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Processes of Evolution
Chapter 16
Overwhelming Evidence for
Evolution
• A) From Biogeography
• B) From Comparative anatomy
• C) From Geologic discoveries
A) Biogeography - is the science which
deals with geographic patterns of species distribution and the
processes that result in such patterns.
• 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?
B) 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
C) 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
Darwin’s Voyage
• At age 22, Charles Darwin began a fiveyear, 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
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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
Lyell Provided the foundation
for Darwin to draw his
conclusions
• Lyell’s Principles of Geology
• Subtle, repetitive processes of change,
had shaped Earth
• Challenged the view that Earth was only
6,000 years old, to millions of years
Malthus - Struggle to Survive this too shaped Darwins’
views
• 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
Darwin put together the short
term struggle for existence
(Malthus) with the great
geological timescale (Lyell)
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
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Darwin’s Theory
Survival of the fittest
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
Populations Evolve
• Biological evolution does not change
individuals
• It changes a population
• Traits (features) in a population vary
among individuals
• Evolution is change in frequency
of traits
When does evolution not occur?
When 5 conditions are met
• No mutation
• Random mating
• Gene doesn’t affect survival or
reproduction
• Large population
• No immigration/emigration
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
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
Gene Mutations
• Infrequent but inevitable
• Every gene is subject to mutation
to some degree
• Changes the frequency of alleles
in a population
= EVOLUTION
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
Giraffes
• Baby giraffes in Africa
• Some have inherited long necks others
shorter ones
• When food is limited only the ones with
longer necks can feed
• The others die of starvation
• Now the population has individuals with
longer neck alleles on average
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 traittrait
at time
at time
22
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)
Number of individuals
in population
Disruptive
Selection
• Forms at both ends of
the range of variation
are favored
• Intermediate forms
are selected against
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
Range of values for wing-color trait at time 3
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
• Heterozygotes are
more resistant to
malaria than
homozygotes
HbS
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
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
• Prevents the two populations forming new
species!!!