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.
LITERATURE CITED
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.
