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Processes of Evolution
Chapter 16
Impacts and Issues
“Rise of the Super Rats”
Rats Are Notorious Pests
The rodenticide warfarin was very
effective when it was first introduced
in the 1950s
Within a few years, rats developed
resistance – the chemical would no
longer kill
Rats Evolved
The resistant rats happened to
inherit a gene, which made the
chemical ineffective
The survivors passed on the gene to
their offspring
Soon resistant rats were the norm
This is an example of evolution
Section 16.1
Early Beliefs,
Confounding Discoveries
Confounding Evidence
Biogeography
Comparative anatomy
Geologic discoveries
Biogeography
Size of the known world expanded
enormously in the 15th century
Discovery of new organisms in
previously unknown places could not
be explained by accepted beliefs
– How did species get from center of
creation to all these places?
Travel Generates Ideas
When global voyages of the 16th
century revealed unusual species not
known in Europe, the students of
biogeography began to question,
“Where do all these species ‘fit’ in the
great Chain of Being”
Species’ Distribution
Furthermore, if all species had been
created at the same time and place,
why were certain species found in
only some parts of the world but not
others
And, how did so many species get
from the center of creation to islands
and isolated places?
Big Birds
Similar-looking species, like the
South American rhea, the African
ostrich and the Australian emu are
native to three continents
Why are they so much alike?
African Ostrich
Fig. 16-2c, p.238
Australian Emu
Fig. 16-2b, p.238
South American Rhea
Fig. 16-2a, p.238
Comparative Morphology
Study of similarities and differences
in body plans of major groups
Puzzling patterns:
– Animals as different as whales and bats
have similar bones in forelimbs
– Some parts seem to have no function
fossilized ankle bone
Fig. 16-3b, p.239
Geological Discoveries
Similar rock layers throughout world
Certain layers contain fossils
Deeper layers contain simpler fossils
than shallow layers
Some fossils seem to be related to
known species
Section 16.2
A Flurry of New Theories
19th Century - New Theories
Scientists attempt to reconcile
evidence of change with traditional
belief in a single creation event
Two examples
– Georges Cuvier - multiple catastrophes
– Jean Lamarck - inheritance of acquired
characteristics
Natural Selection Defined
These are Darwin’s key observations and
conclusions about evolution:
1. Natural populations have an inherent
reproductive capacity to increase in numbers
through successive generations
2. No population can indefinitely grow in size,
because its individuals will run out of food,
living space, and other resources
3. Sooner or later, individuals will end up
competing for dwindling resources
Natural Selection Defined
These are Darwin’s key
observations and conclusions about
evolution:
4. Those individuals generally have the same
genes encoding the same shared traits. Genes
are the population’s pool of heritable
information.
5. Mutations have given rise to alleles, or
slightly different molecular forms of genes,
which are a source of differences in phenotypic
details.
Natural Selection Defined
These are Darwin’s key observations and
conclusions about evolution:
6. Some phenotypes are better than others at helping an
individual compete for resources, and to survive and
reproduce. Alleles for those phenotypes increase in the
population, and other alleles decrease. In time the
genetic changes lead to increased fitness – an increase
in adaptation to the environment.
7. Natural selection among individuals of a population
is an outcome of variation in traits that affect which
individuals survive and reproduce in each generation.
This microevolutionary process results in adaptation, or
increased fitness to the environment.
Darwin’s Voyage
At age 22, Charles Darwin began a
five-year, round-the-world voyage
aboard the Beagle
In his role as ship’s naturalist he
collected and examined the species
that inhabited the regions the ship
visited
Voyage of the Beagle
Galapagos
Islands
Darwin
Wolf
• Volcanic islands
far off coast of
Ecuador
Pinta
Genovesa
Marchena
• All inhabitants
are descended
from species that
arrived on islands
from elsewhere
Santiago
Bartolomé
Fernandia
Seymour
Baltra
Rabida
Pinzon
Santa Cruz
Santa Fe
Tortuga
San Cristobal
Isabela
Española
Floreana
Section 16.3
Darwin’s Theory
Takes Form
The Theory of Uniformity
Lyell’s Principles of Geology proposed
a theory of uniformity –the notion of
a gradual, lengthy molding of the
earth’s geologic structure
Challenged the view that Earth was
only 6,000 years old
Allowed for enough time for
evolution to work
The “Species Problem”
Darwin returned after five years at
sea and began pondering the
“species problem”:
What could explain the remarkable
diversity among organisms?
