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9/2/2015
Unhelpful adaptations can speed up evolution | Science News
News: Evolution
Unhelpful adaptations can speed up evolution
Guppies moved to predator­free zones show changes in gene activity
By Tina Hesman Saey 1:13pm, September 2, 2015
GUPPY GAINS Trinidadian guppies from the Guanapo River basin (shown) make unhelpful changes in gene activity when
they first enter a new environment. Those missteps speed up evolution, a new study suggests.
When organisms enter a new environment, they’re bound to make some missteps. A new study
suggests those initial flubs may speed up evolution.
Trinidadian guppies transplanted from predator­infested waters to streams devoid of predators
responded by changing activity of some genes in the brain. Although some changes were
helpful, most were disadvantageous. But genes that got off on the wrong foot by changing
activity in unhelpful ways evolved the fastest, researchers report September 2 in Nature.
“When the environment has a nurturing effect that’s helpful to you, you don’t have to evolve,”
explains lead author Cameron Ghalambor, an evolutionary ecologist at Colorado State University
in Fort Collins. But when the environment prompts unhelpful gene activity responses, organisms
are forced to change their DNA to compensate, he says.
Scientists have debated how important plasticity — the ability to change traits in response to
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Unhelpful adaptations can speed up evolution | Science News
environmental influences — is for driving evolution, says evolutionary biologist Carl Schlichting of
the University of Connecticut in Storrs. Researchers have previously found that helpful, or
adaptive, trait changes can be made permanent with DNA mutations. But the new study
demonstrates that nonadaptive alterations are the ones that really drive evolution, at least when
organisms first move into a new environment, says Schlichting, who was not involved in the
study.
Ghalambor and colleagues took guppies (Poecilia reticulata) from a part of the Guanapo River in
Trinidad where the little fish are regularly eaten by pike cichlids. The researchers moved the
guppies to two streams without cichlids.
After a year, the guppies had produced three or four generations. The researchers then captured
some of the fish from both safe streams as well as guppies from the original location rife with
predators, plus guppies that lived in a stream historically free of predators. In the lab,
researchers bred each guppy group for two more generations. Second­generation fish from each
location were split into two groups of siblings. One sibling group’s water supply first passed
through a tank containing cichlids that were fed two nonexperimental guppies a day. That water
contained chemical signatures of the cichlid predators and distress pheromones from the dinner
guppies.
Researchers then analyzed the activity
of more than 37,000 genes in the
guppies’ brains. Gene activity patterns
distinguished fish from the high­predator
river from those that came from low­
predator streams. Guppies transplanted
to the two predator­free streams had
similar gene activity patterns. But those
patterns differed from those in fish from
the high­predator and low­predator
streams.
The researchers found 135 genes that
changed activity in the same way in
both groups of transplanted guppies.
Those changes made the transplanted
fish more similar to low­predator than
high­predator guppies, indicating that
those genes were influenced by natural
selection. If random chance rather than
natural selection was at work, the
genes’ activity changes would not have
matched up so neatly, says Schlichting.
And if the fish hadn’t evolved, their gene
PATTERNS OF CHANGE Guppies transplanted from a part of the
activity patterns would still resemble
Guanapo River where cichlids prey on the small fish to streams free of
those of the high­predator guppies.
predators evolved different gene activity patterns (blue). Those patterns
are more similar to guppies from a low­predator stream (green) than to
fish that stayed in the high­predator region (red). Dots connected by solid
lines indicate individual families from each population.
C.K. Ghalambor et al/Nature 2015
To determine if the gene activity
changes were adaptive, researchers
compared siblings raised in tanks that
got the cichlid­tainted water to guppies that didn’t get the predator cues.
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Of the 135 evolved genes, 120 (89 percent) changed gene activity in response to predator cues
in unhelpful ways, the study found. Only 15 genes altered their activity in helpful ways, and those
changes were usually small. The results indicate that plasticity can be important for evolution,
but not because it helps organisms adjust to the environment right away. Instead, nonadaptive
plasticity may force natural selection to work harder on misbehaving genes and to select DNA
mutations that help organisms survive. Those DNA changes should increase fitness over the
long haul, the researchers theorize.
The study’s strength stems from the large number of genes the researchers analyzed, says
Gregory Grether, an evolutionary ecologist at UCLA. Many studies focus on one particular trait,
but analyzing many genes allows the researchers to pick out global patterns of evolution, he
says. It’s clear that nonadaptive changes fueling natural selection isn’t a fluke, Grether says.
“This is the predominant pattern.”
PREDATOR PERIL Cichlids (one shown) prey on guppies. Guppies transplanted to
streams free of these predators evolve different gene activity patterns than do fish
constantly in danger of being eaten.
Paul Bentzen
How the gene activity changes influence behavior isn’t known, and the researchers haven’t
tracked down the DNA changes that natural selection presumably favors in the guppies,
Ghalambor says.
It will be important to understand how the environment causes these changes in gene activity,
and which physiological systems are affected, says geneticist Gene Robinson of the University
of Illinois Urbana­Champaign.
Nonadaptive plasticity may spur evolution in the short term, but may not be a driving force in
later generations, Ghalambor says. The pattern may also hold only for organisms entering a
more benign environment. When the researchers tried the opposite experiment, transplanting
fish from low­predator streams to high­predator locations, the transplanted guppies were quickly
wiped out. Says Ghalambor: “Extinction is an option.”
Citations
Further Reading
C.K. Ghalambor et al. Nonadaptive plasticity
potentiates rapid adaptive evolution of gene
expression in nature. Nature. Published
online September 2, 2015. doi:
10.1038/nature15256.
T.H. Saey. Human evolution tied to a small
fraction of the genome. Science News
Online, January 19, 2015. https://www.sciencenews.org/article/unhelpful­adaptations­can­speed­evolution
S. Milius. Dangerous times: Guppies don't
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follow rules for old age. Science News. Vol.
166, October 30, 2004, p. 276.
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