Evidence for Evolution
Graduate Seminar
Introduction and overview
The three main claims of
Darwinian evolution
• Living species are related by common
ancestry
• Change through time occurs at the
population not the organism level
• The main cause of adaptive evolution is
natural selection (and related mechanisms)
The three main claims of
Darwinian evolution
• Living species are related by common
ancestry
Common
ancestor
The importance of common
ancestry
• If two different species descended from a
single ancestor then change (=evolution) is
implied
All differences evolved
along the lineages
What did people believe before
Darwin?
• Separate ancestry (many versions)
A special case:
Jean-Baptiste Lamarck
• French Naturalist (17441829)
• The first scientific theory
of evolution
• Struggled to reconcile
evolution and the Scala
naturae (great scale of
being)
A special case: Lamarck’s
theory
• Life progresses upward due to
an internal drive towards
perfection
A special case: Lamarck’s
theory
• Life progresses upward due to
an internal drive towards
perfection
• Why are “primitive” organisms
still around?
A special case: Lamarck’s
theory
• Life progresses upward due to
an internal drive towards
perfection
• Why are “primitive” organisms
still around?
– Spontaneous generation of new
life constantly
Lamarck’s view
Because all species follow the
same trajectory of origin: they will
form a ladder of advancement
Lamarck’s view is basically
separate ancestry
First articulation of common ancestry
Lyell, C. Principles of Geology, Vol. II, Chap. 1
Species 1
Species 2
Species 3
Common Ancestor
Common Ancestor
Darwin envisaged evolution as
a tree
The affinities of all the beings of the same class have
sometimes be represented by a great tree. I believe
this simile largely speaks the truth……
…The green and budding twigs may represent
existing species; and those produced during former
years may represent the long succession of extinct
species…..
….the great Tree of Life….covers the earth with
ever-branching and beautiful ramifications
Charles Darwin, On the Origin of Species; pages 131-132
The only figure in The Origin of Species
Two claims
• That major groups of organisms share
descent from common ancestry
– vs. separate ancestry
• That all living organisms share descent from
common ancestry
– vs. several origins
Evidence for common ancestry
(against separate ancestry)
•
•
•
•
•
•
•
Fossil record
Homology
Vestigial Structures
Classification
Hierarchical distribution of traits
Agreement between gene trees
Evolution during domestication
Evidence for common ancestry
• Fossil record
– Transitional fossils (sometimes in temporal
sequence): consistent with the existence of real
common ancestors
Living sister
group
Transitional 
fossils



Major clade
Traits
Deep Homology
• Distantly related
organisms share
structural similarities
• Function varies
• Explicable by common
ancestry
grasping
leaping
flying
swimming
running
human
“fish”
whale
Amphibia Reptilia
Pentadactyl limb
bat
Vestigial structures
• Structures that are non-functional (but functional in related
species). For example:
–
–
–
–
Human appendix, tail bones,
Gill slits in mammal embryos
Hip bones of whales and snakes
Eyes in cave fish
Trees explain patterns in trait
distribution
Fur; milk
Amnion
Four legs; lungs
Vertebral column
Trees explain patterns in trait
distribution
Fur; milk
Amnion
Four legs; lungs
Vertebral column
Applies a forteriori to molecular data
Biogeography: closely related
species live near each other
Molecular
phylogeny of
Hawaiian and
other
Campanulaceae
(Givnish et al.)
Hawaii
Correlation among gene trees
(Penny et al. 1982. Testing the theory of evolution by comparing phylogenetic
trees constructed from five different protein sequences. Nature 297: 197-200.)
• When we estimate the phylogeny from
different genes, we get trees that are much
more similar than could happen by chance
during separate ancestry
• Amenable to statistical analysis
We see diverse forms that are
descended from single ancestor
Brassica oleracea
What about the claim of a single
ancestor of all living organisms?
• Shared biochemistry (e.g., same 4 nitrogenous bases, same
20 L-amino acids, ATP)
– There are many possible nitrogenous bases and amino acids; Many
sugars could have form the NA backbone
– No chemical reason for L- vs. D-amino acids
• Shared structures (ribosomes, lipid bilayer membranes)
• Shared metabolic pathways (e.g., glycolysis)
• Share information processing (genetic code)
– The code is a “frozen accident”
The three main claims of
Darwinian evolution
• Living species are related by common
ancestry
• Change through time occurs at the
population not the organism level
– No organism level mechanism is currently
plausible
• The main cause of adaptive evolution is
natural selection (and related mechanisms)
The main cause of adaptive
evolution is natural selection
• Claim 1: Natural selection happens
– vs. Natural selection does not/cannot happen
• Claim 2: Natural selection is sufficient to
explain even the most complex traits of
living organisms
– vs. natural selection is not sufficient
The main cause of adaptive
evolution is natural selection
• Claim 1: Natural selection happens
• Evidence:
– Artificial selection and rapid natural selection
– Theoretically must occur if only minimal
assumptions are met
• Genetic variation
• Limited resources
The main cause of adaptive
evolution is natural selection
• Claim 2: Natural selection is sufficient to
explain even the most complex traits of
living organisms
• “Evidence”
– On short time scales it is very effective
– Time is long
– No other natural mechanisms are known