RATE AND MODE OF THERMAL NICHE EVOLUTION ACROSS AND WITHIN LINEAGES OF CENOZOIC PLANKTONIC FORAMINIFERA

A major aim of macroevolutionary research is determining where trait evolution occurs along phylogenies and at what rate(s). There may be gradual trait change along branches, abrupt change at speciation events, or both. Abiotic niche breadth and position are important traits to track through time and across species because they dictate patterns of migration, extinction, and perhaps speciation. Species with static niches must move apace with climate change velocity to find suitable habitat, whereas those with labile niches can adapt to new conditions in place. Rate and frequency of niche lability have been reconstructed back along tree nodes from living species’ data and measured along tree branches on the scale of millions of years from fossil data. However, rarely have within- and between-lineage rates been estimated for the same clade from continuous observations through time.

Here we quantified abiotic niche lability across a phylogeny of planktonic foraminifera, at 8-ka resolution, over Pleistocene glacial–interglacial fluctuations. We focused on mean annual temperature as a niche axis. Temperatures were derived both from species’ depth habitats and at the sea surface based on atmosphere–ocean global circulation models of paleoclimate. We estimated within- and between-lineage evolutionary rates using fossil observations directly, and then using observations at the tips to infer past trait change across the tree. Hence, we were able to compare results drawn from fossil evidence at many time steps against results of approaches typically applied to living species.

We relate our findings to present discourse about the incidence of punctuated equilibrium. For instance, evolutionary trait models fit to series of occupied temperature values indicated strong niche stasis within lineages, leaving a large amount of trait variation to be explained by speciation. Moreover, intraspecific niches were equally similar through time regardless of the magnitude of global temperature change, even between extreme contrasts of glacial and interglacial intervals. The findings of abiotic niche stasis suggest that warming and ocean acidification over the next hundreds to thousands of years could redistribute or reduce populations of foraminifera and other calcifying plankton, which are primary components of marine food-webs and biogeochemical cycles.