Molecular Ecology (2004) 13, 2471–2479 doi: 10.1111/j.1365-294X.2004.02240.x © 2004 Blackwell Publishing Ltd Blackwell Publishing, Ltd. Hybridization and contemporary evolution in an introduced cichlid fish from Lake Malawi National Park J. TODD STREELMAN,* S. L. GMYREK,* M. R. KIDD,* C. KIDD,* R. L. ROBINSON,† E. HERT,§ A. J. AMBALI ‡ and T. D. KOCHER * * Hubbard Center For Genome Studies, 4th Floor, Environmental Technology Building, University of New Hampshire, 35 Colovos Road, Durham, NH 03824, USA, † Department of Biology University of Hull, Hull HU6 7RX, UK, ‡ Department of Biology, Chancellor College, The University of Malawi, Zomba, Malawi, § Max-Planck-Institut für Verhaltensphysiologie, Seewiesen, 8130 Starnberg, Germany Abstract Rapidly evolving systems offer the chance to observe genetic and phenotypic change in real time. We exploit a well-characterized introduction of cichlid fish into Lake Malawi National Park to document a short history of habitat colonization and the evolution of genes and colour pattern. In the early 1960s, a fish exporter introduced individuals of Cyno- tilapia afra to a single site (Mitande Point) of Thumbi West Island and, as late as 1983, the species was confined to this location. In 2001, C. afra had colonized the entire perimeter of Thumbi West. In July of that year, we sampled C. afra individuals from six sites around the island and scored variation in dorsal fin colour as well as allelic diversity at six microsatel- lite loci. We found that, in two decades, C. afra had diverged into genetically distinct, phe- notypically different northern and southern populations. We observed a high proportion of hybrids between the introduced C. afra and the native Metriaclima zebra on the southern coast of Thumbi West, and speculate that hybridization is facilitated by low water clarity at these windward sites. The short history of C. afra at Thumbi West is a microcosm of con- temporary evolutionary divergence and may provide the opportunity to study the process from start to finish in genetic detail. Keywords: cichlid, colour pattern, hybridization, introduced species, rapid evolution Received 12 November 2003; revision received 18 March 2004; accepted 21 April 2004 Introduction Thousands of cichlid species have evolved within the great lakes of Tanganyika, Malawi and Victoria in the last 2 million years (Turner et al. 2001). Cichlids are noteworthy because populations are often genetically subdivided on very small geographical scales (Van Oppen et al . 1997; Rico & Turner 2002) and sites can harbour a variety of endemic colour forms (Arnegard et al . 1999; Smith & Kornfield 2002). Colour pattern is thought to evolve in concert with the visual environment and novelties are sometimes produced by hybridization (Seehausen et al . 1997; Smith & Kornfield 2002; Smith et al . 2003) The role of hybridization in cichlid evolution has been debated, and new data suggest that the phenomenon is more common than previously believed. Recent synthesis points to introgression as a source of genetic diversity and novel phenotypes, especially after changes in environment (Rüber et al . 2001; Salzburger et al . 2002; Smith & Kornfield 2002). Given that environmental change (e.g. the rise and fall of water level) is a frequent occurrence in East African lakes (Sturmbauer et al . 2001), hybridization may be a natural evolutionary force contri- buting to cichlid biological diversity. As humans continue to perturb these lacustrine environments, the opportun- ity for human-induced evolution through hybridization increases (e.g. Seehausen et al . 1997). The potential for human-induced ‘contemporary evolu- tion’ is a growing concern for ecologists, and for evolution- ary and conservation biologists (Thompson 1998; Palumbi 2001; Stockwell et al . 2003). Reports have documented extremely rapid rates of phenotypic evolution, or repro- ductive isolation, following human introductions (Hendry Correspondence Present address: J. Todd Streelman, School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230. E-mail: todd.streelman@biology.gatech.edu