New date for speciation of Malawi cichlids

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Scientists have found new evidence for the date of the event that may have caused the explosive speciation of Lake Malawi cichlids.

A study published in a recent issue of the journal Proceedings of the National Academy of Sciences by scientists from the USA and Norway has shown that extremely arid conditions in tropical Africa about 70,000 to 135,000 years ago caused water levels in Lake Malawi to drop to extraordinarily low levels, possibly triggering the explosive speciation of cichlids in the lake.

The study by Andrew Cohen, Jeffery Stone, Kristina Beuning, Lisa Park, Peter Reinthal, David Dettman, Christopher Scholz, Thomas Johnson, John King, Michael Talbot, Erik Brown and Sarah Ivory studied fossil and sedimentological data from drill cores taken from Lake Malawi in order to understand the palaeoclimatological and palaeolimnological conditions of the lake.

The authors found that ...the Lake Malawi region experienced extreme aridity episodically between 135 and 70 ka, with diminished climatic and ecologic variability after 70 ka...

During the most arid intervals, Lake Malawi became a shallow alkaline, saline lake, with lake levels at least as low as -580 m, and its surrounding watershed was converted into a semidesert...

The results have implications for cichlid evolution within the lake: ...the early Late Pleistocene conversion of Lake Malawi to a relatively shallow lake would have eliminated most rocky shoreline habitat in the lake, leaving the majority of the lake s shoreline with a muddy or sandy bottom.

This alters our understanding of the physical setting for evolution of the lake s endemic fauna. The estimated 500"1,500 species of cichlid fishes in Lake Malawi are intensively studied as model systems for speciation, with sexual selection based on visual mate choice as a preferred hypothesis for the elevated diversity...

A shallow and probably turbid, well mixed lake with reduced visibility (as suggested by the diatom flora) might impede cichlid mate choice and negatively impact species maintenance and coexistence...

These conditions could have significantly affected the rocky dwelling littoral cichlid communities (mbuna) that currently account for 20"50% of the extraordinary diversity of the lake s cichlids...

The megadrought low-stand demonstrated here for Lake Malawi provides a much more robust constraint on genetic divergence within the cichlid phylogeny, implying rates of evolution four to eight times slower than those that assume a major role for the LGM in Lake Malawi speciation...

For more information, see the paper: Cohen, AS, JR Stone, KRM Beuning, LE Park, PN Reinthal, D Dettmann, CA Scholz, TC Johnson, JW King, MR Talbot, ET Brown and SJ Ivory (2007) Ecological consequences of early lat Pleistocene megadroughts in tropical Africa. Proceedings of the National Academy of Sciences 104, pp. 16422"16427.