Are cichlids evidence for evolution?

31 10 2009

Well of course they are! Every single living organism on earth is but it seems that the cichlids of Lake Malawi are interesting enough to get a mention in the latest book by evolutionary biologist Richard Dawkins. The book is titled The Greatest Show On Earth and is a summary of the evidence for evolution covering physics to geology to anatomy to genetics. There’s also an important semantic disambiguation between ‘theory’ and ‘hypothesis’ which is worth repeating to anyone daft enough to think that ‘yeah but evolutions only a theory’ is a knock out refutation in itself. That said, I still prefer the shorter if more disparaging retort ‘gravity is only a theory too, go jump off a cliff and test it out’.

Due to the massive success of  The God Delusion, Dawkins is most famous for being an ardent advocate of atheism. So much so that its often forgotten that he is also a very highly respected scientist. The Selfish Gene, although an aggregation of other peoples research, turned popular perception of evolution on its head, despatching group selection and the uniqueness of human altruism along the way. The Extended Phenotype is a significant contribution to evolutionary science exploring the external effects of genes on landscapes (termite mounds, chalk etc) and on the genes of other organisms. While his latest book represents a return to science, its not entirely unconnected to his god slaying hobby. Its been written in response to what he perceives as a growing movement for creationism but also increased scientific illiteracy and general anti-scientific sentiment. I suppose this is a little poignant at this time with the UK governments top drug scientist getting sacked yesterday for having the audacity to publish (in a scientific journal) evidence that contradicts government policy.

Its a very entertaining read but I do feel its been written for a very much wider audience than previous books. This isn’t necessarily a bad thing, but if you are already well informed about evolution there’s nothing conceptually new here. I didn’t mind this, the book is full of interesting examples and oddities all delivered with the succinct brutality of the logic hammer.

Anyway, back to the fish – here is the bit from the book…

Each of the great African lakes has its own fish fauna, dominated by the group called cichlids. The cichlid faunas of Lake Victoria, Lake Tanganyika and Lake Malawi, each several species strong are completely distinct from each other. They have evidently evolved separately in the three lakes which makes it all the more fascinating that they have converged on the same range of ‘trades’ in all three. In each lake, it looks as though one or two founder species somehow made their way in, perhaps from rivers, in the first place. And in each lake these founders then speciated and speciated again, to populate the lake with the hundreds of species that we see today. How, within the confines of a lake, did the budding species achieve the initial geographical isolation that enabled them to split apart?

When introducing islands, I explained that, from a fish’s point of view, a lake surrounded by land is an island. Slightly less obviously, even an island in the conventional sense of land surrounded by water can be an ‘island’ for a fish, especially for a fish that only lives in shallow water. In the sea, think of a coral reef fish, which never ventures into deep water. From its point of view, the shallow fringe of a coral island is an ‘island’, and the great barrier reef is an archipelago. Something similar can happen even in a lake. Within a lake, especially a large one, a rocky outcrop can be an ‘island’ for a fish whose habits confine it to shallow water. This is almost certainly how at least some of the cichlids in the African great lakes achieved their initial isolation. Most individuals were confined to shallow water around islands, or in bays and inlets. This achieved partial isolation from other such pockets of shallow water linked by occasional traversings of the deeper water between them to form the watery equivalent of a Galapogos-like ‘archipelago’.

There’s good evidence (for example from sediment core samples) that the level of Lake Malawi (it was called Lake Nyasa when i spent my first bucket and spade holidays on its sandy beaches) rises and falls dramatically over the centuries, and reached a low point in the 18th century, more than 100m lower than the present level. Many of its islands were not islands at all in that time, but hills on the land around the then smaller lake. When the lake level rose, in the 19th and 20th centuries, the hills became islands, ranges of hills became archipelagoes, and the process of speciation took off among the cichlids that live in shallow water, known locally as Mbuna. ‘Almost every rocky outcrop and islands has a unique Mbuna fauna, with endless colour form and species. As many of these islands and outcrops were dry land within the last 200-300 years, the establishment of the faunas must have taken place in that time.’

Such rapid speciation is something the cichlid fishes are extremely good at. Lake Malawi and Lake Tanganyika are old, but Lake Victoria is extremely young. The lake basin was formed only 400,000 years ago, and it has dried up several times since then, most recently about 17,000 years ago. This seems to mean that its endemic fauna of 450 species of cichlid fishes have evolved over a time-scale of centuries, not the millions of years that we usually associate with evolutionary divergence on this scale. The cichlids of Africas’ lakes impress us mightily with what evolution can do in a short space of time. They almost qualified for inclusion in the ‘before our very eyes’ chapter.

I never really thought about the significance of the rift lakes’s geography. It makes so much sense considering the huge number of variants within species. Labidochromis caeruleus, the humble ‘yellow’ lab, can be yellow, white or even blue depending on where you look. Its not something that would happen if the populations were to mix readily. Cichlids are renowned for their ability to evolve rapidly but its really the geological processes that created the lake in the first place, combined with the complex causes of climate change that have given us the multitude of shapes, sizes and colours of the fish that we keep in our tanks.

The yo-yoing of the water level which results in destruction and creation of habitats home to a plethora of endemic species / variants also makes me wonder what colourful fish have been lost and and what endless forms are yet to develop.




4 responses

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21 12 2009

Nice review. I enjoyed this book even although, as you point out, there is nothing much new in it. Jerry Coyne’s “Why Evolution Is True” is possibly an even better rebuttal to creationists.

28 12 2009

Thanks for reading Graham, Why Evolution is True is a great book too!

7 12 2009

Dawkins is an bigot and anyone who seriously thinks that something can come from nothing is a fool

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