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Chapter 15 Notes – Darwin’s Theory of Evolution
Section 15-3 Darwin Presents His Case (p. 381-386)
Descent with Modification
Natural selection produces organisms that have different
_______________________, establish different ________________, or occupy
different ________________________.
•
As a result, species today look different from their
________________________.
•
Each living species has descended, with ____________________, from
other species over time.
o Darwin referred to this principle as descent with
_______________________________.
•
Descent with modification implies that all living organisms are
_________________ to one another = ___________________ descent.
o According to this principle, all _________________—living and
extinct—were derived from common ancestors.
•
Think of a single “______________ of ________________” that links all
living things on Earth.
What evidence of evolution did Darwin present?
Darwin argued that living things have been evolving on Earth for millions of
years. Evidence for this process could be found in:
•
the ____________________ record
•
the geographical _______________________________ of living species
•
____________________________ structures of living organisms
•
similarities in early _____________________________
The Fossil Record
Before Darwin’s time, it was known that fossils were the remains of
___________________ life, and that different layers of _____________ had been
formed at different times in Earth’s history.
•
Darwin saw fossils as a record of _________________________.
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•
Darwin proposed that the Earth was ____________________ (rather than
thousands) of years old.
•
Over this time, countless different species appeared on Earth,
_______________ for a time, and then disappeared.
•
By comparing fossils from ________________ rock layers with fossils from
__________________ layers, scientists could document that life on Earth
has ______________________ over time.
Geographic Distribution of Living Species
Darwin wondered how the organisms he observed and collected in the
Galápagos became similar, yet _______________________ different species.
•
Could the island finches have changed over time, as populations in
different ___________________ adapted to different _______________
environments?
•
Darwin decided that all Galápagos finches could have descended with
modification from a common ____________________ ancestor.
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Remember the puzzle about different species being found in Argentina and
Australia even though they had similar grassland __________________________?
Pink fairy armadillo - Argentina
•
Kangaroo – Australia
Darwin’s theory was that species now living on different continents had
each _______________________ from different ancestors.
o However, because some animals on each continent were living
under similar ecological ________________________, they were
exposed to similar pressures of natural selection.

Because of these similar selection pressures, different
animals ended up evolving certain _____________________
in common.
•
Ex) beaver (N. America) – capybara (S. America)
muskrat (N. America) – coypu (S. America)
Homologous Body Structures
Further evidence of evolution could be found in living _____________________.
By Darwin’s time, researchers had found striking similarities among the body
parts of _________________________ (animals with backbones).
•
Reptiles, birds, and mammals had similar bone ______________________,
but different ____________________—as seen in their arms, wings, legs, or
flippers.
o Each of these limbs has adapted in ways that enable organisms to
survive in different ____________________________.
o Despite these different functions, however, these limb bones all
develop from the same clumps of cells in growing
_________________________.
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•
Structures that have different mature forms but develop from the same
embryonic tissues are called ________________________________
__________________________.
o Homologous structures provide strong evidence that all fourlimbed animals with backbones have descended, with
modification, from __________________ ancestors.
o Similarities and differences in homologous structures help
biologists __________________ animals according to how recently
they last shared a common ancestor.
Not all homologous structures serve important _______________________.
•
The organs of many animals are so reduced in size that they are just
vestiges, or traces, of homologous organs in other species =
________________________ _________________
o Why not get rid of them? Vestigial organs may not affect an
organism’s ability to ___________________ and reproduce; natural
selection would not eliminate them.
o Example of vestigial organs in humans?
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Similarities in Embryology
The early stages, or ___________________, of many
animals with backbones are very similar.
•
The same groups of embryonic
______________ develop in the same order
and in similar patterns to produce the
__________________ and
__________________ of all vertebrates.
o These common cells and tissues
produce
_____________________________ structures.
Summary of Darwin’s Theory
1. Individual organisms _________________, and some of this variation is
heritable.
2. Organisms produce more _________________________ than can survive,
and many that do survive do not reproduce.
3. Because more organisms are produced than can survive, they
______________________ for limited resources.
4. Individuals best suited to their environment (highest level of
_________________) survive and reproduce most successfully.
5. These organisms pass their heritable ___________________ to their
offspring. Other individuals die or leave fewer offspring.
6. This process of ______________________ ________________________
causes species to change over time.
7. Species alive today are descended with
______________________________ from ancestral
species that lived in the distant past.
8. This process, by which diverse species
____________________ from common ancestors, unites
all organisms on Earth into a single tree of life.
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