Extinctions apparently promote not only invasion but evolutionary diversification in the fossil record, the classic case being the impressive radiation of mammals after the demise of the (nonavian) dinosaurs and other marine and terrestrial vertebrates at the end of the Cretaceous [e.g., Alroy (1999, 2000) and Cifelli and Gordon (2007); for general discussions, see Erwin (2001) and Jablonski (2001, 2005, 2008)]. These macroevolutionary observations are often seen as two sides of the same coin, as they intersect nicely with ecological work on the potential for incumbency or priority effects to resist extinction or damp diversification (Jablonski, 2007, 2008). The three most prevalent explanations for both invasions and diversifications today and in the geologic past are (i) extinction or at least suppression of incumbents, already mentioned, (ii) superior competitive ability of the invaders, not least owing to their escape from their own competitors, predators, and pathogens when they enter a new area (Sax et al., 2007) (although this may be a transient effect and therefore less likely to play a macroevolutionary role), and (iii) changing climatic and other environmental conditions, such as those that drove the extensive invasions and reshuffling of Pleistocene biotas.
One of the most pervasive spatial patterns of invasions, seemingly independent of mass extinction events, underlies the marine latitudinal diversity gradient, wherein morphologies, species, and higher taxa are richest in the tropics and decline toward the poles. Although the gradient has been known for a long time and is documented for many groups and regions, the processes underlying this pervasive biodiversity pattern remain poorly understood (Hillebrand, 2004; Mittelbach et al., 2007). The “out of the tropics” model for the marine gradient has taxa preferentially originating in the tropics, and then expanding their latitudinal ranges over time without actually abandoning their tropical cradle (Jablonski et al., 2006). The tropics are thus a diversity source, containing both young and old taxa, which accumulate to high richness. The poles are a diversity sink, mainly containing older taxa that have moved in from lower latitudes, and the temperate zones have intermediate richness and taxon ages, at least for the marine invertebrates where direct fossil evidence is available. If this model is generally true, then invasion is a basic factor in the latitudinal deployment of life on Earth.
The out-of-the-tropics model is strongly supported in the marine bivalve fossil record. For each of three time slices (Pleistocene, Pliocene, and late Miocene), roughly twice as many bivalve genera first occur in the tropics than in higher latitudes (Jablonski et al., 2006). Because the extratropical fossil record is far better sampled than that of the tropics (Allison and Briggs, 1993; Jackson and Johnson, 2001; Bush and Bambach, 2004;