MOLECULAR CLOCKS AND TECTONIC BLOCKS

Establishing an evolutionary timescale is the fundamental yet elusive goal of the earth and life sciences. Molecular clock methodology has usurped completely the role of the incomplete fossil record in establishing an evolutionary timescale but, ironically, it remains reliant on paleontological data for calibration. Not surprisingly, it has become popular to eschew the fossil evidence entirely, instead calibrating divergence time analyses using geochronologically dated tectonic events that have left a phylogenetic footprint of divergence in evolutionary lineages. Unfortunately, tectonic calibrations have not enjoyed the same scrutiny and, therefore the development, as fossil calibrations.

The profound accuracy and precision of geochronological dates of rock units belies their accuracy and precision in dating divergence events because: (i) these biogeographic calibrations are rarely, if ever, justified; (ii) in eschewing fossil evidence, biogeographic calibrations assume the biogeography of living organisms is a faithful reflection of their ancestral lineages; (iii) age evidence is equally rarely established; (iv) tectonic episodes are protracted and so they should be represented by spans of time, not single dates; (v) biogeographic events have a different impact on lineages dependent on their ecology.

These limitations can be overcome or, at least controlled for since, like fossil calibrations, vicariance-based calibrations can be implemented as probabilistic constraints that span an interval of time, entertaining (i) the probability that the tectonic event was causal to the calibrating node, (ii) errors in the accuracy of dating the tectonic event, (iii) the temporal extent of the tectonic episode, (iv) the differential impact of the tectonic event on organisms with different ecologies. Finally, we would argue that paleontological evidence can add to knowledge of the historical biogeography and ecology of evolutionary lineages supplementing insights provided by the extant biodiversity.