DNA; it
appears to be random with respect to
the adaptive needs of an organism.
Most mutations are harmful or neutral,
but if conditions change, could become
advantageous.
22.2 What Are the Mechanisms of Evolutionary Change?
Mutations can also restore alleles that
other processes remove.
Mutation rates are low—about one per
locus in a million zygotes.
Creates a lot of variation because of the
number of genes that can mutate,
chromosome rearrangements that can
change many genes simultaneously,
and large numbers of individuals.
22.2 What Are the Mechanisms of Evolutionary Change?
Because mutation rate is low, mutations
in themselves result in only minor
deviations from Hardy-Weinberg
equilibrium.
If large deviations are found, it is
appropriate to look for other
mechanisms.
22.2 What Are the Mechanisms of Evolutionary Change?
Gene flow is a result of the migration of
individuals and movements of gametes
between populations.
New alleles can be added to the gene
pool, or allele frequencies changed.
22.2 What Are the Mechanisms of Evolutionary Change?
Genetic drift results from random
changes in allele frequencies.
In large populations, genetic drift can
influence frequencies of alleles that
don’t affect survival and reproduction.
If populations are reduced to a small
number of individuals—a population
bottleneck, genetic drift can reduce the
genetic variation.
Figure 22.8 A Population Bottleneck
22.2 What Are the Mechanisms of Evolutionary Change?
A population forced through a bottleneck
is likely to lose much genetic variation.
Example: Greater prairie chickens in
Illinois were reduced to about 50 birds
in the 1990s; California fan palms are
now restricted to a few oases in
southern California.
Figure 22.9 Species with Low Genetic Variation (Part 1)
Figure 22.9 Species with Low Genetic Variation (Part 2)
22.2 What Are the Mechanisms of Evolutionary Change?
Genetic drift also effects small
populations that colonize a new region.
Colonizing population is unlikely to have
all the alleles present in the whole
population.
Founder effect—equivalent to a
bottleneck.
22.2 What Are the Mechanisms of Evolutionary Change?
Example of founder effect:
Populations of European fruit fly D.
subobscura began in Chile, and then in
Washington state.
Both populations grew and expanded their
ranges.
These populations have very similar genetic
structure, and much less variation than the
European populations.
Figure 22.10 A Founder Effect
22.2 What Are the Mechanisms of Evolutionary Change?
Nonrandom mating occurs when
individuals choose mates with particular
phenotypes.
If individuals choose the same genotype
as themselves, homozygote frequencies
will increase.
22.2 What Are the Mechanisms of Evolutionary Change?
Nonrandom mating in primroses
(Primula):
Two flower types—pin and thrum. Pollen
from one type can fertilize only flowers
of the other type.
Figure 22.11 Flower Structure Fosters Nonrandom Mating
22.2 What Are the Mechanisms of Evolutionary Change?
Selfing, or self-fertilization, is a common
form of nonrandom mating.
Selfing reduces the frequency of
heterozygotes, and increases
homozygotes, but does not change
allele frequencies in the population.
22.3 What Evolutionary Mechanisms Result in Adaptation?
Adaptation occurs when some individuals
in a population contribute more offspring
to the next generation.
Allele frequencies change in a way that
adapts individuals to the environment
that influenced that reproductive
success.
This is natural selection.
22.3 What Evolutionary Mechanisms Result in Adaptation?
Natural selection acts on phenotype.
Fitness is the reproductive contribution
of a phenotype to subsequent
generations.
Changes in the relative success of
different phenotypes in a population
leads to change in allele frequencies.
22.3 What Evolutionary Mechanisms Result in Adaptation?
Fitness of a phenotype is determined by
the average rates of survival and
reproduction of individuals with that
phenotype.
22.3 What Evolutionary Mechanisms Result in Adaptation?
Most characters are influences by alleles
at more than one locus.
Such characters often show quantitative
variation instead of qualitative.
Example: The distribution of body size of
individuals in a population is likely to
resemble a bell-shaped curve.
22.3 What Evolutionary Mechanisms Result in Adaptation?
Natural selection can act on characters
with quantitative variation in three ways:
• Stabilizing selection preserves average
phenotype.
• Directional selection favors individuals
that vary in one direction.
• Disruptive selection favors individuals
that vary in opposite directions from the
average.
Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 1)
Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 2)
Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 3)
22.3 What Evolutionary Mechanisms Result in Adaptation?
Stabilizing selection reduces variation
in a population, but does not change the
mean.
Rates of evolution are slow because
natural selection is usually stabilizing.
Example: human birth weights
Figure 22.13 Human Birth Weight Is Influenced by Stabilizing Selection
22.3 What Evolutionary Mechanisms Result in Adaptation?
Directional selection occurs when
individuals at one extreme are more
successful.
If directional selection operates over
many generations, an evolutionary trend
occurs.
Example: resistance to tetrodotoxin
(TTX) in garter snakes
Figure 22.14 Resistance to TTX Is Associated with the Presence of Newts
22.3 What Evolutionary Mechanisms Result in Adaptation?
Disruptive selection: Variation is
created when individuals at either
extreme are more successful than
average individuals.