Glyptodonts & Armadillos
In Argentina, Darwin observed fossils of
extinct glyptodonts
Animals resembled living armadillos
Of all places on Earth, armadillos live
only where glyptodonts had lived
Would it be reasonable to think that
armadillos had “descended with
modification” from glyptodonts
Fig. 16-6b, p.242
Fig. 16-6a, p.242
A Key Insight – Variation in Traits
Thomas Malthus wrote an essay that
Darwin read that suggested that as a
population outgrows its resources, its
members must compete for what is
available
Some won’t make it
Those that do…
Darwin felt that if some normally
variant members of a population had
traits that increased their survival,
then nature would select those same
individuals to survive, reproduce,
and possibly change future
populations’ traits
Galapagos Finches
Darwin observed finches with a
variety of lifestyles and body forms
On his return he learned that there
were 13 species
He attempted to correlate variations
in their traits with environmental
challenges
Fig. 16-7b, p.243
Fig. 16-7a, p.243
Fig. 16-7c, p.243
Fig. 16-7d, p.243
Darwin’s Theory
A population can
change over time
when individuals differ
in one or more
heritable traits that
are responsible for
differences in the
ability to survive and
reproduce
Alfred Wallace
Naturalist who arrived at the same
conclusions Darwin did
Wrote to Darwin describing his
views
Prompted Darwin to finally present
his ideas in a formal paper
Fig. 16-8, p.243
On the Origin of Species
Darwin’s book
Published in 1859
Laid out in great detail his
evidence in support of the theory
of evolution by natural selection
Section 16.4
The Nature of Adaptation
Adaptation: Defined Variously
1. Short-term adaptations, such as
an individual plant’s stunted growth
on a windy plain, last only as long
as the individual does
2. Long-term adaptations have
some heritable aspect that
improves the odds for surviving and
reproducing
Said a slightly different way…
Long-term adaptation is any heritable
aspect of form, function, behavior, or
development that contributes to the
fit between an individual and its
environment
An adaptive trait improves the odds
of surviving and reproducing, or at
least it did so under conditions that
prevailed when genes for the trait first
evolved
Salt-Tolerant Tomatoes
Tomatoes originated in South
American soils with a high salt
content
– They were adapted to these conditions
Commercial tomatoes in today’s
markets will not tolerate salt
However, gene transfers can yield a
salt-tolerant tomato that will grow in
irrigated plots (see pg. 244)
Section 16.5
Individuals Don’t Evolve,
Populations Do
Populations Evolve
Biological evolution does not
change individuals
It changes a population
Traits in a population vary among
individuals
Evolution is change in frequency
of traits
Variation in Populations
All individuals have the same genes
that specify the same assortment of
traits
Most genes occur in different forms
(alleles) that produce different
phenotypes
Some phenotypes compete better
than others
The Gene Pool
All of the genes in the
population
Genetic resource that is
shared (in theory) by
all members of
population
Variation in Phenotype
Each kind of gene in gene pool
may have two or more alleles
Individuals inherit different allele
combinations
This leads to variation in
phenotype
Offspring inherit genes, not
phenotypes
Change over Time
Over time, the alleles that produce
the most successful phenotypes will
increase in the population
Less successful alleles will become
less common
Change leads to increased fitness
– Increased adaptation to environment
What Determines Alleles
in New Individual?
Mutation
Crossing over at meiosis I
Independent assortment
Fertilization
Change in chromosome
number or structure
On to part II of “Galapagos”
Genetic Equilibrium
Allele frequencies at a
locus are not changing
Population is not
evolving
Five Conditions
No mutation
Random mating
All members survive, mate
and reproduce (no selection)
Large population
No immigration/emigration
Genetic Equilibrium
Seldom Attained
Because these five conditions are
rarely fulfilled in natural populations,
any deviation from the reference
point established by the “rule” will
indicate evolution is occurring
Microevolution is the change in
allele frequencies brought about
by mutation, natural selection, gene
flow and genetic drift.
