Darwin’s puddle

Cichlid fish, despite sharing a habitat, are evolving separately Cichlid fish, despite sharing a habitat, are evolving separately Shutterstock
27 Jul
Blue and yellow fish living in a small lake in Tanzania are helping scientists understand basic mechanics of evolution

Speciation is the slow break of a single species into two. Geographical barriers are thought to be the main drive of these splits, when rivers, seas and mountain ranges prevent the genes from intermixing any further than they already have. Take the Asiatic and American black bear for instance – though they share a common ancestor, the two species have become genetically distinct on their respective continents.

But what happens when the geographical location is entirely contained? In a small volcanic lake in Tanzania, what was once a single species of cichlid fish is slowly becoming two, a process known as sympatric speciation. While similar phenomena have been noticed in Lake Malawi – dubbed ‘Darwin’s Pond’ – where more than 875 cichlid species have evolved from just a handful of ancestors, it is the far smaller Lake Masoko – nicknamed ‘Darwin’s Puddle’ – that the secrets of sympatric speciation are beginning to be realised. At just 700 metres wide, it is a mystery how the pond’s fish can possibly separate their gene pools. Yet that is precisely what is happening.

We may begin to see continents and oceans as active places where evolution of diversity is continually in progress

‘The two cichlids now have different features,’ says George Turner, a biologist at the University of Bangor who has been studying their genetics. ‘They have different colours, habitat preferences, behaviour, diet and morphology.’ The ‘littoral’ cichlid is yellow and prefers shallower water, while the ‘benthic’ cichlid is blue and prefers a deeper habitat. However, the difference in depths doesn’t count as geographic isolation, as both species frequently move between the two and are often found alongside each other. So why did they stop mating? ‘It must have something to do with sexual selection and mate choice by females,’ theorises Turner. ‘What we want to know is whether sexual selection initiates speciation, or if it comes into play later after some other process has started it off.’

Turner believes that the Darwin’s Puddle cichlids could shake-up our understanding of evolution. ‘If we can figure out how sympatric speciation can happen in a tiny crater lake, then we have to ask ourselves whether we think the same thing can happen on continents and in oceans, where most species live,’ he says. ‘Without proposing scenarios of geographic barriers every time a new species evolves, we may begin to see continents and oceans as active places where evolution of diversity is continually in progress.’

This was published in the August 2016 edition of Geographical magazine.

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