Use of Panamanian sea urchins to test the molecular clock

Abstract

THE ‘molecular clock’ hypothesis of protein evolution holds that each protein changes at a constant rate, so that the degree of molecular divergence between two species is linearly related to the time for which their lineages have remained separate¹. This assertion, however, has been challenged repeatedly by authors who discovered taxa and peptides in which the proposed uniformity of molecular evolution did not hold^2,3, who noted that biochemically and palaeontologically determined dates of separation between lineages conflicted^4,5, introduced tests that pointed to significant variation in the rates of evolution of the same proteins^6,7, or dismissed the hypothesis as a confusion of averages with constants⁸. Others have postulated that, although the same proteins evolve at different rates in different lineages, the average amount of molecular change over many proteins is sufficiently uniform to provide approximate dates for the splitting of two lines of descent^9,10. Here I present evidence from sea urchins separated by the Isthmus of Panama which indicates that even this compromise position is not tenable.