Phylogeny and the fossil record
The fossil record: trends and rates
Chapter 4
Evolutionary trends
• Strong correspondence
between phylogenetic
branching order and
order of appearance in
the fossil record
Horse size increased steadily
• Cope's rule states that
evolution tends to
increase body size over
geological time in a
lineage of populations
Grazers evolved high-crowned
teeth
Some lineages undergo reversal
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Dollo’s Law
• Dollo's Law is also known as
the Law of Irreversible
Evolution
• Complex traits are unlikely to
re-evolve
• Dollo essentially states that
organisms cannot re-evolve
along lost pathways, but must
find alternative routes
(because the same fortuitous
train of mutational events,
being totally random, will
never repeat)
Gaps in the fossil record
• Most paleontologists ascribe
the lack of transitional forms
showing gradualism to gaps
in the fossil record
• Eldredge and Gould proposed
a controversial explanation
called punctuated equilibrium
– Stasis is the real pattern in the
fossil record and that most
morphological change occurs
during speciation
2
Punctuated equilibrium
• Niles Eldredge and Stephen
Jay Gould (1972) hypothesized
that species remained stable
for many millions of years
before the sudden appearance
of new species in a very short
time and become stable again
for another long period before
another change
• In contrast to Darwin’s
gradualism
Test of punctuated equilibrium
3 components to punctuated
equilibrium
1. Most phenotypic characters
change little over extended
spans of geological change
(equilibrium, or stasis)
2. When phenotypic change
occurs, it moves rapidly from
one static state to another
3. Rapid change occurs during
speciation events
Phyletic gradualism is common
1. Is stasis & punctuation the
most common pattern in
the fossil record?
Testing punctuated equilibrium
• Demonstrating stasis in bryozoans
How common is PE?
• Erwin and Anstey (1995)
reviewed 58 studies to test for
PE
– “Evidence overwhelmingly
supports that speciation is
sometimes gradual and sometimes
punctuated, and that no one mode
characterizes this very
complicated process in the history
of life.”
– 25% show BOTH gradualism and
stasis
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Punctuated equilibrium
•
•
3. New morphology does
not evolve except when a
small population becomes
a new, reproductively
isolated species
BUT: Microevolutionary
studies show that
morphology can evolve
rapidly without speciation
Rates of evolution
• Rates vary with lineage,
characters and over time
• Evolutionary rates are
proportional rather than
absolute
• Evolutionary rates are
slow on average
Punctuated gradualism
• Change happens, but not
necessarily during speciation
Darwin
• Change by a factor of
2.718 per million years
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Haldane
• Number of standard deviations
by which a character mean
changes per generation
Problem: Sampling interval
• Widely-spaced sampling
intervals makes change
look punctuational
• Testing pattern requires
finely-spaced samples
• Sticklebacks:
Gasterosteus doryssus
• Strata laid down
annually for 110,000
yrs
• Sampled at 5,000 yr
intervals.
Pelvic structure ranges
from fully developed to
vestigial
Note: If Bell had
sampled less often
(20,000 yrs), change
would appear more
abrupt
Salamander larval stage
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