IV.8.2 Group selection involves competing populations.

Group selection is encountered wherever the species forms a large number of more or less independent social groups, i.e. herds, flocks or bands, and where the survival or reproduction success of the individual is closely connected with the survival and success of its social group.If group selection exists in the given species, then its action can lead to preference for those properties of organisms that are advantageous for the group as a whole, but need not provide any advantage or can even be harmful for the bearer.The pattern of altruistic behaviour is a typical example of the second category of traits, i.e. behaviour that is useful for the group as a whole but is harmful for its bearer.

            For example, if a predator appears in the vicinity of a flock of jackdaws, the first jackdaw that notices its presence lets out a warning cry and the whole flock tries to escape or defend itself.From the standpoint of the individual, the issuing of the warning signal and participation in protection of the flock is highly irrational and disadvantageous behaviour.The individual would have a much better chance of survival if it were to selfishly use the information about the presence of the predator for itself alone and, according to the circumstances, either crouch down or inconspicuously move to the other side of the flock and leave some other individual, perhaps its potential competitor, to be eaten.But, instead of this, it warns the rest, gives up its advantage, as there is, at the very least, the same probability that the predator will attack it as against any other member of the flock.

            However, from the standpoint of the group, this altruistic behaviour is useful, because it reduces the probability that the predator will be successful in attacking the flock.Attacking the centre of a scattering flock, amongst a great many moving targets, is difficult and frequently unsuccessful.A flock that contains altruistic individuals thus has a better chance than a flock of the same size that does not contain altruists.At the end of a certain period of time, for example a season, it will thus be more numerous and it is thus more probable that it will split off a greater number of daughter flocks.

            However, in this case, individual andgroup selection act in the opposite direction and there is substantial selection within the flock against altruistic individuals.It is disadvantageous for the individual to be an altruist, but much more advantageous to utilize the advantages provided by altruistic individuals, to behave selfishly and not warn the others about the predator.Whether altruists or selfish individuals predominate in a particular species is determined primarily by the population structure of the particular species, the manner of forming and disbanding of social groups, the degree of advantageousness and disadvantageousness of altruistic behaviour for the individual and the group and other properties of the particular biological system (Fig. IV.8).


Fig. IV.8. Group and individual selection. If new groups are formed by division of the original groups into large parts (a), the effect of individual selection gradually predominates in all the groups, so that selfish behaviour becomes fixed (black circle). However, if the new groups are formed from a small number of individual migrants coming from different groups (b), group selection may predominate and altruists will predominate in the population (white circles).

            In most cases, individual selection is much stronger than group selection.Consequently, until the 1980’s, biologists were mostly of the opinion that group selection is almost never an important factor in nature.However, new results of analysis of theoretical models clearly indicate that, under conditions where individual subpopulations regularly emerge and disappear in the framework of the population as a whole, group selection can be an important factor and, within a certain range of population parameters, can even predominate over individual selection (Alexander & Borgia 1978; Shanahan 1998).

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.