Survival of the weakest in twist to Darwins legacy

On the 200th anniversary of Darwin - few are in doubt what his observations and insight meant for the advancement of biology, and it can be summed up in one famous phrase - “survival of the fittest”. Now, however new work suggests that sometimes it is the weakest that triumph.

Could cyclic competition explain some cases of extinction? Credit: Ballista

German researchers have now simulated the progression of a cyclic competition of three species, where each participant is superior to one other species, but will be beaten by a third interaction partner.

“In this kind of cyclical concurrence, the weakest species proves the winner almost without exception,” reports Professor Erwin Frey of the Ludwig-Maximilians-Universitaet Muenchen, who headed the study. “The two stronger species, on the other hand, die out, as experiments with bacteria have already shown. Our results are not only a big surprise, they are important to our understanding of evolution of ecosystems and the development of new strategies for the protection of species.”

Such cyclical interaction is also familiarly termed “rock-paper-scissors” interaction. This is where the rock blunts the scissors, which cut the paper, which in turn wraps around the rock. Together, these non-hierarchical relationships form a cyclical motion. “The game can help describe the diversity of species,” explains Frey. The background is a branch of mathematics called game theory, and in this case evolutionary game theory. It helps analyse systems that involve multiple ‘players’ whose interactions are similar to those in parlor games.

In their study, the scientists working with Frey developed elaborate computer simulations in order to calculate the probabilities with which species in cyclical competition will survive. The games started off with three species coexisting in the systems, and ran until two species became extinct - with the third being the only remaining survivors. “What we saw was that in large populations, the weakest species would - with very high probability - come out as the victor,” says Frey.

“This result was just as unexpected for us,” reports Frey. “But it shows once more that chance plays a big part in the dynamics of an ecosystem. Incidentally, in experiments that were conducted a couple of years ago on bacterial colonies, in order to study cyclical competition, there was one clear result: The weakest of the three species emerged victorious from the competition.”