Lecture of the Week: Part I: Planetary-scale Patterns

The Evolutionary Biology Lecture of the Week for April 10, 2006 is now
available at:

http://aics-research.com/lotw/

The talks center primarily around evolutionary biology, in all of its
aspects: cosmology, astronomy, planetology, geology, astrobiology,
ecology, ethology, biogeography, phylogenetics and evolutionary
biology itself, and are presented at a professional level, that of one
scientist talking to another. All of the talks were recorded live at
conferences.

=====================================

April 17, 2006

Part I: Planetary-scale Patterns


The Dynamics of Global Biodiversity:
Insights from the Fossil Record
David Jablonski, University of Chicago
35 min. (requires QCShow Player)


If an alien civilization should come to Earth and take a survey of the
life they find here, one pattern would immediately stand out above all
of the rest. The most striking biodiversity pattern on this planet is
the latitudinal diversity gradient, with its maximum richness of
species, higher taxa, and diversity of body plans clustered near the
equator and a stepwise decline in all three metrics towards the poles.

Surprisingly, however, uncovering the root underlying causes of the
latitudinal diversity gradient has been one of the most contentious and
persistent debates in evolutionary ecology.

David Jablonski of the University of Chicago argues in this talk that
sea surface temperature is perhaps the best physical predictor of the
biodiversity that will be found at each latitude.

Correlations across latitude between diversity and mean annual
temperature (or other covariates with solar energy input) occur in a
wide variety of marine and land-based taxa, suggesting that energy or a
closely related variable shapes these pervasive trends on a global
scale. Unfortunately, this static picture tells us little of the
dynamics underlying the origin and maintenance of this gradient.

The presistent question has been whether the tropics are a cradle or a
museum of biodiversity? The museum hypothesis speculates that extinction
rates are merely lower in the tropics and thus biodiversity naturally
accumulates there as a consequence. The contending hypothesis argues
that the tropics are the point of origination of the Earth's
biodiversity and that diversity spreads outward across the planet from
there.

Jablonski and his colleagues argue primarily for the second view,
presenting an "Out of the Tropics" model.

In a preliminary analysis in which they have begun to integrate the
fossil record of marine bivalves — a molluscan group they chose for
their high diversity, excellent fossil record and increasingly
standardized taxonomy — with the group’s present-day biogeography, they
argue that the cradle vs. museum debate hinges on a false dichotomy: the
tropics are both the primary diversity source and accumulator. Taxa
first appear in the tropics and then expand outwards without losing
tropical occupancy, while the high latitudes are primarily a diversity
sink.

The tropics are so rich today they argue not only because they are the
source of young taxa, but because the geographic ranges of old, mostly
widespread taxa overlap there with young and spatially restricted taxa.

=====================================