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Size matters as fish species splits in two

Scientists report evolution in action as small fish in a big pond lose out.

Competition between big and small versions of one fish species in Lake Tanganyika in East Africa has caused them to diverge, launching the process of forming a new species.

A team that includes researchers at the Museum announced the findings in a paper published last week in the journal Nature Communications.

Big bullies

The fish, Telmatochromis temporalis, consists of two ecomorphs, or different varieties. There is a big ecomorph that lives on the rocky shoreline, and a small ecomorph, about half the size of the other, that lives among shells on nearby sand.

The bigger fish out-compete the smaller ones, driving them away from the preferred rocky habitats and into the neighbouring sand, where the smaller fish find shelter for themselves and their eggs in empty snail shells.

The big and small fishes live in different habitats and, because of this segregation, usually mate with other fish of similar size.

Breeding between the two ecomorphs did not produce normal offspring, suggesting the fish are on their way to becoming separate species.

‘There is virtually no genetic exchange between the large- and small-bodied ecomorphs,’ said Dr Kai Winkelmann, who conducted the research as part of his PhD studies at the Museum.

Best evidence

The team drew their conclusions by observing the fish in the wild, looking at their behaviour in aquaria at the Museum, and by testing how far they had separated genetically.

‘Our study is really one of the first to clearly show the role competition plays in the origin of new species,’ said researcher Dr Lukas Rüber, also from the Museum.

Evolution in action

Competition for a limited resource, such as food, space or shelter, was one of Darwin’s mechanisms for the formation of new species.

Although there are many theoretical studies of this process and there is plenty of species having separated in this way, there has been little experimental evidence of the process in action.

The researchers can’t say yet whether the fish will definitely become two separate species, but the evidence points towards it happening in the future.