If I was asked to name a few of the most compelling systems in evolutionary biology, I’d certainly start with Darwin’s Finches. Next might come peppered moths, African cichlids, stickleback, Caribbean Anolis lizards, or Lenski’s E. coli. What’s interesting about this (very short and incomplete) list is that 4 of the 6 examples represent adaptive radiations.
Adaptive radiation is when one ancestor occupies different environments and undergoes rapid phenotypic divergence to exploit the resources of those environments (for a full discussion of the topic, read Schluter 2000). Perhaps one of the reasons adaptive radiations are so well known as evolutionary examples is because, on the surface, it is conceptually quite clear how these processes may occur. However, our understanding of the actual underlying processes is still very much in progress.
The African cichlid system is a particularly incredible example of an adaptive radiation. In Lakes Victoria, Malawi, and Tanganyika, radiations have produced between 250 and 500 species of fish per lake. These three radiations range from recent, 15,000-100,000 years ago, to ancient, 10-12 million years ago (Brawand et al., 2014), and have provided much insight into the processes of adaptive radiations. While these big lakes may get most of the press, Ole Seehausen pointed out in 2006 that there are lots of other small radiations of cichlids that have been largely ignored (Seehausen 2006). Moreover, Dolph Schluter states in 2000 that understanding processes of speciation and reproductive isolation requires studying the process when it is underway. He follows this by saying that:
A microevolutionary focus amounts to the study of macroevolution in action
Kavembe et al. take advantage of one such small recent radiation of cichlids in East Africa to reconstruct the evolutionary history during diversification into novel niches. They argue that the presence of fewer species in their study rather than the hundreds of species in the larger African Great Lakes makes reconstructing demographic history (and evolutionary history) less complicated. For example, it is pretty difficult to determine the relative effects of hybridization and population splits when you’re looking at a few hundred species. Also, by focusing on recently diverged populations, they hope to gain insight into the processes that have driven past adaptive radiations.
Map of sampling locations and example individuals from each population. From Kavembe et al. 2016.
The authors work on tipalia in Lake Magadi and its satellite lake, Lake Little Magadi. In this system, fish are restricted to lagoons and there are three divergent populations present: FSL and ROM in Lake Magadi; LM in Lake Little Magadi (see map).
The study consisted of three main analyses:
- Geometric morphometrics to identify morphological divergence between populations
- Stable isotope analysis for assessment of ecological niche
- Population genomics (RADseq) and simulations to reconstruct demographic history
The results show solid evidence that the populations are diverging morphologically, ecologically, and genetically. LM individuals had an upturned mouth, presumably for feeding on the water surface. There were also significant differences in stable isotope compositions between populations. Finally, genetic results revealed much about the demographic history of the populations: (1) it seems that all populations split simultaneously about 1100 generations ago; and (2), while there is ongoing gene flow between FSL and ROM in Lake Magadi, LM in Lake Little Magadi is isolated.
In my opinion, the authors demonstrate their main point. Harnessing the simplicity of the system, they managed to thoroughly elucidate the evolutionary history of these very recently derived populations. This should lay the groundwork to further understand the early stages of adaptive radiations and help us understand how microevolutionary processes lead to macroevolutionary patterns.
References:
Brawand, David, et al. “The genomic substrate for adaptive radiation in African cichlid fish.” Nature 513.7518 (2014): 375-381.
Kavembe, Geraldine D., et al. “Eco‐morphological differentiation in Lake Magadi tilapia, an extremophile cichlid fish living in hot, alkaline and hypersaline lakes in East Africa.” Molecular ecology (2016).
Schluter, Dolph. The ecology of adaptive radiation. OUP Oxford, 2000.
Seehausen, Ole. “African cichlid fish: a model system in adaptive radiation research.” Proceedings of the Royal Society of London B: Biological Sciences 273.1597 (2006): 1987-1998.