The fossil record: trends and rates - Chapter 4
Phylogeny and the fossil record
• Strong correspondence between phylogenetic branching order and order of
appearance in the fossil record
Evolutionary trends
• Cope's rule states that evolution tends to increase body size over geological
time in a lineage of populations
Horse size increased steadily
Some lineages undergo reversal
Dollo’s Law
• Dollo's Law is also known as the Law of Irreversible Evolution
• 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)
Seemingly irreversible characters may carry a burden
• Burden is a measure of the degree of systemic integration of specific
characters within the developmental process
• The more integrated a character is within development, the higher its burden
and the more stable the character
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
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
3 components to punctuated equilibrium
• Most phenotypic characters change little over extended spans of geological
change
(equilibrium, or stasis)
• When phenotypic change occurs, it moves rapidly from one static state to
another
• Rapid change occurs during speciation events
Test of punctuated equilibrium
• Is stasis & punctuation the most common pattern in the fossil record?
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 5000 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
Phyletic gradualism is common
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
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
Punctuated gradualism
• Change happens, but not necessarily 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
Darwin
• Change by a factor of 2.718 per million years
Haldane
• Number of standard deviations by which a character mean changes per
generation