THE PLAGUE OF FALSE DICHOTOMIES IN EVOLUTIONARY BIOGEOGRAPHY

An impediment to understanding a number of major patterns in evolutionary biogeography has been the tendency to narrow focus to a single causal factor in opposition to others, instead of a synthetic, integrative approach. Using marine bivalves as a model system we present empirical support for two supposedly opposing views, that latitudinal diversity gradient (LDG) -- the most pervasive large-scale biotic pattern on Earth -- is shaped primarily (a) by local environmental factors that determine the number of species and higher taxa present at a given latitude (in-situ hypotheses), or (b) the entry of lineages arising elsewhere into a focal region (spatial-dynamics hypotheses). Supporting (a), bivalves show significant correlations between present-day regional diversity and climatic variables, and a strong positive relation between net regional temperature changes since the Pliocene and regional extinctions in Pliocene faunas. Supporting (b), bivalve faunas at both poles show a pronounced lack of young genera relative to the tropics, and all three assemblages share a suite of old, widespread genera, indicating that genera preferentially originate in tropics and expand poleward over geologic time (and showing that the tropics are both cradle and museum, another long-standing false dichotomy). Further, southeast Australia and southern Japan both harbor more bivalve species than the Caribbean, despite the many in-situ factors that should promote greater richness in the tropical province than in two temperate ones – an imbalance derived from the expansion of species and lineages from the tropical west Pacific. Thus, both in-situ and spatial-dynamics processes must be interacting strongly to shape the bivalve LDG. The two processes may operate at different strengths in other groups showing similar LDGs, but comparative analyses will require improved methods of integrating fossil data with molecular phylogenies. Many of the most dramatic biotic patterns, past and present, may also be generated by diverse, mutually reinforcing drivers, including the Cambrian explosion, the end-Paleozoic mass extinction, the massive diversifications of angiosperms and insects, and the tropical West Pacific global diversity peak.