ENVIRONMENTAL CHANGE AND NICHE EVOLUTION: WHICH TYPES OF CHANGE PROMOTE ADAPTIVE RESPONSE?

Environmental change, including both biotic and abiotic change, is the primary driver of evolutionary innovation and extinction. Adaptive responses to environmental change that result in morphological divergence are concentrated during speciation events; whereas, most species exhibit morphological stability during the remainder of their existence. Understanding the mechanisms that control when a species does and does not demonstrate adaptive response is, therefore, a critical component of understanding evolutionary dynamics.

This study examines niche stability within a diverse suite of marine invertebrates that occupied a shallow marine basin centered on present day Cincinnati, OH, USA during the C3, C4, and C5 sequences of the Katian Age (Late Ordovician). Using spatial distribution modeling, we reconstructed the fundamental niche for 21 taxa of articulated brachiopods, bryozoa, trilobites, crinoids, rugose corals, bivalves, and gastropods across nine temporal intervals. The relative degree of niche stability that taxa exhibited between time slices was assessed in both geographic and environmental space.

Niche stability varied through time. During the C3 sequence, taxa exhibited niche stability (=no adaptive response) to environmental changes. Adaptive response, as indicated by increased niche evolution, became more common during and after an interbasinal invasion event, the Richmondian Invasion. Species adjusted to the increased competition by altering aspects of their niche. Notably, surviving taxa contracted their niche into a subset of their previous niche parameters. This represents an adaptive response, and it was employed most successfully by generalist taxa. Notably, patterns of niche evolution were congruent between clades, trophic group, and at both the specific and generic level. Adaptive response (stability vs. evolution) was related to the tempo, mode, or a combination of both aspects of environmental change. Biotic interactions played a key role in driving biotic divergence via habitat partitioning at the species level in this case study.