Bottom-heavy Clades & the Diversification of Life

1. Bottom-heavy Clades & the Diversification of Life

2. Introduction • What a clade is & transpecific evolution. • Patterns of clade diversity: paleontologists have recently noticed that new clades diversify rapidly and early in their history = bottom-heavy. • Opposed to the preconceived notion: a clade becomes increasingly more diverse later in history = top-heavy. • The iconography associated with this.

3. Traditional Top-Heavy Icons From Gould, 1989. Wonderful Life.

4. Less Traditional Bottom-Heavy Portrayal From Gould, 1989. Wonderful Life.

5. Clade Diversity Diagrams & Center of Gravity From Gould et al., 1987

6. Histograms with Mean, Mode, Median. But time axis reversed.

7. Critical Questions • Are bottom-heavy patterns typical? • And, if so, what’s driving this pattern? • Introduce “probabilistic paleontology” . . .

8. Probabilistic Paleontology • Random versus deterministic causes: explain. • If deterministic, then speciation (p) must be different than extinction (q): p > q or p < q. • Use random computer models with p = q to generate patterns; if random models aren’t capable, then must be deterministic. • Random models to generate clade diversity diagrams – show results (see excel file).

9. Randomly Generated Clades • Computer model ran 119 simulations. • Set speciation rate equal to extinction rate: p = q. • Mean Center of Gravity = 0.496. • Standard Deviation = 0.04. • Not distinguishable from CG = 0.50.

10. Exercise #4 . . . • Are clades that originated at the beginning of the Cambrian bottom-heavy? • Sepkoski’s Cambrian fauna. • Use Sepkoski’s genus-level compendium. • The players among the Cambrian fauna (see list). • The data set (see excel). • Generating a histogram for the clade.

11. Genus-level Compendium

12. Three Great Faunas Cambrian Fauna

13. Symmetrical, Bottom-heavy, and Top-heavy Histograms