Rates of evolution
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What is anagenesis?
The gradual morphological change predicted by Darwin where cladogenesis is the formation of new groups of taxa
What contrasting model of evolution did Gould, Eldridge and Stanley propose?
Evolution is a history of punctuated equilibrium with abrupt transition following long periods of stasis, claimed support from the fossil record where new species appear with intermediates rarely found
What does the punctuated equilibrium model say about the mechanisms of macroevolution?
Occurs via: • The splitting of lineages • Rapidly in small populations • Place/species fringes • Changes in a few key genes • Stable lineages between punctuations
What evidence do trilobites provide against the punctuated equilibrium model?
Sheldon found a gradual increase in the number of ribs in the trilobite fossil record
What other 4 issues are there with the punctuated equilibrium model?
• Measurement timescale - amount of time you look at is related to the amount of change you expect to find • Fluctuating evolution - results may be confounded depending on what timescale a researcher uses • Incomplete records - sometimes intermediate fossils are missing, can only relate speciation to morphological change • Stasis - morphological stasis does not mean evolution has stopped, although horseshoe crabs are very similar they are very genetically diverse