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15-3 Darwin Presents His Case
Inherited Variation and Artificial
Selection
Inherited Variation and Artificial Selection
Members of each species vary from one another
in important ways.
In Darwin’s day, variations were thought to be
unimportant, minor defects. Variations were
inherited and therefore passed on.
Darwin argued that this variation mattered.
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15-3 Darwin Presents His Case
Inherited Variation and Artificial
Selection
Darwin noted that plant and animal breeders would
breed only the largest hogs, the fastest horses, or the
cows that produced the most milk.
Darwin termed this process artificial selection.
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15-3 Darwin Presents His Case
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15-3 Darwin Presents His Case
Publication of On the Origin of
Species
Darwin said that what ever farmers are doing must
work the same way in nature, by itself.
• proposed a mechanism for evolution called
natural selection: process by which
individuals that are better suited to their
environment survive and reproduce most
successfully.
• presented evidence that evolution (change in a
kind of organism over time) has been taking
place for millions of years—and continues in all
living things.
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15-3 Darwin Presents His Case
Evolution by Natural Selection
Evolution by Natural Selection
Darwin compared processes in nature to artificial
selection.
By doing so, he developed a scientific hypothesis
to explain how evolution occurs.
Now, with the work of others, it is a theory:
a well-tested explanation that unifies a broad
range of observations.
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15-3 Darwin Presents His Case
Evolution by Natural Selection
The Struggle for Existence
Darwin realized that high birth rates and a
shortage of life's basic needs would force
organisms to compete for resources.
Remember back to ecology in first semester:
Density dependent limiting factors and what
organisms compete for:
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15-3 Darwin Presents His Case
Evolution by Natural Selection
The struggle for existence means that members of
each species compete regularly to obtain food, living
space, and other necessities of life.
The struggle for existence was central to Darwin's
theory of evolution.
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15-3 Darwin Presents His Case
Evolution by Natural Selection
How is natural selection related to a
species' fitness?
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15-3 Darwin Presents His Case
Evolution by Natural Selection
Survival of the Fittest
The ability of an individual to survive and
reproduce in its specific environment is fitness.
Darwin proposed that fitness is the result of
adaptations.
An adaptation is any inherited characteristic
that increases an organism's chance of
survival; NOT something learned within an
organisms lifetime.
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15-3 Darwin Presents His Case
Evolution by Natural Selection
Successful adaptations means organisms are better
suited to their environment and better able to survive
and reproduce.
Individuals with characteristics that are not well suited
to their environment either die or leave few offspring.
Individuals that are better suited to their environment
survive and reproduce most successfully.
Darwin called this process survival of the fittest.
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15-3 Darwin Presents His Case
Darwin’s tenets:
OVERPRODUCTION: Organism’s produce more offspring
than their environment can support. Why?
HERITABLE VARIATION: Offspring vary in their
appearance and function, some of these variations are
heritable.
COMPETITION: Offspring must compete for survival,
food and mates (reproduction), living space, etc.
SURVIVAL OF THE FITTEST: Offspring who have the
highest fitness for their environment will live longer
and leave more offspring than those less suited for the
environment.
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15-3 Darwin Presents His Case
Evolution by Natural Selection
Over time, natural selection results in
changes in the inherited characteristics
of a population. These changes
increase a species' fitness in its
environment.
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15-3 Darwin Presents His Case
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15-3 Darwin Presents His Case
Evolution by Natural Selection
Descent With Modification
Natural selection produces organisms that have
different structures, establish different niches, or
occupy different habitats.
Each living species has descended, with changes,
from other species over time.
Darwin referred to this principle as descent with
modification.
Decent w/ modification implies relatedness. This is
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known as common decent.
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Differences among individuals of a single
species are referred to as
a. artificial selection.
b. genetic variation.
c. survival of the fittest.
d. environmental adaptation.
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Changes that increase a species' fitness in its
environment over time are due to
a. the principle of common descent.
b. the geographic distribution of that species.
c. natural selection.
d. habitat selection.
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An inherited characteristic that increases an
organism's chance of survival is called a(an)
a. homologous structure.
b. vestigial organ.
c. adaptation.
d. analogous structure.
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Evidence used by Darwin to support the idea of
evolution included all the following EXCEPT
a. fossils that demonstrate change over time.
b. the genetic mechanism by which useful traits
are inherited.
c. the geographic distribution of living things.
d. the presence of many homologous
structures in plants and animals.
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