Simple versus complex models of trait evolution and stasis as a response to environmental change

Abstract

Previous analyses of evolutionary patterns, or modes, in fossil lineages have focused overwhelmingly on three simple models: stasis, random walks, and directional evolution. Here we use likelihood methods to fit an expanded set of evolutionary models to a large compilation of ancestor-descendant series of populations from the fossil record. In addition to the standard three models, we assess more complex models with punctuations and shifts from one evolutionary mode to another. As in previous studies, we find that stasis is common in the fossil record, as is a strict version of stasis that entails no real evolutionary changes. Incidence of directional evolution is relatively low (13%), but higher than in previous studies because our analytical approach can more sensitively detect noisy trends. Complex evolutionary models are often favored, overwhelmingly so for sequences comprising many samples. This finding is consistent with evolutionary dynamics that are, in reality, more complex than any of the models we consider. Furthermore, the timing of shifts in evolutionary dynamics varies among traits measured from the same series. Finally, we use our empirical collection of evolutionary sequences and a long and highly resolved proxy for global climate to inform simulations in which traits adaptively track temperature changes over time. When realistically calibrated, we find that this simple model can reproduce important aspects of our paleontological results. We conclude that observed paleontological patterns, including the prevalence of stasis, need not be inconsistent with adaptive evolution, even in the face of unstable physical environments.evolutionary mode | stasis | gradualism | punctuated equilibrium | climate change P aleontologists have long sought to document patterns of trait change within fossil species and to infer from these patterns their underlying evolutionary drivers (1-3). However, only recently have sufficient case studies accumulated to assess what the aggregated fossil record has to say about phenotypic evolution occurring on the 10 5 -to 10 7 -y timescales routinely captured in paleontological sequences. Several factors have contributed to this cumulative increase, including advances in geochronology that more readily permit time-calibrating evolutionary sequences, better morphometric practices for documenting trait change, and new analytical tools to examine the resulting data. Perhaps most important, however, is that paleontologists were motivated to capture many additional examples of trait evolution in fossil lineages in response to the intense debate centered around punctuated equilibrium (4-6)-the notion that species do not usually evolve gradually over long periods of time but instead emerge in a discontinuous pattern, in bursts of change associated with cladogenesis followed by longer intervals of morphological stasis.Punctuated equilibrium proponents and critics disagreed about how best to interpret the same patterns of trait evolution, and thus early reviews (7, 8) did little to reso...

Southern Ocean phytoplankton turnover in response to stepwise Antarctic cooling over the past 15 million years

“…Also, as noted elsewhere, we cannot yet discriminate whether turnover was driven by regional or global extinction, and in situ evolution or immigration, or the precise relative timings of these components. We note, however, that our findings may be consistent with recent models of evolution in response to fluctuating but trending global temperature change, which reproduce patterns of bounded trait evolution (stasis) on short time frames with pulses of accelerated evolutionary divergence at time spans of longer than a million years (36).…”

Sex and the shifting biodiversity dynamics of marine animals in deep time

“…We analyzed the temporal pattern of change in the standardized diversity curves using the maximum likelihood methods of Hunt et al (13). Models were fit that corresponded to stasis, directional change, unbiased random walk, an OU-OU process, as well as two-phase models incorporating a switch between simple models.…”

“…Model support was evaluated initially using AICc (Akaike information criterion with correction for finite sample size). Hunt et al (13) found that AICc tended to be overly liberal in favoring complex models and used parametric bootstrapping as a more conservative test of when complex models were favored over simple ones, an approach we follow here (SI Appendix, Maximum Likelihood Modeling).…”

“…To characterize diversity history, we generated sampling-standardized genus richness curves from the Paleobiology Database (PBDB; paleobiodb.org) and used maximum likelihood methods (13) to model diversity history. Models of diversity change include random walk, directional change, and two models that are consistent with diversity dependence [stasis and Ornstein-Uhlenbeck (OU-OU), in which diversity approaches and then fluctuates around an equilibrium value].…”

Getting somewhere with the Red Queen: chasing a biologically modern definition of the hypothesis

“…The Fluctuating-RQH is associated with an oscillating mode of selection, where two antagonist species oscillate backwards and forwards between fitness optima, with one interactor always lagging behind the other [10]. This involves continual, yet non-directional, evolutionary motion for both antagonists, analogous to constrained stasis or a random walk in species morphology that produces no net change over the long term [29,30]. Finally, the Chase-RQH supposes that across the range of two interacting or co-evolving species, respective populations may be responding in different ways to the biotic milieu they experience [10].…”