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 long-standing 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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