Monday, August 11, 2014

Are Distributions of Ancient Lineages of Mollusks Shaped by Vicariance or Dispersal?

                It is thought that the study of animal and plant distributions advanced greatly with the development of vicariance biogeography in the 1970s and 1980s.  Rather than assuming related groups of organisms dispersed around the globe across seemingly insurmountable barriers, it was thought that organisms rafted apart from one another on drifting continents via plate tectonics yielding many groups that shared a common Earth history.  Vicariance biogeography revolutionized the discipline and stimulated many studies with an interest in constructing area cladograms and searching for generalized patterns.  Indeed, its influence was so profound that dispersal was considered less and less likely and some even believed it could not be falsified and therefore was unscientific.  Over the past decade, however, numerous studies have been published showing that the distribution of a number of taxonomic groups formerly thought to be due to vicariance are best explained by dispersal.  Indeed, one maverick, Alan de Queiroz, has proposed that most distributions can be attributed to dispersal.  He tells his very interesting story in a recently published book entitled The Monkey’s Voyage – How Improbable Journeys Shaped the History of Life.
                De Queiroz proposes a new paradigm for historical biogeography that entails three steps (1) acceptance of plate tectonics, (2) building evolutionary trees utilizing cladistic thinking or other statistical means, and the step that was missing in vicariance biogeography (3) time.  Time is an absolutely essential component and comes from molecular dating results.  So for example, the New World and Old World monkey molecular timeline is a split between 26 to 51 million years ago, which means they missed the vicariant event separating Africa from South America.  Therefore, Old World monkeys must have dispersed to South America.
                De Queiroz presents numerous case studies and makes a compelling case for dispersal shaping the history of life in a much more significant way than thought of during the vicariance revolution.  But is it possible, the pendulum will swing too far back from vicariance being dominant mechanism to dispersal without thoroughly investigating the matter?   For example, many of the case studies are vertebrate groups that simply are not old enough to play a part in many vicariant events including the ever popular break-up of Gondwana (although this has not stopped investigators from proposing vicariance during the vicariant movement).  However, de Queiroz cites a few exceptions involving two lineages of mite harvestmen (Giribet et al., 2012) and two lineages of centipedes (Murienne et al., 2010) that appear to be Gondwanan relicts dating back to 90 million years ago or earlier.  What about other ancient invertebrate groups?  What about mollusks?  It is known that dispersal plays a fundamental role in the evolution of biodiversity on oceanic islands including terrestrial gastropods (Cowie and Holland, 2006), but what about the break-up of continental landmasses like Gondwana?   Clearly, many mollusk lineages are ancient enough to possibly be shaped by vicariance, but studies are needed to determine whether this is the case or not.  I encourage malacologists to rise to this interesting challenge.

Cowie, R. H., and B. S. Holland.  2006.  Dispersal is fundamental to biogeography and the evolution of biodiversity on oceanic islands.  Journal of Biogeography 33:193-198.

Giribet, G., et al.  2012.  Evolutionary and biogeographical history of an ancient and global group of arachnids (Arachnida: Opiliones: Cyphophthalmi) with a new taxonomic arrangement.  Biological Journal of the Linnean Society 105:92-130.

De Queiroz, A.  2014.  The Monkey’s Voyage:  How Improbable Journeys Shaped the History of Life.  Basic Books, New York. 

Murienne, J., G. D. Edgecombe, and G. Giribet.  2010.  Including secondary structure, fossils and molecular dating in the centipede tree of life.  Molecular Phylogenetics and Evolution 57:301-313.