A University of Leeds-led study, published in the journal Ecology Letters,
overturns the common assumption that evolution only occurs gradually over
hundreds or thousands of years.
Instead, researchers found significant genetically transmitted changes in
laboratory populations of soil mites in just 15 generations, leading to a
doubling of the age at which the mites reached adulthood and large changes in
population size. The results have important implications in areas such as
disease and pest control, conservation and fisheries management because they
demonstrate that evolution can be a game-changer even in the short-term.
Professor Tim Benton, of the University of Leeds' Faculty of Biological
Sciences, said: "This demonstrates that short-term ecological change and
evolution are completely intertwined and cannot reasonably be considered
separate. We found that populations evolve rapidly in response to environmental
change and population management. This can have major consequences such as
reducing harvesting yields or saving a population heading for extinction."
Although previous research has implied a link between short-term changes in
animal species' physical characteristics and evolution, the Leeds-led study is
the first to prove a causal relationship between rapid genetic evolution and
animal population dynamics in a controlled experimental setting.
The researchers worked with soil mites that were collected from the wild and
then raised in 18 glass tubes. 40% of adult mites were removed every
week from six of the glass tubes. A similar proportion of juveniles were removed
each week in a further six tubes, while no "harvesting" was conducted in the
remaining third of the tubes.
Lead author Dr Tom Cameron, a postdoctoral Fellow in the Faculty of
Biological Sciences at Leeds at the time of the research and now based in Umeå
University, Sweden, said: "We saw significant evolutionary changes relatively
quickly. The age of maturity of the mites in the tubes doubled over about 15
generations, because they were competing in a different way than they would in
the wild. 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. When they did eventually mature, they were so
enormous they could lay all of their eggs very quickly."
Disastrous effect on population
The initial change in the mites' environment - from the wild into the
laboratory - had a disastrous effect on the population, putting the mites on an
extinction trajectory. However, in every population, including those subjected
to the removal of adults or juveniles, the trajectory switched after only five
generations of evolution and the population sizes began to increase.
The researchers found that the laboratory environment was selecting for those
mites that grew more slowly. Under the competitive conditions in the tubes, the
slow growing mites were more fertile when they matured, meaning they could have
Dr Cameron said: "The genetic evolution that resulted in an investment in egg
production at the expense of individual growth rates led to population growth,
rescuing the populations from extinction. This is evolutionary rescue in action
and suggests that rapid evolution can help populations respond to rapid
Short-term ecological responses to the environment - for instance, a
reduction in the size of adults because of a lack of food - and hard-wired
evolutionary changes were separated by placing mites from different treatments
into a similar environment for several generations and seeing whether
Professor Benton said: "The traditional idea 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. However, our study proves that the
evolutionary effect - the change in the underlying biology in response to the
environment - can happen at the same time as the ecological response. Ecology
and evolution are intertwined," he said.
Unpicking evolutionary change from ecological responses is particularly
important in areas such as the management of fisheries, where human decisions
can result in major changes to an entire population's environment and life
histories. The size at which cod in the North Sea mature is about half that of
50 years ago and this change has been linked to a collapse in the cod population
because adult fish today are less fertile than their ancestors.
"The big debate has been over whether this is an evolutionary response to the
way they are fished or whether this is, for instance, just the amount of food in
the sea having a short-term ecological effect. Our study underlined that
evolution can happen on a short time scale and even small 1% to 2% evolutionary
changes in the underlying biology caused by your harvesting strategy can have
major consequences on population growth and yields. You can't just try to bring
the environment back to what it was before and expect everything to return to
normal," Professor Benton said.