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Processes of Evolution
Chapter 12
Evidence
• Biogeography
• Comparative morphology
• Geologic discoveries
Biogeography
• Size of the known world expanded
enormously in 15th century
• Accepted beliefs did not explain discovery
of new organisms in previously unknown
places
Biogeography
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
Comparative Morphology
coccyx
fossilized ankle bone
ankle bone
Comparative Morphology
Comparative pelvic anatomy
Geological Discoveries
• Similar rock layers throughout world
• Certain layers contain fossils
• Deeper layers contain simpler fossils than
shallow layers
• Some fossils resemble known species
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
The Theory of Uniformity
• Lyell’s Principles of Geology
• Earth shaped by subtle, repetitive
processes of change
• Challenged the view that Earth was only
6,000 years old
Darwin’s Voyage
• At age 22, Charles Darwin began a fiveyear, round-the-world voyage aboard
the HMS Beagle
• As ship’s naturalist, he collected and
examined species that inhabited
regions the ship visited
Darwin’s Voyage
Galapagos Islands
The Galapagos Islands
Fossil Evidence
• Darwin found fossil Glyptodont
• Proposed descent with modification
Voyage of the Beagle
EQUATOR
Galapagos
Islands
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
Malthus: Struggle to Survive
• Thomas Malthus, a clergyman and
economist, wrote essay that Darwin
read on his return to England
• Argued that as population size
increases, resources dwindle, the
struggle to live intensifies, and conflict
increases
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
Galapagos Finches
Galapagos Finches
Finches of the Galapagos Islands
Reproductive Capacity
and Competition
• All populations have the capacity to
increase in numbers
• No population can increase
indefinitely
• Eventually, individuals of a population
end up competing for resources
Variation in Populations
• All individuals have the same genes that
specify the same assortment of traits
• Most genes occur in different forms
(alleles), which produce different
phenotypes
• Some phenotypes compete better
than others (fitness)
Change over Time
• Over time, 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
Natural Selection
• Natural selection for various traits
among individuals of a population
affects which individuals survive and
reproduce in each generation
• Process results in adaptation to the
environment (increases fitness)
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
Adaptation
• Some heritable aspect of form, function,
or behavior that improves the odds for
surviving and reproducing
• Environment specific
• Outcome of natural selection
Salt-Tolerant Tomatoes
• An example of adaptation
Adaptation to What?
• Llamas live at high altitude and have
hemoglobin with a high oxygen affinity
• Is this an adaptation to altitude?
Probably not
• Llamas are related to camels, which live at
low altitudes
• Camels also have hemoglobin with high
oxygen-binding capacity
Common Ancestors
• Llama and camel
Adaptation to what?
Populations Evolve
• Biological evolution changes
populations, not individuals
• Traits in a population vary among
individuals
• Evolution: change in the frequency
of traits
• Main thing needed for evolution =
diversity within the population
The Gene Pool
• All the genes in a population
• Genetic resource that is shared
(in theory) by all members of
population
Variation in Phenotype
• Each gene in gene pool may have
two or more alleles
• Individuals inherit different allele
combinations
– leading to variation in phenotype
• Offspring inherit genes, not
phenotypes
Variation in Populations
What Determines Alleles in a
New Individual?
• Mutation
• Crossing over at meiosis I
• Independent assortment
• Fertilization
• Change in chromosome number
or structure
Genetic Equilibrium
• Allele frequencies at a locus
are not changing
• Population is not evolving
Five Conditions of
Genetic Equilibrium
• No mutation
• Random mating
• Gene doesn’t affect survival or
reproduction
• Large population
• No immigration/emigration
Microevolutionary Processes
• Drive a population away from
genetic equilibrium
• Small-scale changes in allele
frequencies brought about by
– Natural selection
– Gene flow
– Genetic drift
Gene Mutations
• Infrequent but inevitable
• Each gene has own mutation rate
• Lethal mutations
• Neutral mutations
• Advantageous mutations
Polymorphism
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
Number of
individuals
Directional
Selection
Range of values at time 2
Number of
individuals
Allele frequencies
shift in consistent
direction over time
Number of
individuals
Range of values at time 1
Range of values at time 3
Directional selection
Pinpointing the Target of
Selection
• Populations of rock
pocket mice have fur
that matches the rocks
on which they live
– Black basalt: dark fur
– Tawny granite: light fur
Pinpointing the Target of
Selection
• DNA comparisons show that the two
populations differ in Mclr gene sequence
Pesticide Resistance
• Pesticides kill susceptible insects
• Resistant insects survive and
reproduce
• If resistance has heritable basis, it
becomes more common with each
generation
Rise of the Super Rats
• Warfarin is a chemical
pesticide used against
rodents
• When introduced the 1950s it
was extremely effective
• Where warfarin was used,
rats became resistant
Impacts, Issues Video
Rise of the Super Rats
Antibiotic Resistance
• Antibiotics first came into use in
the 1940s
• Overuse has led to increase in
resistant forms
• Most susceptible cells died out, while
resistant forms multiplied
Number of
individuals
Range of values at time 1
Number of
individuals
Intermediate
forms are favored
and extremes are
eliminated
Range of values at time 2
Number of
individuals
Stabilizing
Selection
Range of values at time 3
Stabilizing selection
Human Birth Weight
100
20
50
15
30
20
10
10
5
5
percent mortality
percent of population
70
3
2
1
2
3
4
5
6
7
8
birth weight (pounds)
9
10
11
human newborns
rate of death
• Intermediate forms
are selected
against
Number of
individuals
Range of values at time 1
Number of
individuals
• Happens when
forms at both ends
of the range of
variation are
favored
Range of values at time 2
Number of
individuals
Disruptive
Selection
Range of values at time 3
Disruptive selection
African Finches
• Selection favors birds with very large or
very small bill
• Birds with intermediate-sized bill are
less effective feeders
African Finches
lower bill 12 mm wide
lower bill 15 mm wide
Fig. 12-15, p.188
Selection for Bill Size
Disruptive selection among African finches
Sexual Selection
• Selection favors certain secondary
sexual characteristics
• Through nonrandom mating, alleles
for preferred traits increase
• Leads to increased sexual dimorphism
Sexual Selection in Birds
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
• Allele
causes
sickle-cell anemia
when heterozygous
HbS
• 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
Sickle Cell and Malaria
Sickle-Cell Trait
Distribution of sickle-cell trait
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
Genetic Drift
Simulation of genetic drift
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 starts a new population
• By chance, allele frequencies of
founders may not be same as those in
original population
• Effect is pronounced on isolated islands
or in voluntarily isolated human
populations, as in religious isolates
Founder Effect
• Albatross carries seed to island
phenotypes of
mainland population
phenotype of
island population
Fig. 12-19, p.191
Inbreeding
• Nonrandom mating between
related individuals
• Leads to increased homozygosity
• Can lower fitness when deleterious
recessive alleles are expressed
• Old Order Amish
Gene Flow
• Physical flow of alleles into a
population
• Tends to keep the gene pools of
populations similar
• Counters the differences that arise
from mutation, natural selection, and
genetic drift
Gene Flow
• Blue jay carries acorn between oak
populations
Table 12-1, p.192
Designer Dogs