Evolution is unique in that two similar scenarios can yield vastly different results. Natural selection does not work with future information, it simply selects on individuals that have traits that are currently or have previously selected them over others. An example of this is in the East African Rift Lakes, or more specifically, Lake Tanganyika. The oldest of the lakes (Brown ET. Al, 2010), Lake Tanganyika is famed for being the second largest freshwater lake in the world (N.A. 2016), with the countries of Zambia, Tanzania, DR Congo, and Burundi bordering its boundaries. Here, adaptive radiation has caused for the explosive speciation of the Cichlid Family of fishes, in which the multiple niche levels began to be filled by the family. The causes of the increase in fish species diversity in Lake Tanganyika is likely due to external factors such as shifting lake levels, but could also be attributed to internal factors such as sexual selection and predation.
Estimated at being nearly 9-12 million years old (Brown et. Al, 2010), nearly 60% of the animal species that inhabit the lake originated in the body of water (Sweke ET. Al, 2016). With such an extended time of geographic isolation, the lake has experienced multiple water level fluctuations throughout its history. Major lake level fluctuations could thereby explain intralacustrine allopatric speciation; low water levels in the lake would have left the Cichlid fishes geographically divided by different sub-basins (Koblmüller ET. Al, 2008). The fact that many Cichlid species are only found in small, separated sections of the lake also supports this idea (Salzburger and Meyer, 2004). Many of the rock-dwelling Cichlids of the lake are divided due to habitat separation. Even minor lake level fluctuations can have significant impacts on fish lineages. Littoral or shoreline dwelling species could be significantly affected by minor fall in water level (Salzburger and Meyer, 2004) since their habitat would be the first to be effected by lake fluctuations.
|Adaptive radiation of east African cichlid fish.|
In this case, the extremely large size of the lake would have a greater effect on the speciation of the fish than the age of the lake would. The emergence of multiple deep-water habitats could then act as a barrier to fish populations, while also separating the spawning sites of the Cichlid fishes, further influencing allopatric speciation (Salzburger and Meyer, 2004) . Other species have also experienced unique speciation in Lake Tanganyika as well, such as the Mastacembelid eels. While Mastacembelid eels can be found throughout Asia and Africa, a separate lineage has formed in the lake (Brown ET. Al, 2010). It is believed that low water levels in the lake nearly 7-8 million years ago began the diversification of the lineage.
Other factors besides environmental have also played a role on the speciation of the Cichlid family as well. Female Cichlids are known for choosing their mates based on their coloration; “Fisherian runaway sexual selection”, as Meyer calls it, could describe how speciation of the lineages further occurred. Short separation of the fish could cause for gradual coloration shifts in males due to preferred selection by females. These changes, even small, could have dramatic effects on how females would select their mates, further seeding the separation of species flocks. While predation of Cichlid fishes has not be explained for their rapid speciation, it has been known to affect other organisms in the lake. For the freshwater snails of the lake, predation from the freshwater crab Potamonautes lirrangensis has affected both their shell size and shape, and has given rise to their thalassic shells (Weigand ET. Al, 2014).
Unique habitat fluctuations on the lake has led to the lacustrine diversification of fish species in Lake Tanganyika. While other factors may have also played a smaller role in shaping the diversity of aquatic species in Lake Tanganyika, evidence shows that lake levels were a major contributing factor. The lake is remarkable in that it has allowed scientists the opportunity of an isolated study environment in which the effects of allopatric speciation has been greatly induced. In more recent times, influences such as overfishing and climate change (N.A. 2016) have been affecting the lake, putting the unique diversification of species in question. Perhaps we can use these factors to study if overfishing can further induce the effects of allopatric speciation on the Cichlid fishes, but only time will tell how the formation of Lake Species will end. By studying this extraordinary ecosystem now, we can better understand the effects that allopatric speciation will have on future species and ecosystems to come.
|Cichlid fishes of Lake Tanganyika. Animal Press. Source|
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Sweke, E.A., J.M. Assam, A. I. Chande, A.S. Mbonde, and M. Magnus. 2016. Comparing the performance of protected and unprotected areas in conserving freshwater fish abundance and biodiversity in Lake Tanganyika, Tanzania. International Journal of Ecology 2016
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