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. (slideshow requires QCShow Player)
Audio only (mp3 format)
View as a webpage (quicktime, real player) (notes)

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 (as a proxy for total energy input) 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.

— Wirt Atmar

About the Speaker

David Jablonski is the Chair of the Committee on Evolutionary Biology at the University of Chicago. He is also a professor in the Department of Geophysical Sciences at the university.

His research emphasizes the combining of data from living and fossil organisms to study both the origins and the fates of lineages and adaptations. His main areas of interest include:

• The environmental histories of higher taxa in post-Paleozoic marine invertebrates, based on the primary literature and museum collections. Orders and other major groups tend to arise in onshore environments, but this trend is weak or absent at lower taxonomic levels. An understanding of how these patterns at diferent levels compare to the underlying dynamics of speciation and extinction along bathymetric and latitudinal gradients could lead to a general theory of evolutionary novelty.

• Related work on the role of larval development and other intrinsic biological factors in determining speciation rates and patterns in mollusks of the Coastal Plain Cretaceous and the eastern Pacific Cenozoic.

• Understanding background patterns, so that they can be compared to patterns of extinction and survivial during mass extinctions to gain a better picture of the evolutionary significance of extinction events.