The Geography of Speciation in Marine Organisms

nucellaUnderstanding the geographic scale at which new species arise in the sea has challenged marine biogeographers for well over a century, and our lab has been investigating the geographic circumstances of speciation in several different systems: continuously-distributed rocky-shore faunas of the north Pacific, sister-species pairs currently separated by the Isthmus of Panama, and anti-tropical species in the eastern Pacific.

Research on rocky-shore Nucella suggests that even though areas of sympatry among living species can be extensive, sympatric distributions on rocky shores likely represent secondary contact between species that evolved in geographic isolation. These species often also show significant morphological differentiation where they co-occur, indicating that secondary contact and competitive interactions between recently diverged sister-species plays an important role driving morphological evolution.

In tropical America, so-called “geminate” species pairs found on either side of the Central American Isthmus provide a unique system to understand the geography speciation. However, molecular sequence data gathered in my lab indicates that many of these so-called geminates split significantly earlier than 3 million ago and that more species have evolved on either side of the Isthmus in just the last 3-4 million years. Speciation clearly happens without such dramatic barriers, so the challenge to marine biogeographers is to understand what other factors contribute to population divergence that ultimately results in new species.

nsfThis work was supported by grants from the National Science Foundation.

Marko, P. B. & A. L. Moran. 2009. Out of sight, out of mind: High cryptic diversity obscures the identities and histories of geminate species in the marine bivalve subgenus Acar. Journal of Biogeography 36: 1861-1880. [PDF],

Marko, P. B. 2008. Allopatry. Pp. 131-138 in The Encyclopedia of Ecology by S. V. Jørgensen (ed.). Elsevier, Oxford, UK.

Marko, P. B. 2008. Sympatry. Pp. 3450-3458 in The Encyclopedia of Ecology by S. V. Jørgensen (ed.). Elsevier, Oxford, UK.

Marko, P. B. 2005. An intraspecific comparative analysis of character divergence between sibling species. Evolution 59: 554-564.[PDF]

Marko, P. B. 2002. Fossil calibration of molecular clocks and the divergence times of geminate species pairs separated by the Isthmus of Panama. Molecular Biology & Evolution 19: 2005-2021.[PDF]


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