Russell Powell - Contingency and Convergence: Toward a Cosmic Biology of Body and Mind (Vienna Series in Theoretical Biology)

The Burgess Shale and surrounds. (Top) Waterfalls cascading down Michael Peak just after emerging onto the Burgess Highline from Yoho Pass. (Middle) View of Emerald Lake from Burgess Highline just past the switchbacks leading to the Burgess Shale. (Bottom) Guided group ascending to the Walcott Quarry outcrop, taken from the southern portion of the Burgess Highline. Photos by author.

Dorsal and lateral views of Opabinia. From H. B. Whittington, The Enigmatic Animal Opabinia Regalis, Middle Cambrian, Burgess Shale, British Columbia, Philosophical Transactions of the Royal Society of London. B, Biological Sciences 271, no. 910 (1975): 143.

(a) Double-wedge diversity pattern, with one clade replaced by another through competitive interactions. (b) Mass extinction diversity pattern, in which one clade is extinguished or severely depleted in a major biotic perturbation and only then replaced by another clade. Redrawn from M. J. Benton, The Late Triassic Tetrapod Extinction Events, in The Beginning of the Age of Dinosaurs: Faunal Change across the Triassic-Jurassic Boundary, ed. K. Padian, 303320 (Cambridge University Press, 1986).

Decimation-diversification pattern showing large gaps in extant morphospace occupation due to early extinction events. Redrawn from S. J. Gould, Wonderful Life: The Burgess Shale and the Nature of History (Norton, 1989).

Illustration of the stem taxa concept in cladistic approaches to taxonomy, which can accommodate many of the Cambrian Problematica as members of the total (or pan) group that branched off before divergence of the crown group. Redrawn from R. P. S. Jefferies, The Origin of Chordatesa Methodological Essay, in The Origin of Major Invertebrate Groups, ed. M. R. House, 443477 (Academic Press, 1979).

(a) A radically contingent system in which paths not taken at earlier times (dotted lines) cause evolutionary outcomes to become inaccessible at later times. (b) A convergent system in which many outcomes remain accessible from even distant evolutionary trajectories. From R. Powell and C. Mariscal, Convergent Evolution as Natural Experiment: The Tape of Life Reconsidered, Journal of the Royal Society Interface Focus 5, no. 6 (2015): 113.

The great white shark (Carcharodon carcharias, top sketch, a fish) and the ichthyosaur (Stenopterygius quadricissus, bottom photo, a Mesozoic marine reptile) have converged on numerous features of the body plan, including a teardrop shape, a heavy dorsal fin and high-aspect-ratio caudal fin for swimming at speed, and a crossed-fibered architecture of the skin composed of the same chemical fibers, as well as a specialized caudal peduncle and ligament force-transmission system. Redrawn from T. Lingham-Soliar, Convergence in Thunniform Anatomy in Lamnid Sharks and Jurassic Ichthyosaurs, Integrative and Comparative Biology 56, no. 6 (2016): 13231336. Photo of specimen in Senckenberg Museum, Germany. Courtesy of Wikimedia Commons.

A mixed contingent-convergent system that exhibits path independence and predictability at finer grains of taxonomic resolution, but path dependence and unpredictability at coarser grains. For comparison and discussion, see . From R. Powell and C. Mariscal, Convergent Evolution as Natural Experiment: The Tape of Life Reconsidered, Journal of the Royal Society Interface Focus 5, no. 6 (2015): 113.

Recurrent evolution of Anole ecomorphs on isolated Caribbean islands. Photograph panel courtesy of the Luke Mahler laboratory at the University of Toronto. From left to right, the top row depicts twig specialists