Embargoed until 2 P.M EST
NSF PR 02-06 - February 6, 2002
Predators Drive Evolution of Virtual Prey Appearance
Two University of Nebraska-Lincoln biologists, in a
three-year experiment using live blue jays and evolving
virtual moths, have made the first direct observations
to support a longstanding idea: that predators' behavior
can promote variation in the appearance of insect
prey species, particularly if the predators use "searching
images" and restrict their search to the most abundant
prey types.
The research results of Alan Bond and Alan Kamil are
published in the February 7 issue of Nature.
Bond and Kamil, whose research is supported by the
National Science Foundation (NSF), describe using
four blue jays as predators in a virtual ecology --
a population of 200 virtual moths whose wings had
relatively similar cryptic markings. The moth images
were overlaid on a complex, granular background that
was projected on a computer display mimicking the
appearance of live moths on a tree trunk. The appearance
of the moths was developed from virtual genomes that
were based on the way wing patterns are coded for
in real moth genetics.
"The behavior of predators, in this case blue jays
preying on computer-generated images of moths, has
the potential to promote diversity among populations
of prey," says Steve Vessey, program director in NSF's
division of integrative biology, which funded the
research. "Such behavior can lead to the evolution
of diverse morphs of the prey. It remains to be seen,
however, if this diversity could lead to discrete
populations or even species of prey."
The jays started by "hunting" the parental population
one moth at a time. Each moth in the population was
presented to one of the jays once in the course of
a daily session. Half of the displays seen by each
bird had no embedded moths.
At the end of each day, the accuracy and speed of the
birds were scored and entered into a genetic algorithm,
which favored the moths that were the most difficult
to detect as parents for the next generation. The
moth population was allowed to "reproduce" and the
birds hunted the new generation on the succeeding
day. Bond and Kamil continued the process for 100
generations and repeated it two more times. For 30
days between runs, the jays were exposed only to the
original parental population to return them to a consistent
baseline.
In each run, Bond and Kamil found unequivocal evidence
that jay predation had resulted in selection for increasing
both the crypticity and the variability of wing markings.
Two sets of control lineages did not show similar
effects.
"What we found is that the moths not only get more
difficult to find when the birds are searching for
them, but they also get significantly more diverse
than you would expect on the basis of a similar predatory
process that didn't involve searching images," Bond
said. "The fact that the birds are looking for something
similar to what they've seen before actually encourages
the development of novel-appearing moths."
The experiment, Bond and Kamil said, illustrates what
British entomologist Edward Poulton had predicted
in 1890: Since it's harder for a predator to search
for two things at the same time than it is to search
for one, the source of polymorphism (variance in physical
appearance) of insect prey might be that it makes
it harder for the predator to find prey.
"What's amazing is that it has taken this long to
be able to put together a preparation that allows
us to demonstrate it clearly in circumstances in which
prey appearance is actually evolving," Bond said.
"This sort of question is very difficult to do with
real prey animals. In the field, there are so many
other things going on that you have no way of parsing
out the changes that are due to predation; and in
the laboratory, there's another set of complicated
problems just trying to keep both predator and prey
alive through multiple generations."
Bond said the experiment's design overcame those difficulties,
and was sufficiently complex and carried enough key
features of the real predator-prey system that he
and Kamil are confident their results show a meaningful
effect.
"Although the prey are not real animals, the predators
are," Kamil said. "So we're looking at what real predators
do to a very reasonable simulation of what this type
of prey is like."
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