The Fossil Record: Trends and Rates - Chapter 4

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 doryssusStrata laid down annually for 110,000 yrs

•  Sampled at 5000 yr intervals.Pelvic structure ranges from fully developed to vestigialNote: 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