Scottish fish may hold key to how species evolve

Arctic charr took up residence in Scotland's lochs when the last ice age thawed out around 13,000 years ago. Picture: Getty

Arctic charr took up residence in Scotland's lochs when the last ice age thawed out around 13,000 years ago. Picture: Getty

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Scotland’s oldest freshwater fish could help answer one of the most fundamental questions about life on earth, according to leading scientists.

Ecologists at Glasgow University say studies of the rapidly changing and highly diverse Arctic charr could reveal what drives new species to evolve.

Arctic charr, a relative of Atlantic salmon and brown trout found only in northerly latitudes, invaded Scotland as the country thawed at the end of the last ice age – between 12,000 and 14,000 years ago.

Surveys have found remarkable genetic, physical and behavioural differences between populations, which inhabit more than 250 Scottish lochs.

Professor Colin Adams, director of the university’s Scottish Centre for Ecology and the Natural Environment, and his team are trying to discover why.

“Evolution is the most fundamental driving force in the natural world,” he said. “It is the process that gives rise to new species and gives us the wonderful biodiversity in the natural world that we cherish.

“Despite that we have known about the basic process for over 150 years we still do not fully understand how evolution works in some circumstances. One of the things is how it can, at times, operate very quickly.”

Among Arctic charr, different populations vary in a range of characteristics – so that some experts believe the fish should be reclassified into distinct species.

Some of the obvious differences include colouring, which varies from subtle mottling to vivid red, and head shapes that can be slim or more robust. These characteristics have developed in adaptation to conditions in particular lochs.

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Prof Adams said: “In these places we have two charr populations which differ in a wide range of characteristics, but particularly in feeding ecology and habitat use. This is usually correlated with differences in head and mouth shape – the apparatus a fish uses for feeding.”

This process is known to have occurred Loch Dughaill and Loch Maree in Wester Ross, Loch Awe in Argyll and Loch Stack in Sutherland. “Each of these have two populations of charr that are independent of each other,” said Adams. “For some, such as the Stack, Awe and Maree populations, these two groups comprise separate gene pools. They are, to all intents and purposes, operating like separate species in all but name.”

The researchers found the most striking populations that have fragmented into closely related but separate genetic groups in Loch Rannoch, where there are three forms that differ in colour, shape, feeding ecology and habitat use.

“One element of why this species makes such an important study species for [evolutionary] scientists is the speed at which these populations have diverged – most within the last 12,000 years, which is the blink of an eye in evolutionary time,” Prof Adams said. “What we and colleagues have discovered is that Arctic charr are able to evolve quickly because they are able to respond to rapidly changing environments, as they would have experienced immediately after the last glaciation. They do this by modifying the normal processes of development from egg to adult to express new characteristics, which natural selection can then act upon.

“In practice, charr do this by switching genes off or on in different sequences during development in response to the environment they find themselves in. This allows them to shortcut the earliest stages of the normal evolutionary processes. It is impossible to predict with certainty but in time it would be reasonable to expect that what we currently know as Arctic charr will become a broad diverse group of different fish species.”

He added: “Had Darwin spent more time in Scotland, perhaps we would now be calling Arctic charr Darwin’s fishes.”

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