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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
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
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
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
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
Populations Evolve
• Biological evolution changes
populations, not individuals
• Traits in a population vary among
individuals
• Evolution: change in the frequency
of traits
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
What Determines Alleles in a
New Individual?
• Mutation
• Crossing over at meiosis I
• Independent assortment
• Fertilization
• Change in chromosome number
or structure
Reproductive Isolation
• Cornerstone of the biological
species concept
• Speciation is the attainment of
reproductive isolation
• Reproductive isolation arises as a
by-product of genetic change
Biological Species Concept
• “Species are groups of interbreeding
natural populations that are
reproductively isolated from other
such groups.”