Community level selection as an explanation of competitive restraint in RPS ecosystems One interesting aspect of RPS ecosystems is the evolution of competitive restraint discovered by Johnson and Seinen (2002). In an experiment which takes place on a SCA model of a RPS ecosystem, the invasion rate of one of the three species (rock) is allowed to evolve, and the invasion rates of other two species, scissors and paper, are fixed at their initial levels. As the grid is updated, the mean invasion rate of rocks increases up to a point but eventually stops rising. Although there is selection pressure for rocks to increase their speed of invasion in order to compete against other rocks, when they become too competitive, they deplete all the scissors individuals in their local area of the grid and are slowly overrun by paper. So the competitiveness of rocks eventually becomes constrained. The competitive restraint that evolves in this model is an example of individual altruism, at least in the short term. Rocks restrain their competitiveness, thereby producing fewer offspring than they otherwise would have, and the primary beneficiaries of the restraint are completely unrelated individuals of a different species (scissors). Johnson and Seinen claim that the evolution of restraint in these ecosystems is a case of community level selection. Natural selection between individuals towards greater invasiveness is competing with a second process of natural selection at the level of the ecological multispecies subcommunity, which selects among subcommunities for the ones that are the most stable
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