The Darwin-Wallace theory of organic change
• 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 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.
• In addition, Darwin drew on evidence
• Comparative Anatomy
• Comparative Embryology
• 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
• 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
• 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
• Darwin looked at and described:
• Homologous structures
• Analogous structures
• Vestigial 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
• 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
Insects and birds evolved flight independently.
Ancestral seals and penguins that had a streamlined
body form were better able to survive.
• Vestigial structures are those that
were well-developed in an ancestor
but are much reduced in a
• Vestigial structures show that
different selection pressures shape
All higher vertebrates descend from ancestors with
four limbs. What happened in the snake and the
• 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.
Can you tell these three mammals apart?
Which one is human?
• 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.
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
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.
• 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
• Peppered Moth simulation:
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