Soil mites evolve in a few generations
9 April 2013, by Tom Marshall
A changing environment can make animals evolve much faster than we'd ever thought possible, scientists have found.
When researchers put soil mites into new conditions in the lab, they underwent significant genetic shifts within just 15 generations. This doubled the age at which the bugs reached adulthood and had a major effect on population size.
The findings challenge the common assumption that evolution happens only over centuries or millennia. They could have implications in areas from pest control to fisheries management.
'We found that populations evolve rapidly in response to environmental change and population management,' says Professor Tim Benton of the University of Leeds, one of the paper's authors. 'This can have major consequences such as reducing harvesting yields or saving a population heading for extinction.'
'We saw significant evolutionary changes relatively quickly,' adds lead author Dr Tom Cameron of Umeå University in Sweden. 'The age of maturity of the mites in the tubes doubled over about 15 generations, because the competition for resources was much more intense than in the wild - they had to adapt or die.'
Sancassania berlesei mites
Previous research suggested that animals could evolve new characteristics surprisingly quickly, but this is the first lab study that's proved a causal relationship between rapid genetic evolution and an animal's population dynamics.
The group collected mites from the wild and put them in 18 glass tubes, dividing them into three groups of six. From the first group they removed forty per cent of adults each week; from the second, they harvested the same proportion of juvenile mites. The third group went unmodified.
Initially it looked like the new environment would lead to extinction; all groups quickly declined by around half. But after about 30 weeks – about six generations – all adapted and started growing again. It also turned out that the tube environment was selecting for late-maturing mites that produced more eggs. The average age of mites in the unharvested group on reaching sexual maturity rose from 12.5 to 22 days.
'Removing the adults caused them to remain as juveniles even longer because the genetics were responding to the high chance that they were going to die as soon as they matured,' Cameron explains. 'When they did eventually mature, they were so enormous that they could lay all of their eggs very quickly.' Maturing late would normally be a disadvantage, but in the hyper-competitive tube environment it became helpful.
The mites' response to harvesting was weaker than the simple effect of the changed environment, but there was an impact. The adult-harvested populations ended up with adult population sizes of around 86 per cent of the unharvested ones, while for the juvenile-harvested population that figure was about 70 per cent.
Female mite in the lab
The study sheds light on the relationship between two kinds of change that organisms can undergo in response to their environment. On the one hand there's ecological change without any real shift in its inhabitants' genes, on the other there are true evolutionary changes in a population's genetic makeup.
'The traditional view would be that if you put animals in a new environment they stay basically the same but the way they grow changes because of variables like the amount of food,' says Benton. 'However, our study proves that the evolutionary effect – the change in underlying biology in response to the environment – can happen at the same time as the ecological response. Ecology and evolution are intertwined.'
The implications of the research in unpicking evolutionary from ecological change could be particularly important in managing fisheries, where our actions have major effects on whole populations. For example, North Sea cod are now about half the size on reaching maturity as they were 50 years ago. This has been linked to the collapse in their population as these smaller adults are also less fertile.
There is a major debate over whether such changes are purely ecological or involve evolution. Are we just making the average cod smaller because we're harvesting the big ones, for example - an ecological change that would fade if we stopped for a while? Or is fishing causing them to evolve, changing their biology in a way that will take much longer to reverse, if it is even reversible at all?
Fisheries scientists have tended towards the former view, but Cameron thinks the possibility that evolution is taking place must be taken seriously. 'The harvesting we did to these mites isn't exactly like fishing for cod, but we have shown that harvesting can cause evolution over short periods,' he says. 'This needs more attention, as we could be basing fishing policies on false expectations.' For example, policy-makers might argue for a moratorium on fishing endangered species to let stocks recover. But if fishing has caused their basic biology to evolve, it could be that recovery will be much longer and more difficult.
The paper appears in Ecology Letters.
Cameron, T. C., O'Sullivan, D., Reynolds, A., Piertney, S. B., Benton, T. G. (2013), Eco-evolutionary dynamics in response to selection on life-history. Ecology Letters. doi: 10.1111/ele.12107
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