Group Selection.

Abstract: Variations in local population densities are common in many fish, amphibian, bird, and smaller mammal species. Aggregations attract predators, the main factor of nonselective elimination in schooling fish shoals, smaller bird flocks, and aggregations of disseminating underyearling anuran amphibians and smaller rodents. Very high degrees of similarity of the external phenotypic traits are characteristic of monospecific fish shoals and smaller bird flocks. Aggregations are adaptive, because they facilitate locomotion and the location of abundant food sources irregularly distributed in space. The "multiple eyes" effect allows a timely detection of danger. Phenotypic monomorphism, including behavioral, hampers the predator's choice of prey, often rendering an attack unsuccessful. A group attack of predators disorganizes the defensive maneuvering of the aggregation and ensures hunting success. The "dilution effect" plays crucial roles in these conditions. The more numerous the aggregation, the higher the probability of survival of any individual member. The dilution effect is inherent in any aggregation irrespective of the degree of monomorphism of its constituent organisms, on the assumption of their equal availability to predators. This peculiarity is characteristic of many bird colonies, dispersing young smaller rodents, underyearling anurans leaving their native pond, etc. Aggregations are adaptations to two very important factors: food supply and defense. The more numerous the aggregation, the higher its adaptive value. Inside an aggregation, the fitness of all of its members is the same, but the more numerous the aggregation and the greater the dilution, the higher the chances for the survival of any individual. The larger aggregations are more fit than the less numerous ones. The reproductive success and thus the contributions to abundance of the next generation are higher in larger aggregations. That is what group selection is, groups rather than the constituent organisms being selected for. [ABSTRACT FROM AUTHOR]