A new molecular study on the cichlids of South America has provided evidence to suggest that geophagine cichlids have undergone an adaptive radiation.
Lopez-Fernandez, Honeycutt and Winemiller sequenced the nucleotides of the mitochondrial ND4 gene and the nuclear RAG2 gene to produce what they claim is the most extensive molecular phylogeny to date for the Geophaginae subfamily.
In their paper in the January issue of the journal Molecular Phylogenetics and Evolution, the team explains how they tested a number of evolutionary theories on South American cichlids using the tree they constructed from the genetic data.
The new phylogeny, a sort of evolutionary family tree, provides support for the theory that Geophagus (sensu lato), Biotodoma, Mikrogeophagus, Crenicara, Dicrossus and Gymnogeophagus are all related members of a single big clade.
The tree also suggests that Satanoperca, Apistogramma, Apistogrammoides and Taeniacara are all members of a smaller clade, which shows that these fish are more closely related to each other than to other cichlids in the region.
The study also supports the Acarichthyini tribe clade that was proposed by Dr Sven Kullander.
The team also believe the molecular phylogeny provides evidence to support an adaptive radiation for geophagines:
"Both branch lengths and internal branch tests revealed extremely short basal nodes that add support to the idea that geophagine cichlids have experienced an adaptive radiation sensu Schluter that involved ecomorphological specializations and life history diversification."
For more details see the paper: Lopez-Fernandez H, Honeycutt RL, Winemiller KO. (2005) - Molecular phylogeny and evidence for an adaptive radiation of geophagine cichlids from South America (Perciformes: Labroidei). Mol Phylogenet Evol. 2005 Jan;34(1):227-44.