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Darwin Presents His Case
Chapter 15, Section 3
Lyell’s Influence
In attempt to explain the past in terms of present
day processes, Darwin went to local farmers and
animal breeders and observed…
 Variation in the organisms that could be
 After many generations, organisms appear very
different from ancestors
Darwin called this process ARTIFICIAL SELECTION
Darwin was convinced a similar process occurred
in nature:
Variation in Nature
No two organisms are exactly alike!
Struggle for Existence (Malthus)
High birth rates and limited resources will force organisms to
Selective pressures (predation, competition, parasitism, disease,
pesticides, etc)
Survival of the Fittest
Fitness: an organism’s ability to survive and reproduce
Adaptation: any inheritable characteristic that increases an
organism’s chance of survival
Organisms with the best adaptations survive and reproduce more
Reproduction of Viable Offspring
Offspring must be fertile and reproduce
Descent with Modification
Darwin then proposed that over long periods of
time, natural selection produces organisms that
look different from their ancestors.
 This implies that all living things are related to
one another.
Common Descent: all species, living and extinct,
were derived from common ancestors.
Darwin’s Evidence of Evolution
The Fossil Record
Fossils are buried in sedimentary
rock→ Relatively, deeper fossils
are older
Fossils provide evidence of
changing life
Paleontologists search for
transitional fossils
Geographic Distribution of Living
The geographic
distribution of animals
serves as a clue to how
modern species evolved
Two islands in different
parts of the world (with
similar environments) will
house species similar to
the nearest mainland
rather than the other
Ex: Australia’s unique
plants and animals
evolved from isolation
from other continents
Similarities in Structure
The forelimbs of all
mammals are made of
bones that are similar in
origin (skeletal structure),
but may vary in function
→ called homologous
Vestigial structures are
remnants of structures in
an animal that may have
had a function or purpose
in ancestral species, but
serve no apparent
function or purpose in
modern species
Similarities in Development
Embryology: the
study of embryo
Animals that are more
closely related will
have more similarities
in the stage of