Example: bill size in black-bellied seed
crackers
Figure 22.15 Disruptive Selection Results in a Bimodal Distribution
22.3 What Evolutionary Mechanisms Result in Adaptation?
Sexual selection is a special type of
natural selection, which acts on
characters that determine reproductive
success.
If an individual survives but does not
reproduce, it makes no contribution to
the next generation.
Sexual selection favors traits that
increase the chances of reproduction.
22.3 What Evolutionary Mechanisms Result in Adaptation?
Traits such as bright colors, long horns,
and elaborate courtship displays, may
improve ability to compete for mates
(intrasexual selection);
or to be more attractive to the opposite
sex (intersexual selection).
Such traits are costly, but reliably
demonstrate the fitness of the bearer to
the choosing sex.
22.3 What Evolutionary Mechanisms Result in Adaptation?
This has been shown experimentally in
long-tailed widowbirds.
Male tails were shortened or lengthened.
Both were able to successfully defend
their territories, but males with
lengthened tails attracted more females.
Long tails indicate the health and vigor of
the male.
Figure 22.16 The Longer the Tail, the Better the Male (Part 1)
Figure 22.16 The Longer the Tail, the Better the Male (Part 2)
22.3 What Evolutionary Mechanisms Result in Adaptation?
Sexual selection has also been shown in
zebra finches.
Brightness of male’s bill is an indicator of
health.
Color is due to carotenoids, which are
also important in the immune system. A
brighter bill indicates more carotenoids
and greater overall health.
Figure 22.17 Bright Bills Signal Good Health (Part 1)
Figure 22.17 Bright Bills Signal Good Health (Part 2)
Figure 22.17 Bright Bills Signal Good Health (Part 3)
22.4 How Is Genetic Variation Maintained within Populations?
Many mutations do not affect the function
of the resulting proteins.
An allele that does not affect fitness is a
neutral allele. They tend to accumulate
in a population.
Molecular techniques allow neutral
alleles to be identified and used to
estimate rates of evolution (see Chapter
24).
22.4 How Is Genetic Variation Maintained within Populations?
Sexual reproduction results in new
combinations of genes through crossing
over and independent assortment, and
the combination of gametes.
Sexual recombination produces genetic
variety that increases evolutionary
potential.
22.4 How Is Genetic Variation Maintained within Populations?
But sexual reproduction has
disadvantages:
• Recombination can break up adaptive
combinations of genes.
• Reduces rate at which females pass
genes to offspring.
• Dividing offspring into genders reduces
the overall reproductive rate.
22.4 How Is Genetic Variation Maintained within Populations?
How did sexual reproduction arise?
Possible advantages:
• Sexual reproduction facilitates repair of
damaged DNA. Damage on one
chromosome can be repaired by
copying intact sequence on the other
chromosome.
22.4 How Is Genetic Variation Maintained within Populations?
• Permits elimination of deleterious
mutations.
In asexually reproducing species,
deleterious mutations can accumulate;
only death of the lineage can eliminate
them—Muller’s ratchet.
22.4 How Is Genetic Variation Maintained within Populations?
• Sexual recombination produces some
individuals with many deleterious
mutations, some with few. The
individuals with few deleterious
mutations are more likely to survive.
22.4 How Is Genetic Variation Maintained within Populations?
• The variety of genetic combinations
possible in sexually reproducing species
may be especially valuable in defense
against pathogens and parasites.
22.4 How Is Genetic Variation Maintained within Populations?
• Sexual recombination does not affect the
frequency of alleles, but generates new
combinations of alleles on which natural
selection can act.
22.4 How Is Genetic Variation Maintained within Populations?
Frequency-dependent selection: A
polymorphism can be maintained when
fitness depends on its frequency in the
population.
Example: a scale-eating fish in Lake
Tanganyika. “Left-mouthed” and “rightmouthed” individuals are both favored;
the host fish can be attacked from either
side.
Figure 22.18 A Stable Polymorphism
22.4 How Is Genetic Variation Maintained within Populations?
Environmental variation also helps to
preserve genetic variation.
Example: Colias butterflies live in an
environment with temperature extremes.
The population is polymorphic for an
enzyme that influences flight at different
temperatures.
Heterozygotes are favored because they
can fly over a larger temperature range.
Figure 22.19 A Heterozygote Mating Advantage (Part 1)
Figure 22.19 A Heterozygote Mating Advantage (Part 2)
22.4 How Is Genetic Variation Maintained within Populations?
Subpopulations in different geographic
regions maintain genetic variation.
The subpopulations may be subjected to
different environmental conditions and
selective pressures.
Example: populations of white clover that
produce cyanide as defense against
herbivores. Plants that produce cyanide
are more likely to be killed by frost.
Figure 22.20 Geographic Variation in a Defensive Chemical
22.5 What Are the Constraints on Evolution?
Lack of genetic variation can prevent
evolution of potentially favorable traits.
If the allele for a given trait does not exist
in a population, that trait cannot evolve,
even if it would be favored by natural
selection.
