Evolutionary trends. Horse size increased steadily. Phylogeny and the fossil record

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1 Phylogeny and the fossil record The fossil record: trends and rates Chapter 4 Strong correspondence between phylogenetic branching order and order of appearance in the fossil record Evolutionary trends 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 1

2 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

3 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 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 Test of punctuated equilibrium 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 3

4 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 during 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 per million years 4

5 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 5