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Processes of Evolution
Chapter 16
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Overwhelming Evidence for Evolution
• A) From Biogeography
• B) From Comparative anatomy
• C) From Geologic discoveries
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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?
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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
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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
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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
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Voyage of the Beagle
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GalapagosIslands
Isabela
Darwin
Wolf
Pinta
Marchena Genovesa
Fernandia
SantiagoBartolomé
RabidaPinzon
SeymourBaltra
Santa Cruz
Santa Fe
Tortuga
Española
San Cristobal
Floreana
• Volcanic islands far off coast of Ecuador
• All inhabitants are descended from species that arrived on islands from elsewhere
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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
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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
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Darwin put together the short term struggle for existence (Malthus) with the great geological timescale (Lyell)
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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
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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
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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 Evolution is change in frequency of traitsof traits
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When does evolution not occur?
• No mutation
• Random mating
• Gene doesn’t affect survival or
reproduction
• Large population
• No immigration/emigration
When 5 conditions are met
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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
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No Change through Generations
STARTING POPULATION
490 AA butterfliesdark-blue wings
420 Aa butterfliesmedium-blue wings
90 aa butterflieswhite wings
490 AA butterfliesdark-blue wings
THE NEXT GENERATION
420 Aa butterfliesmedium-blue wings
90 aa butterflieswhite wings
THE NEXT GENERATION
490 AA butterfliesdark-blue wings
420 Aa butterfliesMedium-blue wings
90 aa butterflieswhite wings
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Gene Mutations
• Infrequent but inevitable
• Every gene is subject to mutation to some degree
• Changes the frequency of alleles in a population
= EVOLUTION
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Directional Selection
• Allele frequencies shift in one direction
Range of values for the trait at time 1
Number of individuals
in population
Range of values for the trait at time 2
Number of individuals
in population
Range of values for the trait at time 3
Number of individuals
in population
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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
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Natural Selection
• A difference in the survival and
reproductive success of different
phenotypes
• Acts directly on phenotypes and indirectly
on genotypes
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Stabilizing Selection
• Intermediate forms are favored and extremes are eliminated
Range of values for wing-color trait at time 1
Number of individuals
in population
Range of values for the trait at time 2
Number of individuals
in population
Range of values for wing-color trait at time 2
Number of individuals
in population
Range of values for wing-color trait at time 3
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Stabilizing Selection: Another Example
• Weight distribution for 13,370 human newborns (yellow curve) correlated with death rate (white curve)
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Disruptive Selection
• Forms at both ends of the range of variation are favored
• Intermediate forms are selected against
Number of individuals
in population
Range of values for wing-color trait at time 1
Number of individuals
in population
Range of values for wing-color trait at time 2
Number of individuals
in population
Range of values for wing-color trait at time 3
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Balanced Polymorphism
• Polymorphism - “having many forms”
• Occurs when two or more alleles are maintained at frequencies greater than 1 percent
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Sickle-Cell Trait: Heterozygote Advantage
• Allele HbS causes sickle-cell anemia when heterozygous
• Heterozygotes are more resistant to malaria than homozygotes
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
Malaria case
Sickle cell trait
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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
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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
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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
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Inbreeding
• Nonrandom mating between related individuals
• Leads to increased homozygosity
• Can lower fitness when deleterious recessive alleles are expressed
• Amish, cheetahs
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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!!!