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Descent with Modification:
Darwin’s Theory of Natural
Selection
AP Biology
Early Theories
 Catastrophism (Georges Cuvier)—each
boundary between strata correspond in time to a
catastrophe
Destroyed many of the species = extinction
Early Theories
 Catastrophism (Georges Cuvier)
 Gradualism (James Hutton)—significant
changes are a result of the cumulative product of
slow but continuous processes
Early Theories
 Catastrophism (Georges Cuvier)
 Gradualism (James Hutton)
 Uniformitarianism (Charles Lyell)—geological
processes have not changed throughout Earth’s
history
If geological changes result from slow, continuous
actions, then the Earth must be very old
Lamarck’s Theory of
Acquired Inheritance
(early 1800s)
 Jean Baptiste Lamarck observed fossil
records and the current diversity of life and
formulated his theory
 Suggested giraffes acquired long necks
because ancestors stretched higher and
higher into the trees to reach leaves
Lengthened neck was passed to offspring
Charles Darwin
Evolution: Descent with
Modification
 Compared South American
fossils with living species
there and elsewhere
 Observed organisms and
their distributions on
Galápagos Islands
Darwin’s Theory of Natural
Selection

Observations:
Overproduction of offspring leads to competition of
limited resources (food, space, breeding partners)
Individuals of a population vary in characteristics, and
many such traits are passed on to offspring

Conclusions:
Individuals with inherited characteristics make them best
adapted to survive in their environment and reproduce
and leave more offspring than less fit individuals
Natural Selection (Summary)
 Individuals have unequal ability to survive and
reproduce
 Occurs through the interaction between the
environment and the variability in characteristics
of individuals
 Product of natural selection is the adaptation of
populations of organisms to their environment
Natural Selection
 Prominent force in nature
 Support in the results of artificial selection—
selective breeding of domesticated
plants/animals
 Populations tend to evolve in response to
environmental conditions
Cactus eater. The long,
sharp beak of the
cactus ground finch
(Geospiza scandens)
helps it tear and eat
cactus flowers and
pulp.
Seed eater. The large
ground finch (Geospiza
magnirostris) has a large
beak adapted for cracking
seeds that fall from plants
to the ground.
Insect eater. The green warbler finch
(Certhidea olivacea) used its narrow,
pointed beak to grasp insects.
Evidence of Evolution
 Fossil Record
 Biogeography
 Comparative anatomy
 Comparative embryology
 Molecular Biology
Fossilization
 Most fossils are actually casts of animals or plants.
 Animal dies and sinks to the sea floor.
 Tissue begins to decay and is buried under layers of
sediment such as mud or sand.
 These layers become rock.
 The hard parts of the animal are
replaced with minerals such as
pyrites or silica.
 These minerals form the fossil.
iron
 Usually fossils show the hard parts of the animal or
plant - such as shell or bones.
 Trace fossils—evidence of living plants or animals,
such as worm burrows or dinosaur footprints.
 Most fossils are found in sedimentary rocks - rocks
which were created when shells or small loose bits
of rock are laid down in layers (limestone,
sandstone, clay and chalk)
Fossil Record
Evidence: Biogeography
 Geographical distribution of species suggests
organisms evolve from common ancestors
 Island forms are most similar to forms found on
the closest mainland, rather than forms on
ecologically similar, but more distant islands
Continental Drift
Pangea >> Gondwanaland + Laurasia >> modern continents
Many biogeographic patterns of phylogentic groups can be explained on
a global scale (via continental drift) or on a local scale by climate change
with isolation of populations and divergence, or other factors. Other
patterns may be explained by dispersal, invasion and spread. For
example species colonizing oceanic islands, or the formation of land
bridges.
Comparative Anatomy
 Comparison of body structures between different
species
Similarities give signs of common descent
 Homologous structures—features that have
similar structure but have different functions
Comparative Anatomy
 Comparison of body structures between different
species
 Homologous structures
 Vestigial structures—Small body structures that may
have been functional in the ancestors of a species,
but has no real function at the present time
(appendix, tail bone)
Comparative embryology
 Different organisms go through similar
embryonic stages
 All vertebrates have an embryonic stage in
which gill pouches appear in the throat region—
evidence of a common ancestor
Ontogeny Recapitulates
Phylogeny
 Ontogeny—development of an organism
 Phylogeny—evolutionary history of the species
 Ontogeny provides clues to phylogeny
http://www.utm.edu/staff/nlillega/phil120_files/image010.gif
Molecular Biology
 Study of molecular basis of genes and gene
expression
 Universality of genetic code
 Conservation of amino acid sequences in
proteins such as hemoglobin