Climate change, body size evolution, and Cope's Rule in deep-sea ostracodes

Abstract

Causes of macroevolutionary trends in body size, such as Cope's Rule, the tendency of body size to increase over time, remain poorly understood. We used size measurements from Cenozoic populations of the ostracode genus Poseidonamicus, in conjunction with phylogeny and paleotemperature estimates, to show that climatic cooling leads to significant increases in body size, both overall and within individual lineages. The magnitude of size increase due to Cenozoic cooling is consistent with temperature-size relationships in geographically separated modern populations (Bergmann's Rule). Thus population-level phenotypic evolution in response to climate change can be an important determinant of macroevolutionary trends in body size.

Fig. 1.

Body size evolution in the genus Poseidonamicus. (A) Population-level body size patterns, shown for individual specimens (small orange circles) and population means (larger open squares). (B) Populations shown connected according to their phylogenetic relationships. Populations from four species indicated by colored circles: Poseidonamicus rudis (red), Poseidonamicus dinglei (green), P. major (blue), and P. pintoi (orange); all other species are shown in gray. (C) Mean body size within million-year temporal bins (dotted lines; numbers indicate how many populations were included in each bin), plotted with a bottom-water temperature curve inferred from Mg/Ca paleothermometry (light-blue solid line).

Fig. 2.

Slope of the body size-paleotemperature relationship within nine species of Poseidonamicus. Most species show a significantly negative slope between body size and temperature, indicating a tendency for body size to increase in response to climatic cooling (horizontal lines show 95% confidence interval on the slope estimate; vertical dotted line indicates a slope of zero). Also shown is the body size-temperature slope among modern populations of P. major (gray line, open circle; see also Fig. 4). The last row shows the summary estimate of within-species slope according to a metaanalysis (filled square, with a line indicating the 95% confidence interval).

Fig. 3.

Body size in Poseidonamicus, with populations divided into bathymetric categories. Lines indicate lowess smoothing function for the shallowest localities (1,000-1,800 m, solid line) and localities from intermediate depths (1,800-3,000 m, dotted line). The deepest depth bin (>3,000 m) includes only samples <3 million years old; its lowess function is quite similar to the intermediate depth group and is not shown for clarity.