The evolution of altruistic behavior through group selection is generally viewed as possible in theory but unlikely in reality, because individual selection favoring selfish strategies should act more rapidly than group selection favoring cooperation. Here we demonstrate the evolution of altruism, in the form of conditional reproductive restraint based on an explicitly social mechanism, modulated by intrapopulation communication comprising signal and evolved response, in a spatially distributed predatory͞para-sitic͞pathogenic model system. The predatory species consistently comes to exploit a signal implying overcrowding, individuals constraining their reproduction in response, with a corresponding increase in equilibrium reproduction rate in the absence of signal. This signaled restraint arises in a robust way for a range of model spatial systems; it outcompetes non-signal-based restraint and is not vulnerable to subversion by noncooperating variants. In these systems, communication is used to evaluate population density and regulate reproduction accordingly R esearchers from Darwin on have speculated about the evolutionary origins of cooperative behavior. In recent decades, evolutionary explanations have been rooted in individual-and gene-level selection, with selection above these levels considered too weak to play any significant role. Altruistic behavior is explained using inclusive fitness theory, through fundamentally selfish mechanisms such as kin selection, in which individual reproductive success is augmented by success of relatives with shared genes (1-4). However, experimental and theoretical metapopulation studies, with explicit partitioning into subpopulations, have shown that a lower reproduction rate can confer a long-term selective advantage with regard to population persistence (4-7). More recent studies of the evolution of reproductive restraint in spatially extended models (8-13) have demonstrated populations evolving such that individuals have lower reproduction ratios than they might. This restraint results in lower reproductive success for the individual, but over many generations, spatially and genetically correlated lineages avoid extinction that would otherwise result from exhaustion of all available resources. This strain extinction mechanism thus operates as a form of selection above the level of individuals. Whereas metapopulation studies display limited reproduction under restricted conditions, the spatially extended models demonstrate such behavior over a very wide range of model parameters (see figure 2 of ref. 13).Although these studies indicate a kind of altruism, they do not reveal much about interactive social behavior and its effects on the outcomes of selection. Communication is ubiquitous in nature, with organisms using every available medium (auditory͞ vibrational, visual, olfactory͞chemical, electrical, and even tactile signals) for social integration (14, 15), and it is a necessary aspect of true social community. Moreover, social interactions very commonly affect reprod...