Aisling Irwing reports from the British Association for the Advancement of Science festival at Newcastle University.

Nature may operate more like a hippie commune than a Darwinian free market in some places, according to evidence from the plankton societies of the sea. Plankton, which forms the largest ecosystem in the world, has flouted the Darwinian law of survival of the fittest throughout history, a scientist told the British Association for the Advancement of Science annual festival at Newcastle University.

Susan Rigby, lecturer in the department of geology and geophysics at the University of Edinburgh, whose research is to be published in the journal Geology in November, said that plankton, which ranges from jellyfish to tiny microplankton species, is the least-studied ecosystem in the world.

She studied plankton fossils preserved in rocks in Scotland and Newfoundland, Canada, which lived between 450 and 370 million years ago. Her discoveries relate to large-scale changes in groups of organisms over millions of years. When she mapped the appearance and disappearance of these groups, or phyla, she found that the patterns were at odds with typical Darwinian mappings.

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In Darwinian ecosystems the number of different organisms rises through history until all the ecological niches are full. At this point a newcomer can only settle in by fighting another species for its ecological niche - and winning if it is more adapted for survival. In addition, the number of species sometimes plunges, because of a mass extinction.

But Dr Rigby found that the plankton ecosystem is not full and there appears to be no competition between species for survival. New species enter the system with ease and groups of species do not become extinct.

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Instead of a few dominant species, there is generally a huge variety of species, each present in small numbers. Organisms that are identical in all relevant ways co-exist happily - rather than one of them making some crucial adaptation and ousting the others.

"If an ecosystem doesn't fill up then that destroys some of our ideas of evolution," she said.

"It's a world that appears to be without competition. Any individual might be outcompeted by another but no species appears to be outcompeted by another. It's not a Thatcherite environment. It's a kind of communist ideal.

"I'm not saying that Darwinian evolution doesn't happen in plankton but the larger scale patterns are conditioned by the environment," she said.

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"The question is: is Darwinian evolution the motor that drives selection and controls its results or is the environment so important that it controls the patterns that evolve within it?"