BIOL360 Lecture Notes - Lecture 18: Exponential Growth, Ordovician, Paleozoic
12 May 2018
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Two models of diversification
• Logistic model -> global equilibrium and stops at max carrying capacity (can
only accommodate a certain number of species)
• Exponential model -> diversity expands with no max carrying capacity
Equilibrium -> fixed carrying capacity
• Assumes some limiting factors such as space or food limited diversification
• Raup Model probably correct -> one equilibrium in Ordovician
• Arguments for equilibrium
o Expansion of marine animals slowed after early Cambrian
o Rapid rebounds after mass extinctions, but returned to pre-extinction
levels shortly after
o Later diversifications are due to increases in extinctions and
decreases in new species
o Evidence is “Paleozoic Plateau” -> only evident at family-level, not
genus or species
Expansion -> no carrying capacity for Earth or has not yet been reached
• Diversification can be repeated by new adaptations, habitat changes and
recovery following extinctions
• Arguments for expansion
o How many equilibria? 3 or 1? Could be expansion
o Radiation of life on land and some parts of ocean show exponential
growth; no slowing down
o Modern fauna radiated and shows no slowing down
Natural selection -> survival of the fittest and fittest must be better than
predecessors
Patterns -> appearance and disappearance of species in diff geographic regions =
radiation
Document Summary
Logistic model -> global equilibrium and stops at max carrying capacity (can only accommodate a certain number of species: exponential model -> diversity expands with no max carrying capacity. Later diversifications are due to increases in extinctions and decreases in new species: evidence is paleozoic plateau -> only evident at family-level, not genus or species. Could be expansion: radiation of life on land and some parts of ocean show exponential growth; no slowing down, modern fauna radiated and shows no slowing down. Natural selection -> survival of the fittest and fittest must be better than predecessors. Patterns -> appearance and disappearance of species in diff geographic regions = radiation. Processes -> hypotheses that explain patterns and adaptation is a hypothesis. Trends -> one-way change in features in a lineage through time (just b/c something lives today does not mean it is better than its ancestors) Biotic replacements -> times when one group obviously replaces another (ex brachiopods replaced by bivalves)
