CHARACTER PARTITIONS, RATE SHIFTS, AND PARTITIONED RATE SHIFTS: EVALUATING TEMPORAL HETEROGENEITY IN RATES AMONG CHARACTER SUITES IN CAMBRIAN ECHINODERMS

Both evolutionary-developmental and ecomorphological theory posit that different suites of anatomy evolve at different rates. A related but more complex possibility is that shifts in rates over time or across phylogeny can differ among those suites. For example, “ecologic release” might allow high rates of change among some characters tied to basic ecology without encouraging elevated rates in other characters. Similarly, alterations of integration might increase evolvability for some modules without encouraging elevated rates among character in conserved modules. The distinctions are important, as either might look like a general difference between character partitions and/or general shifts in rates in some parts of a phylogeny when they truly reflect more particular events.

The theoretical questions pose methodological challenges with fossil data: ideally, we would test these hypotheses in a phylogenetic context, but the predictions of these hypotheses correspond to differences in rates among character partitions and local rates under “relaxed clock” models. As these are basic models in phylogenetic analysis, the phylogeny most consistent with one model will differ from the phylogeny most consistent with another. Moreover, the particular model of interest is one rarely considered in phylogenetic analyses: different "relaxed clocks" for different character suites. Here, we employ reversible jump Markov Chain Monte Carlo (rjMCMC) analyses of the Cincta (Echinodermata; Cambrian) to contrast a range of possible models, with the simplest being the same "strict" clock and rates for all character partitions and the most complex being different "relaxed" clocks and rates for different partitions. The rjMCMC controls for phylogeny by integrating over different plausible phylogenies without assuming a “known” tree. A corollary outcome of our approach is that we also evaluate possible phylogenetic relationships while allowing (rather than assuming) a wide range of possible character evolution models. Thus, the posterior distributions simultaneously assess complex macroevolutionary scenarios and phylogeny.