TEMPO AND MODE IN EVOLUTION REFRAMED IN THE LIGHT OF PHYLOGENETIC THINKING

At the grand scale explaining macroevolution is about understanding and squaring two basic types of patterns: Why are there so many different types of species, and how do complex structures evolve? My emphasis will be on the former, considering a pattern Dobzhansky named discontinuity and an approach Eldredge termed taxic. Species are individuals with distinct births and deaths, with macroevolution comprising the patterns and processes relating to the birth, death, and persistence of species. Here, the focus is going to be on presenting data pertaining to tracking the birth of species - cladogenesis - using temporal, distributional, and phylogenetic data from fossil clades running the gamut from arthropods in general to trilobites in particular. These data, when leavened with analytical data pertaining to patterns across Metazoa, make it possible to consider some of the abiotic and biotic factors involved with when and why new species appear and also when and why speciation rates wax and wane. Deducing the specific constellation of factors that conspire to modulate speciation rates is pertinent to G. G. Simpson’s twin themes of tempo and mode. Data suggest that fundamental factors are how are opportunities for allopatry promoted (or restricted), and how quickly do species respond to such opportunities. Here is where microevolution is channeled or directed up to the level of macroevolution. Any attempt to ponder the birth of species must consider issues relating to species stasis and its converse, and there are many mechanisms that cause stasis operating at several hierarchical levels, but the juxtaposition between single populations and multiple populations is a crucial link. Macroevolution happens most easily when species are actually, or effectively via allopatry, winnowed down to a single population. Biotic factors that conspire to produce this vary across and within clades and will be elaborated on. Abiotic factors vary through time and by place; the specifics of these will be discussed emphasizing oscillating tectonic and climatic regimes, and insular versus cosmopolitan settings. Any study tracking species history through time in a phylogenetic context, buttressing it with information about where and when those species lived, can expand our understanding of macroevolution.