Transcript E3_Selection_2011 Part 3

The postulates are satisfied:
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Postulate 1: Daphnia from different
ponds differ in their tolerance of UVR.
Postulate 2: Daphnia clones display
similar tolerance to UVR.
Postulate 3: Daphnia from more
transparent ponds have higher UVR
tolerance (survive better under UVR).
Brooks tested postulate 3 by
isolating animals from different
ponds, growing them in a common
garden and exposing them to
different levels of UVR.
Question: What do you think is the
basis for the differences in tolerance?
UV-A
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UV-B
Visible
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UV-Resistance Can Evolve by Natural Selection
Natural Selection
Lecture Outline
• Artificial selection
• Case studies of natural selection
• Dr. Pangloss and some caveats
• Summary
The Best of All Possible Worlds?
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Darwin’s original insight was that natural
selection not only led to phenotypic
change, but also adaptation.
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Evolutionary biologists Lewontin and
Gould were critical of what they called the
“adaptationist program”:
Richard Lewontin
Voltaire
– Natural selection is the only important evolutionary
force
– Selection acts on “atomized” traits
– Selection optimizes (“Panglossian Paradigm”)
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Their critique of these views:
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Other evolutionary forces (e.g., drift) are
important in evolutionary change.
Selection works on integrated phenotypes, not
atomized pieces.
Direction of selection is always constrained by
history (or phylogenetic constraints).
They argued that many biologists
were guilty of adaptive storytelling.
Stephen Jay Gould
“All is for the best
in the best of all
possible worlds”
Caveats about Selection
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Selection is one evolutionary force, a set
including drift, migration and mutation.
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Phenotypic change need not be attributed
to the operation of selection (e.g., in small
populations, drift can strongly influence
phenotypic change over time).
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Organisms are integrated and selection for
one trait (e.g., tameness) can alter other
traits (e.g., tail, coat, and ears).
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Selection is constrained by history (e.g., all
tetrapods go through an embryonic stage
with five-digit limbs).
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These caveats are important; nonetheless
selection can be a powerful force. To
demonstrate it most fully, one would:
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Confirm that the postulates hold.
Study the genetics/development of phenotype.
Study the reasons for fitness differences.
Natural Selection
Lecture Outline
• Artificial selection
• Case studies of natural selection
• Dr. Pangloss and some caveats
• Summary
Summary
•
Darwin suggested that natural selection could play a part in
the emergence of new species as well as the adaptation of
organisms to their environment.
•
The process of selection requires heritable phenotypic
variation with fitness consequences (“Darwin’s postulates”).
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Darwin made his case for natural selection through discussion
of artificial selection: humans have dramatically changed
animals and plants, sometimes in short periods (e.g., foxes).
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The postulates have been confirmed in a number of natural
systems as well (e.g., finches, mice, and daphnids).
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While a powerful force, selection is not solely responsible for
all evolutionary change and always works with constraints.
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