22.5 What Are the Constraints on Evolution?
Evolution must work within the boundaries
of universal constraints such as:
• Cell size, constrained by surface areato-volume ratios
• Protein folding, constrained by types of
bonding that can occur
• Laws of thermodynamics that constrain
energy transfers
22.5 What Are the Constraints on Evolution?
Developmental processes also constrain
evolution.
All evolutionary innovations are
modifications of previously existing
structures.
22.5 What Are the Constraints on Evolution?
Example: two lineages of bottom-dwelling
fishes
Skates and rays evolved from a common
ancestor with sharks. They started with a
flattened body plan, and can swim along
the ocean floor.
Sole and flounder evolved from laterally
flattened bony fishes. They can’t swim
well, but lie still on the bottom. The eyes
gradually shifted to one side.
Figure 22.21 Two Solutions to a Single Problem (Part 1)
Figure 22.21 Two Solutions to a Single Problem (Part 2)
22.5 What Are the Constraints on Evolution?
Adaptations involve both fitness costs and
benefits.
Benefit must outweigh cost if adaptation is
to evolve—the trade-offs must be
worthwhile.
Example: Garter snake resistance to TTX
only occurs where poisonous newts are
common.
22.5 What Are the Constraints on Evolution?
Conspicuous features used by some
males to compete with other males are a
trade-off with reproductive success.
Species in which males have multiple
mates are polygynous. Males are usually
larger than females and often have
weapons—horns, antlers, etc.
Dramatic differences between sexes are
known as sexual dimorphism.
22.5 What Are the Constraints on Evolution?
In polygynous species, males must
defend their mates against other males.
Defensive structures require a lot of
energy, but the male has a lot of
reproductive success.
22.5 What Are the Constraints on Evolution?
Short-term changes in allele frequencies
can be observed and manipulated to
demonstrate the processes by which
evolution occurs.
But patterns of long-term evolutionary
change can be influenced by infrequent
events (e.g., meteorites) or very slow
processes (e.g., continental drift). Other
types of evidence are used to study
these long-term changes.
22.6 How Have Humans Influenced Evolution?
As humans have changed their
environments, selective forces that act
on human populations have changed
also.
Today, survival and reproductive success
are related to genes that confer defense
against diseases such as malaria and
AIDS, and stresses of modern industrial
life such as hypertension.
22.6 How Have Humans Influenced Evolution?
Humans have also influenced the
evolution of other species:
Efforts to control populations of “pests”
make humans agents of evolutionary
change.
Sport hunters that seek large trophy
animals are removing the biggest and
healthiest animals from populations.
Figure 22.22 Trophy Hunting Selects for Smaller Males
22.6 How Have Humans Influenced Evolution?
Humans also move species around the
globe, and modify species through
breeding and biotechnology.
Humans are also changing the climate;
and have caused the extinction of many
species.
Photo 22.1 Western sandpipers (Calidris mauri), CA; all look the same to unpracticed eyes.
Photo 22.2 Fall gathering of ladybird beetles (Hippodamia convergens).
Photo 22.3 Brussel sprouts, emphasizing lateral buds.
Photo 22.4 Cauliflower, selected for flower clusters.
Photo 22.5 Broccoli, selected for stems and flowers.
Photo 22.6 Ornamental kale, emphasizing appearance of leaves.
Photo 22.7 Canola, or rape, selected for seed.
Photo 22.8 Northern elephant seals (Mirounga angustirostrus) experienced a population bottleneck.
Photo 22.9 Primrose (Primula sp.) flowers in cross section. Left: pin flower; Right: thrum flower.
Photo 22.10 Large blue mussels (Mytilus edulis) are better adapted to low salinity than young.
Photo 22.11 Buddhist monks at Hemis Gompa; Ladakh, northern India.
Photo 22.12 Ouighur women, men, and children gape at foreign tourists; Turfan, China.
Photo 22.13 Food vendors along the Irawaddy River, Burma.
Photo 22.14 Formal family photograph at Shinto wedding ceremony; Tokyo, Japan.
Photo 22.15 Members of the Yali tribe; Baliem Valley, Irian Jaya, Indonesia.
Photo 22.16 Headman and young women of Naitawoli Village, Fiji.
Photo 22.17 A Ja'aliyin mother and her children; Nile River, northern Sudan.
Photo 22.18 Village women preparing feast; Toulfe Village, Yatenga, Burkina Faso.
Photo 22.19 Teachers and native children; Hachipancha Village, Andean foothills, Peru.
Photo 22.20 Modern Mexican family; Janitzio Island, Patzcuaro, Mexico.
Photo 22.21 Family portrait: parents and children of European ancestry; United States.
Photo 22.22 Harbor, San Cristobal Is.
Photo 22.23 Shoreline, Darwin Bay, Tower Is.
Photo 22.24 Volcanic features, Bartolome Is.
Photo 22.25 Punta Moreno lava field, Isabella Is.
Photo 22.26 Crater of Vulcan Alcedo, Isabella Is.
Photo 22.27 "Pit Crater," sinkhole caused by receding lava, Highlands, Santa Cruz Is.