The cichlid radiation of Lake Malawi
In the bonefish example above, genetic studies show that cryptic species can be revealed by mtDNA sequence divergence, most especially in cases where morphological differences are slight or absent. The reverse can also be the case, in which morphological divergence and speciation can outpace mtDNA divergence. The cichlid species flocks of the African Great Lakes have fascinated fish biologists and (more recently) evolutionary geneticists. According to some estimates, Lake Malawi in eastern Africa contains over 600 species, most in the lineage of haplochromine cichlids, with a diversity in form and function that includes eye biters, scale eaters, crab eaters, sediment sifters, plankton eaters, egg robbers, a species that picks parasites off catfish, and one that catches flies near the water’s edge. Taxonomists spent decades sorting these fishes into genera and species, until mtDNA studies upended the whole classification scheme in the 1990s. First, genetic studies demonstrated that the haplochromine cichlids of Lake Malawi are very closely related, d <0.06 in mtDNA sequence comparisons (Albertson et al. 1999). Second, these species descended from a single common ancestor that colonized the lake a few million years ago. Third, many of the species are indistinguishable in mtDNA surveys, indicating speciation events no older than a few thousand years (Kornfield & Parker 1997; Won et al. 2003).
Fish species flocks exist elsewhere in the world, but none are as diverse as the cichlids of Lake Malawi. What could promote such rapid and extreme diversifi cation? Kocher (2004) describes two factors that seem to promote this process. In the first step, the cichlids move into habitats that require some specialization. Fishes in each habitat will benefit from breeding with similar individuals, to reinforce the genetic and morphological features that allow successful feeding and reproduction. This ecological selectionpromotes isolation from cichlids in other habitats, and promotes specialization of feeding morphology and other adaptive traits. The next step is diversifi cation in coloration, a step that can apparently happen on a scale of dozens or hun dreds of generations. Malawi cichlids are nest builders and many are female mouth brooders, behaviors that promote sexual selection wherein females choose a mate based on coloration and behavior. Hence coloration determines which fish interbreed and which ones do not, the foundation of speciation. When the genes for an ecological adaptation are coinherited (perhaps on the same chromosome) with the genes under sexual selection (for distinct coloration), speciation can occur very rapidly. Therefore the composition of the cichlid genome is a third factor that promotes rapid speciation. Kocher (2004) concludes that this plurality of genetic, behavioral, and ecological factors, all of which drive speciation in other organisms, are combined in cichlids to produce the greatest diversity in freshwater fishes. Notably, the cichlid model of speciation does not require geographic isolation (allopatry).
In summary, molecular systematics has revealed much about the history of fishes (and ourselves), and also key points about fish diversity. First, molecular phylogenetics is especially valuable for determining the pattern and pace of evolutionary changes. In the last 20 years the field of systematics has switched from morphology-based trees, to mapping morphological changes on molecular trees. The molecular studies provide a time dimension for these morphological changes, if a calibrated molecular clock is available.
Second, speciation can occur very rapidly, as is the case for African cichlids. Some species of cichlids and other fishes are distinguished by morphology, behavior, and coloration, yet are indistinguishable with mtDNA sequences (Bowen et al. 2006a). A related point is that these rapidly evolving fishes are not isolated by physical barriers, defying the conventional model of allopatric speciation (Wiley 2002; Coyne & Orr 2004). Instead, much of the speciation in fishes seems to occur in adjacent habitats, along ecological rather than geological partitions (Rocha & Bowen2008).
Third, some sister species may be unrecognized because they retain very similar morphology across millions of years, and these hidden species can be revealed with DNA surveys. Cryptic species continue to be discovered, even among the large and well-studied fishes: the numbers of species of ocean sunfishes (genus Mola), goliath groupers (genus Epinephelus), and hammerhead sharks (genusSphyrna) have all expanded after genetic appraisals (Bass et al. 2005; Quattro et al. 2006; Craig et al. 2008). Very often the genetic difference is accompanied by subtle morphological differences that become apparent upon re-examination. Molecular genetic surveys have also been useful in identifying emerging species, those that seem to be in the process of speciation (McMillan & Palumbi 1995; Campton et al. 2000; Craig et al. 2006).
Finally, the discovery of unrecognized species and cryptic evolutionary diversity can be especially important if these species are scarce, endangered, or heavily exploited. Recall the case of Hawaiian bonefish, a favorite with anglers that was once thought to be a single species, now known to be two species. Consider the implications if the two species, one more common than the other, are managed as a single fishery stock. The less abundant species could be severely depleted without any sign of distress in the overall fishery.