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Ancient lakes have been an intensely studied area in evolutionary biology (Brooks 1950; Martens 1997). This is because ancient lakes contain a continuous record of biotic change over long periods of geologic time in their basin sediments, and because ancient lakes generally have high levels of endemism in their biological communities. Lake Malawi is located in the East African Rift Valley and its basin dates to 8.6 million years ago (MYA) with modern deep water conditions attained at ~ 4.5 MYA (Delvaux 1995). Of the three major lakes that comprise the East African Great Lakes, Lake Malawi boasts the highest diversity of endemic cichlid fish, 49 endemic genera containing over 1000 species (Turner 1994; Moran et al. 1994; Danley and Kocher 2001). These cichlid fish stocks do not date to the period of lake formation, and instead date to an invasion of cichlids from Lake Tangyanika to Lake Malawi that was thought to have occurred between 2 MYA and 700,000 years before present (Genner et al. 2007; Danley and Kocher 2001).
The cause of the accumulation of diversity in African rift valley cichlid species has been a source of debate for several decades (Keenleyside 1991). This debate has been centered on whether allopatric or sympatric speciation is the cause of the explosive radiation seen in the African Great Lakes. Early experiments focused on the Mayr (1963) model of complete geographic separation of populations leading to speciation (McKaye and Gray 1984). Other, more recent research has shown speciation with gene flow as a plausible form of sympatric speciation in Lake Malawi (Turner 1994; Shaw et al. 2000). I will show that these two processes are not mutually exclusive, and that a combination of both mechanisms has likely occurred in Lake Malawi over different time periods and different scales. I will consider models developed by Danley and Kocher (2001) and Kocher (2004) to specifically look at changing environments, feeding habits, and mate choice as the main drivers of speciation of cichlid fish in Lake Malawi. In addition, I will discuss several genetic factors that arise in the speciation of cichlids as well as future directions of cichlid research in the Great Lakes of Africa.
Habitat: Changing environments through time
There are two main groups of cichlid fish in Lake Malawi, pelagic and benthic, and of the benthic group two subgroups have formed, those that are rock-dwellers and those that are sand-dwellers. All of these types of cichlids evolved from a generalist that invaded Lake Malawi from Lake Tangyanika (Danley and Kocher 2001). These groups and subgroups of cichlids each specialize in a broad geographic location and/or ecological niche causing speciation based on locality, an allopatric process. Danley and Kocher (2001) consider this process to be most significant immediately following invasion, playing less of a role after initial ecological speciation. However, I challenge this notion as many rock-dwelling and sand-dwelling benthic cichlid's habitat would be negatively impacted during the known variation in lake level of the African Great Lakes throughout the last several million years (Scholz and Rosendahl 1988; Johnson et al. 1996).
Stumbauer at al. (2001) suggested that major changes in lake levels match genetic divergences in cichlid fish throughout the history of Lake Malawi and Genner et al. (2010) suggest that after lake levels increased, populations of cichlid fish expanded and underwent allopatric speciation because of increased niche environments were created. These studies show that climatic effects impact cichlid speciation by fragmenting populations as lake levels decrease and by increasing niche habitat space as lake levels increase. However, there could be additional environmental factors that have not been as intensely researched. This would include increased turbidity, affecting visual cues for mate choice (Genner et al. 2010). In addition, dramatic aquatic chemistry changes, such as increased salinity that occurs when lake levels decrease, could affect primary production and thus, tropic predation.
Decreases in populations of cichlid fish brought about by ecological and environmental changes from lake level fluctuations could create "bottlenecks" and "founder effects". Danley et al. (2000) found that allelic diversity was reduced in rock-dwelling species of cichlids that inhabited shallower areas when compared to higher allelic diversity in cichlids that inhabited deeper older areas of Lake Malawi. This difference was likely created by the desiccation of the lake and suggests that reduced populations of cichlids were associated with low lake stands, but no conclusive evidence of a bottleneck in the population was found (Danley et al. 2000).
Predation: The quest for food
Following ecological speciation based on geographic location, Danley and Kocher (2001) considered trophic competition to be the next chronological mode of speciation of cichlid fish in Lake Malawi. The authors state that speciation based on predation diversification is the logical progression from ecological speciation. Competition for food further drives diversification of the cichlid fish in each geographic area that they inhabit. This progression happened efficiently in the cichlid fish of East Africa because the jaw apparatus underwent changes that allowed for the pharyngeal jaws to process food, a role once relegated to the oral jaw (Liem 1973). This change allowed for a diversity of jaw movements and freed the oral jaw to solely collect food (Liem 1973). Changes in jaw function permitted the collection of food from of a larger assortment of trophic sources ranging from plankton to scales of fish to whole fish (Kocher 2004). Differentiation of morphology based on trophic competition created niches for cichlid fish usually inhabited by different families of fish (Greenwood 1964).
