Transcript Bottom Heavy Clade Presentation

Bottom-heavy Clades & the
Diversification of Life
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.
Traditional Top-Heavy Icons
From Gould, 1989. Wonderful Life.
Less Traditional Bottom-Heavy Portrayal
From Gould, 1989. Wonderful Life.
Clade Diversity Diagrams & Center of Gravity
From Gould et al., 1987
Histograms with Mean, Mode, Median.
But time axis reversed.
Critical Questions
Are bottom-heavy patterns typical?
And, if so, what’s driving this pattern?
Introduce “probabilistic paleontology” . . .
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).
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.
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.
Genus-level Compendium
Three Great Faunas
Cambrian Fauna
Symmetrical, Bottom-heavy, and Top-heavy Histograms