Sympatric speciation, the formation of species in the absence of geographical barriers, remains one of the most contentious concepts in evolutionary biology. Although speciation under sympatric conditions seems theoretically possible^{1, 2, 3, 4, 5}, empirical studies are scarce and only a few credible examples of sympatric speciation exist⁶. Here we present a convincing case of sympatric speciation in the Midas cichlid species complex (Amphilophus sp.) in a young and small volcanic crater lake in Nicaragua. Our study includes phylogeographic, population-genetic (based on mitochondrial DNA, microsatellites and amplified fragment length polymorphisms), morphometric and ecological analyses. We find, first, that crater Lake Apoyo was seeded only once by the ancestral high-bodied benthic species Amphilophus citrinellus, the most common cichlid species in the area; second, that a new elongated limnetic species (Amphilophus zaliosus) evolved in Lake Apoyo from the ancestral species (A. citrinellus) within less than 10,000 yr; third, that the two species in Lake Apoyo are reproductively isolated; and fourth, that the two species are eco-morphologically distinct.

1. Lehrstuhl für Zoologie und Evolutionsbiologie, Department of Biology, and
2. Center for Junior Research Fellows, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany
3. *These authors contributed equally to this work

Correspondence to: Axel Meyer¹ Correspondence and requests for materials should be addressed to A.M. (Email: Axel.Meyer@uni-konstanz.de). GenBank accession numbers for all mitochondrial control region sequences can be found in Supplementary Table 1.

Received 13 September 2005; Accepted 10 October 2005

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