Why ancient lakes are hotspots for evolutionary biology research

One of the earliest posts I wrote here, way back in the arcane days of 2010, is a distillation of a term paper I wrote on the snails of the Steinheim Basin. This is a Miocene-aged lake that formed in the crater left by a small meteorite (it has a larger brother, the Nördlinger Ries; they both came from the same meteorite that broke in two on atmospheric entry). While it is now only a fossil outcrop, when it was filled, the Steinheim Basin persisted for several million years. Most lakes don’t last longer than twenty thousand years, so those that survive longer are termed ancient lakes. Extant examples of ancient lakes include the African Great Lakes, e.g. Lake Tanganyika, which is 9-12 million years old (Cohen et al., 1993), or the most ancient lake known, the 30 million year old Lake Baikal (Sherbakov, 1999). Lakes generally don’t last very long because sediment accumulation is mostly greater than net water accumulation, making these places exceptional.

Ancient lakes are some of the most sought-after arenas in evolutionary biology. They are often filled with a unique, endemic fauna that has been evolving in isolation in the lake for millions of years. As a stark example, 99% of the cichlid fishes of the African Great Lakes Malawi and Victoria are endemic (Snoeks, 2000). Around 30% of all gammaroid amphipods known are endemic species from Lake Baikal (Kamaltynov, 1999) – and that’s only a conservative estimate (Vainola & Kamaltynov, 1999). In other words, ancient lakes are open evolutionary laboratories where the processes of evolution can be observed in action, in a natural setting. Lakes also provide an excellent record of their past environments through their perfectly layered sediments, allowing for the history of the locality to be tied into the patterns observed.

The Steinheim snails provide a great example of this in fossil form: one ancestral population of Gyraulus kleini became dominant, and selective pressures led to its splitting into G. crescens and G. steinheimensis. The latter ended up eradicating the original G. kleini due to its competitive advantages, while G. crescens went its own way. G. steinheimensis later on suffered the same fate as G. kleini, as did its daughter species, as did their daughter species in turn. All of these changes can be traced and linked to changes in environmental conditions, as hinted at by proxy geochemical data. Such a detailed succession (see my older posts) is exceptional, and serves as a great example of speciation and macroevolutionary processes in action. Specifically, it shows natural selection caused by competition and environmental changes.

If we can get this just from biased fossils, you can imagine what sort of exciting and groundbreaking research we can do when the animals are alive. You will not find an evolutionary biology textbook that does not mention research on the African Great Lakes. What makes them valuable, besides this long-term isolation, is that they have shown that there is no one universal way by which adaptaive evolution proceeds. To demonstrate this, look at the cichlids and paludomid gastropods of Lake Tanganyika, both of which have reached an incredible level of endemicity within the lake as a result of adaptive radiation.

The cichlid fishes of the Great Lakes have undergone, and are still undergoing, an explosive radiation, driven by both classical natural selection and also by sexual selection (Kocher, 2004). In particular, it was trophic specialisations that provided the initial spur (Danley & Kocher, 2001). As an example of the level of specialisation now present among these fishes, one species has specialised to only eating the eyeballs of other fishes. Through the actions of sexual selection compounding on this initial specialisation, the massive speciation occurred.

On the other hand, the paludomid gastropods in Lake Tanganyika were already morphologically diverse even prior to the formation of the lake. What this means is that Lake Tanganyika acts both as a cradle for evolutionary novelty as well as an “evolutionary reservoir” (Wilson et al., 2004), a refuge.

Both of these properties are equally as important for the study of evolution, as they open up several research questions. Are some taxa more prone to speciation, and if so, why? Is the difference between the cichlids and the paludomids merely down to the cichlids’ potential diversity in morphology, colour, and behaviour enabling them to speciate more readily? Are these speciations necessarily adaptive? Are there any unique processes that lead to (adaptive) speciation in these lakes, or to the maintenance of disparity over million of years? Is evolution in ancient lakes merely an extrapolation of evolution in regular lakes, or do different processes come into play, and if so, at what point does the discrepancy arise? What is the role of historical contingency in all of this? For example, would the sexual selection in cichlids have occurred if Lakes Malawi and Victoria had less clear waters (Kocher, 2004)?

Ancient lakes worldwide. Source: Cristescu et al. (2010)
Ancient lakes worldwide. Source: Cristescu et al. (2010)

This creates an impetus to study as many taxa from as many ancient lakes as possible, to see what generalisations can be made. We’re lucky that many ancient lakes exist, in all sorts of climates. While Lakes Tanganyika and Baikal are exceptionally old, the rest range from 5-2 million years of age, allowing direct comparisons between them. For example, Lakes Titicaca (South America), Ohrid (Balkans), Biwa (Japan), and the Malili Lake System (Sulawesi), as well as the Caspian Sea (largest ancient lake by surface area) are all known to be of the same age.

Additionally, a range of fossil ones are known. The afore-mentioned Steinheim Basin is one, and other well-studied examples include the Jurassic Newark Basin (North America), the Miocene Shanwang Lake (China), and the Mio-Pliocene Lake Pannon (Central Europe).

How to study speciation. Source:
How to study speciation. Source: Cristescu et al. (2010)

Every ancient lake allows us to study every aspect of evolution, especially speciation, in an integrative, holistic manner, without the niggly constraints and limitations of pure theoretical modelling or pure lab-based experimental evolution, because they are natural laboratories, much like islands (it’s similar to why I am concentrating on doing research in Cyprus and Japan). Such studies have led not only to very interesting case studies, but to the establishment of new concepts. For example, Gorthner & Meier-Brooke (1985) coined the Ancient Lake Concept based on palaeobiological works on ancient lakes. It states that environmental stability allows directional evolutionary rates to increase linearly. Disturbing the environment leads to a resetting of the previous directionality of evolution, as seen in the evolution of the Steinheim snails. They did not evolve to just become larger over time, but experienced a many changes in direction due to the instability of the Steinheim Basin’s geochemistry and temperature.

References:

Cohen AS, Soreghan MJ & Scholz CA. 1993. Estimating the age of formation of lakes: An example from Lake Tanganyika, East African Rift system. Geology 21, 511-514.

Cristescu ME, Adamowicz SJ, Vaillant JJ & Haffner DG. 2010. Ancient lakes revisited: from the ecology to the genetics of speciation. Molecular Ecology 19, 4837-4851.

Danley PD & Kochler TD. 2001. Speciation in rapidly diverging systems: lessons from Lake Malawi. Molecular Ecology 10, 1075-1086.

Gorthner A & Meier-Brook C. 1985. The Steinheim Basin as a paleo-ancient lake. In: Bayer U & Seilacher A (eds.). Sedimentary and Evolutionary Cycles.

Kamaltynov RM. 1999. On the Higher Classification of Lake Baikal Amphipods. Crustaceana 72, 933-944.

Kocher TD. 2004. Adaptive evolution and explosive speciation: the cichlid fish model. Nature Reviews Genetics 5, 288-298.

Sherbakov DY. 1999. Molecular phylogenetic studies on the origin of biodiversity in Lake Baikal. TrEE 14, 92-95.

Snoeks J. 2000. How well known is the ichthyodiversity of the large East African lakes? Advances in Ecological Research 31, 17-38.

Vainola R & Kamaltynov RM. 1999. Species diversity and speciation in the endemic amphipods of lake baikal: molecular evidence. Crustaceana 72, 945-965.

Wilson AB, Glaubrecht M & Meyer A. 2004. Ancient lakes as evolutionary reservoirs: evidence from the thalassoid gastropods of Lake Tanganyika. Proc. R. Soc. B 271, 529-536.

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