Hardy-Weinberg Rule
At genetic equilibrium, proportions
of genotypes at a locus with two
alleles are given by the equation:
p2 (AA) + 2pq (Aa) + q2 (aa) = 1
Frequency of allele A = p
Frequency of allele a = q
Punnett Square
p
A
q
p A
AA(p2)
Aa(pq)
q
Aa(pq)
aa(q2)
a
a
Frequencies in Gametes
F1 genotypes:
Gametes:
0.49 AA
A
0.42 Aa
A
A
0.09 aa
a
a
a
0.49 + 0.21
0.21 + 0.09
0.7A
0.3a
No Change through
Generations
STARTING POPULATION
THE NEXT GENERATION
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
Medium-blue wings
90 aa butterflies
white wings
90 aa butterflies
white wings
THE NEXT GENERATION
90 aa butterflies
white wings
Microevolutionary Processes
Drive a population away from
genetic equilibrium
Small-scale changes in allele
frequencies brought about by:
– Natural selection
– Gene flow
– Genetic drift
Section 16.6
Mutations Revisited
Gene Mutations
Infrequent but inevitable
Each gene has own mutation
rate
Lethal mutations
Neutral mutations
Advantageous mutations
Number of individuals
in population
Directional
Selection
Allele frequencies
shift in one
direction
Number of individuals
in population
Number of individuals
in population
Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3
Pesticide Resistance
Pesticides kill susceptible
insects
Resistant insects survive and
reproduce
If resistance has heritable basis,
it becomes more common with
each generation
Antibiotic Resistance
First came into use in the 1940s
Overuse has led to increase in
resistant forms
Most susceptible cells died out
and were replaced by resistant
forms
Natural Selection
A difference in the survival and
reproductive success of different
phenotypes
Acts directly on phenotypes and
indirectly on genotypes
Number of individuals
in population
Stabilizing
Selection
Intermediate
forms are favored
and extremes are
eliminated
Number of individuals
in population
Number of individuals
in population
Range of values for wing-color trait at time 1
Range
Range
of values
of values
for wing-color
for the trait
trait
at time
at time
22
Range of values for wing-color trait at time 3
Number of individuals
in population
Disruptive
Selection
Number of individuals
in population
Number of individuals
in population
Range of values for wing-color trait at time 1
Range of values for wing-color trait at time 2
Forms at both ends
of the range of
variation are
favored
Intermediate forms
are selected
against
Range of values for wing-color trait at time 3
Stabilizing Selection:
Another Example
• Weight distribution for 13,370 human
newborns (yellow curve) correlated with
death rate (white curve)
Results of Natural Selection
Three possible outcomes:
A shift in the range of values for
a given trait in some direction
Stabilization of an existing range
of values
Disruption of an existing range of
values
Sexual Selection
Selection favors certain
secondary sexual characteristics
Through nonrandom mating,
alleles for preferred traits
increase
Leads to increased sexual
dimorphism
Balanced Polymorphism
Polymorphism - “having
many forms”
Occurs when two or more
alleles are maintained at
frequencies greater than 1
percent
Sickle-Cell Trait:
Heterozygote Advantage
HbS
Allele
causes
sickle-cell anemia
when heterozygous
Heterozygotes are
more resistant to
malaria than
homozygotes
Malaria case
Sickle cell trait
less than 1 in 1,600
1 in 400-1,600
1 in 180-400
1 in 100-180
1 in 64-100
more than 1 in 64
Genetic Drift
Random change in allele
frequencies brought about by
chance
Effect is most pronounced in small
populations
Sampling error - Fewer times an
event occurs, greater the variance
in outcome
Computer Simulation
1.0
AA in five populations
0.5
allele A lost
from four
populations
0
1
5
10
15
20
25
30
35
40
45
50
Generation
(25 stoneflies at the start of each)
Computer Simulation
1.0
0.5
allele A
neither
lost nor
fixed
0
1
5
10
15
20
25
30
35
40
45
Generation
(500 stoneflies at the start of each)
50
Bottleneck
A severe reduction in population size
Causes pronounced drift
Example
– Elephant seal population hunted down
to just 20 individuals
– Population rebounded to 30,000
– Electrophoresis revealed there is now no
allele variation at 24 genes
Founder Effect
Effect of drift when a small number
of individuals start a new population
By chance, allele frequencies of
founders may not be same as those
in original population
Effect is pronounced on isolated
islands
Inbreeding
Nonrandom mating between related
individuals
Leads to increased homozygosity
Can lower fitness when deleterious
recessive alleles are expressed
Amish, cheetahs
Gene Flow
Physical flow of alleles into a
population
Tends to keep the gene pools of
populations similar
Counters the differences that result
from mutation, natural selection,
and genetic drift