The latest copy of FBA News includes an article I wrote, along with Catherine Duigan, on the scientist Geoffrey Fryer, who celebrates 50 years as a Fellow of the Royal Society this year. Geoffrey Fryer spent his early career working on the cichlid fish of the Great Lakes of East Africa’s rift valley. He moved back to the UK in 1960 and spent the rest of his career working mostly on planktonic crustaceans at the Freshwater Biological Association’s laboratory at Windermere, where I first met him.
When he arrived at Lake Malawi in 1953, the existence of “species flocks” – relatively large numbers of closely related species confined to a narrowly circumscribed region – of cichlid fish had already been recognised but the reasons that led to their evolution was barely understood. These were piscine analogues – albeit significantly more diverse – of the finches that Darwin observed on the Galapagos Islands, and which provided crucial evidence for his theory of natural selection. Swimming in Lake Malawi and observing cichlid fish close quarters through a face mask, then dissecting fish caught by local boys enabled Geoffrey to show how morphology was linked to function (especially feeding behaviour) and explained preferences for sandy or rocky shores. It is one of the great examples of “adaptive radiation”.
Some of the many cichlid fish from Lake Malawi studied by Geoffrey Fryer. Top left: Pseudotropheus greshakei; top right: Ramphochroumus ferox; bottom left: Lethrinops sp.; bottom right: Copadichromus virginalis. Photos: Ken Irvine, Institute of Hydrology, Delft. The photographs at the top of the post show Geoffrey Fryer in the 1950s and earlier this year.
“Species flocks” have been found for other organisms including, some claim, diatoms. The evidence is not quite so clear-cut but most of the criteria (several closely-related species within a limited geographic area) are fulfilled, and there are reasonable grounds for assuming a common ancestor. Interestingly, too, several of the purported flocks come from areas with ancient lakes (the African Rift Valley, Lake Ohrid in North Macedonia and Lake Baikal in Siberia, for example). What is missing from the studies on species flocks in diatoms, however, is the link between form and function. We can infer radiation of several species from a common ancestor, but we cannot explain why it is that one lake is able to support so many close relatives. We are, once again, confronted by Hutchinson’s Paradox of the Plankton (rephrased for surface-dwelling organisms as “Baffled by the benthos”).
The plate below shows four species of the genus Epithemia from Lago di Trasimeno in Italy. These do not represent a “species flock” because they are not confined to a limited geographical area but they serve to make a point about our understanding of diatoms in general. Epithemia is not a particularly common genus but, when you find one species from this genus, there is a good chance that you will find another in the same sample. I’ve found E. sorex and E adnata, in particular, together in samples from the Shetland Islands to Italy and Greece in the Mediterranean Basin. In this sense, they are like Geoffrey Fryer’s cichlid fish: two closely related species living in close proximity.
Epithemia species from Lago di Trasimeno, Italy, September 2022. a. E. sorex; b., c.: E. adnata (two focal planes); d. E. turgida, e. E. frickeri. Scale bar: 10 micrometres (= 100th of a millimetre).
But in another sense, they are nothing like cichlid fish. Fryer, dabbling around in the shallows of Lake Malawi, was able to explain how form linked to function, enabling each species to exploit different resources. The literature on Epithemia mostly focuses on form, also addresses geographical differences but is almost silent on the subject of function. We learn that Epithemia, as a genus, is capable of nitrogen-fixation but not how individual species differ in their habitat preferences. Clearly, if two species occur side-by-side in a sample, then the chemical composition of the water (the go-to explanation for diatom ecologists) is not going to tell us the whole story. And it is not just Epithemia: similar patterns can be found in many other genera.
The study of diatoms is certainly not dormant. We have learned a huge amount about their diversity and evolution over the past few decades. We also know a lot about why apparently closely-related species live in different places (geological/chemical conditions and biogeography). What we are less sure about is why two or more closely-related species share a habitat. Diatom science seems to have become stuck in a cul-de-sac where diversity is celebrated but rarely explained, at least not in the functional ecology terms that motivated Geoffrey Fryer all those years ago in East Africa.
Two good general introductions to “species flocks” are:
Goldschmidt, T. (1996). Darwin’s Dreampond. Drama in Lake Victoria (translated by S. Marx-McDonald). MIT Press, Cambridge, MA.
Weiner, J. (1994). The Beak of the Finch: a Story of Evolution in Our Time. Knopf, New York.
DeYoe, H. R., Lowe, R. L., & Marks, J. C. (1992). Effects of nitrogen and phosphorus on the endosymbiont load of Rhopalodia gibba and Epithemia turgida (Bacillariophyceae) 1. Journal of Phycology, 28(6), 773-777.
Kociolek, J. P., Hamsher, S. E., Kulikovskiy, M., & Bramburger, A. J. (2017). Are there species flocks in freshwater diatoms? A review of past reports and a look to the future. Hydrobiologia, 792: 17-35.
Stelbrink, B., Jovanovska, E., Levkov, Z., Ognjanova‐Rumenova, N., Wilke, T., & Albrecht, C. (2018). Diatoms do radiate: evidence for a freshwater species flock. Journal of Evolutionary Biology 31:1969-1975.
Some other highlights from this week:
Wrote this whilst listening to: And in the Darkness, Hearts Aglow, new album by Weyes Blood
Currently reading: Nadime Gordimer’s A World of Strangers
Cultural highlight: The Wonder, adaptation of a 2016 novel which I have not read, featuring Florence Pugh. The lighting palette often recalls Vermeer’s interiors. –
Culinary highlight: roast broccoli and plum salad. Would not have thought of combining these two ingredients before trying it in the café at the Royal Botanic Gardens in Edinburgh. The secret lies in the dressing, which should evoke hoisin sauce.