Jurassic primates, immobile ducks and other oddities: a reply to Heads’ review of The Monkey’s Voyage

In The Monkey’s Voyage, I focused on the issue of disjunct distributions, and, in particular, on the burgeoning support from molecular-dating studies for long-distance dispersal over vicariance as the most reasonable explanation for many (but by no means all) distributions broken up by oceans. Michael Heads’ assessment of the book is founded on his long-standing belief, following Croizat, that long-distance dispersal is an insignificant process and, therefore, that disjunctions are virtually always attributable to vicariance. In holding to these notions, Heads offered a series of unsound arguments. In particular, to preserve an ‘all-vicariance’ perspective, he presented a distorted view of the nature of long-distance dispersal, misrepresented current applications of fossil calibrations in molecular-dating studies, ignored methodological biases in such studies that often favour vicariance hypotheses, repeatedly invoked irrelevant geological reconstructions, and, most strikingly, showed a cavalier approach to evolutionary timelines by pushing the origins of many groups back to unreasonably ancient ages. The result was a succession of implausible histories for particular taxa and areas, including the notions that the Hawaiian biota is almost entirely derived from ancient (often Mesozoic) central Pacific metapopulations, that the disjunctions of extremely mobile organisms such as ducks rarely, if ever, result from long-distance dispersal, and that primates were widespread 120 million years before their first appearance in the fossil record. In contrast to Heads’ perspective, a central message of The Monkey’s Voyage is that explanations for disjunct distributions should be evaluated on the basis of diverse kinds of evidence, without strong a priori assumptions about the relative likelihoods of long-distance dispersal and vicariance.