Sexual selection: A multitude of color choice
The third mode of speciation Danley and Kocher (2001) describe is driven by sexual selection, and this type of speciation accounts for the bulk of morphological variety in colorization that comprises the majority of recent speciation in extant East African cichlid populations. Sexual selection has been implicated as the cause of male secondary sexual characterizations, including male colorization (McKaye et al. 1984), with the rest of the morphological features remaining largely unchanged. Male traits are chosen by females in lek-breeding type situation, where females chose from many males (Barlow 1991). This skewed system of reproduction can create linkage disequilibrium and rapid diversification of morphological types, which can occur in sympatric or allopatric speciation (Turner and Burrows 1995). Not all types of East African cichlids undergo morphological speciation based on colorization, but lineages that differentiate by means of other morphological or ecological factors are relatively species poor (Danley and Kocher 2001).
Timing and magnitude
Danley and Kocher (2001) indicate that all three speciation types (habitat, predation, and sexual selection) are present during three distinct pulses of radiation. During all three radiation events, all three modes of speciation are present in differing abundances. The first radiation event is dominated by habitat diversification, the second by trophic competition and the third by sexual selection (Danley and Kocher 2001). Speciation based on sexual selection has occurred at the same level of intensity throughout the multiple radiations of cichlid fish in Lake Malawi. Sexual selection only appears to be increasing in impact in the last radiation event because ecological speciation and differentiation based on predation have decreased in impact (Danley and Kocher 2001). Danley and Kocher (2001) suggest that these three types of speciation promoted 'rapid' diversification through a positive feedback loop, where reduced gene flow further promoted divergent selection.
In studies that have elucidated genes responsible for traits, it has been shown that those traits have undergone strong selection (Kocher 2004). For this reason, the author suggests that moving forward there needs to be a better understanding of genes that are responsible for various phenotypes, especially those that are selected under directional pressure. Once the fine scale mapping of the genome is complete, Kocher (2004) believes that coalescent history of alleles will be very informative in reconstructing speciation events.
Kuraku and Meyer (2008) assert that comparative developmental and genomic approaches are the future of African Great Lakes cichlid research. The authors state that this is because many of the cichlid species are very similar genetically. Thus, they can be hybridized and studied to determine the genetic causes of phenotypic expression. Although research has shown that hybridization is not a driving force in speciation of the rock-dwelling cichlids of Lake Malawi (Albertson et al. 1999), hybridization could be a factor in the speciation of sand-dwelling and pelagic varieties of cichlids.
The Danley and Kocher (2001) model follows a logical progression of speciation events; invasion, followed by niche habitat exploitation, then trophic competition, with the remaining diversification left to sexual selection. The fundamental reasoning of this model has merit for the rapid speciation of cichlids in the Lake Malawi, and it is robust, in that it includes multiple types of speciation (sympatric, allopatric, and parapatric) occurring over differing geographic and time ranges. However, it lacks resolution over time scales that include dramatic fluctuations in environmental conditions (i.e. lake level fluctuations).
For instance, sexual selection remains a constant factor throughout speciation in Danley and Kocher's model (2001), but lowered lake level would cause increased turbidity, which would limit the ability of females to chose males based on colorization. The model also relegates habitat and trophic competition to being factors in only the first and second bursts of cladogenesis. This does take into account issues of habitat fragmentation and population expansions associated with lake level changes, or the trophic changes that would occur from nutrient and chemical changes brought about by hydrogeologic variability.
In conclusion, it has been experimentally shown that there are many ways that speciation has occurred - both allopatric and sympatric - in the cichlid fish populations of Lake Malawi. The Danley and Kocher (2001) model summarizes these types of speciation well. However, the model could be further developed to include recent discoveries in cichlid response to lake level changes (Genner et al. 2010; Sturmbauer 2001) as well as increasingly better refined phylogenetic studies. Understanding the history of Lake Malawi's environmental and geologic past and the relationships of cichlid taxonomy will aid in the reconstruction of the causes, drivers, and limitations of rapid speciation in Lake Malawi. This understanding could be invaluable as human populations increase, putting pressure on natural resources, affecting habitats and reducing biological populations and diversity. Looking at periods of lowered lake level could be a powerful analogue for decreased genetic diversity that could inform future perturbations in endemic cichlid fish population of Lake Malawi.