Natural Selection
The Darwin-Wallace theory of organic change over
time.
Thinking Question
• Some evidence exists that over several
centuries, the number of people born
with small wisdom teeth or no wisdom
teeth has increased. Using your best
understanding of Natural Selection,
explain how selection could cause this.
Darwin’s Evidence
• Darwin spent nearly 20 years analyzing
evidence as he developed his ideas. He
drew on:
• His own field work during his voyage
on board the Beagle.
• His work in classifying barnacles,
beetles, and other organisms.
Darwin’s Evidence
• In addition, Darwin drew on evidence
from:
• Fossils
• Comparative Anatomy
• Comparative Embryology
Fossils
• The fossil record shows that:
• Things existed in the past that no
longer exist today and
• Things exist today that did not exist in
the past.
• The fossil record is a record of change in
the composition of the biosphere over
time.
What is a fossil?
Fossils are rare
• Most organisms are eaten or decay after
death.
• Fossils only form in places where there
is little erosion: lake bottoms, deep
oceans, etc.
• We have little fossil record of beach
organisms and alpine communities.
Some fossils show transition
• Because fossils are rare, and change
can be rapid, there are few “missing
links” in the fossil record.
• However, some fossil sequences of
marine organisms do show good
evidence of transition from one form to
another.
Comparative Anatomy
• Darwin looked at and described:
• Homologous structures
• Analogous structures
• Vestigial structures
Homologous structures
• Homologous structures are those that
are shared between related organisms,
but are slightly altered.
• In modern terms, this indicates shared
genes (similar structures), but also
shows genetic differences led to
divergent evolution (modifications of
those structures.
Homology
Analogous Structures
• Analogous structures are similar
solutions to the same environmental
challenges seen in unrelated species
living in similar environments.
• In modern terms, analogous structures
represent convergent evolution. Rather
than shared genes, the organisms are
under similar selection pressures.
Analogy
Insects and birds evolved flight independently.
Analogy
Ancestral seals and penguins that had a streamlined
body form were better able to survive.
Vestigial Structures
• Vestigial structures are those that were
well-developed in an ancestor but are
much reduced in a descendant.
• Vestigial structures show that different
selection pressures shape organisms
differently.
Vestigial Structures
All higher vertebrates descend from ancestors with four
limbs. What happened in the snake and the whale?
Comparative Embryology
• Like homology, embryology reveals
shared traits between species,
demonstrating shared genes.
• As embryos develop, newer genes may
shape the same structures into different
parts: gill arches in fish become gills, but
in mammals become parts of the face,
jaw, and inner ear.
Comparative Embryology
Can you tell these three mammals apart?
Which one is human?
Molecular Comparisons
• Shared traits give us some insights into
shared genes. Today’s technolgy allows
us to analyze genes directly.
• Because small mutations accumulate in
populations over time, more genetic
differences between two different groups
of organisms indicates more time since
they separated from one another.
How Selection Works
1. Variation Exists
• All populations vary as the result of the
accumulation of small, random mutations
over many generations.
2. Inheritance of traits
• Inheritable traits (those coded for by
genes) are passed directly to the
offspring from the parents through
genetic information.
3. Differential Survival
• More offspring are born than can survive.
Many offspring die young. Those with
traits best suited to the environment are
more likely, though not guaranteed, to
survive.
4. Differential Reproduction
• Some survivors fail to reproduce. Some
have traits that better insure reproduction
than others.
5. Differential Inheritance
• Survivors that reproduce pass some of
their traits on to their offspring. Those
with favorable traits may pass those
favorable traits on — or not.
Natural Selection in Action
• Peppered Moth simulation:
• http://www6.district125.k12.il.us/~nfische
r/Moth/default.htm
Wisdom Teeth, Revisited
• Now that we’ve gone through the
process of selection, return to the paper
on which you responded to the wisdom
tooth question.
• Draw a line under your original response,
and write a second one, using what you
just learned about the process of
selection: Variation, Inheritance,
Differential Survival, Differential
Reproduction, Differential Inheritance.