Proc. Nati. Acad. Sci. USA Vol. 75, No. 1, pp. 385-389, January 1978 Evolution Altruism: Its characteristics and evolution (set and group selection/kin selection/human evolution) P. J. DARLINGTON, JR. Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 Contributed by P. J. Darlington, Jr., November 3, 1977 ABSTRACT Altruism is a group phenomenon in which some rower, everyday concept of altruism is related to it at the end genes or individuals, which must be presumed to be selfish, of this paper. benefit others at cost to themselves. The presumption of self- ishness and the fact of altruism are reconciled bykin-group se- Altruism is a group phenomenon. It requires at least a group lection and by reciprocal altruism. Kin-group selection is clearly of two, an altruist that pays a cost and a recipient that receives visible only in special cases; its role even among social insects a benefit. Such groups do form themselves, vary, sometimes may be overestimated; it is probably usually inhibited by com- compete, and undergo selection. Understanding their evolution petition. However, reciproca altruism is ubiquitous. All altruism requires understanding of selection-in-principle, Darwinian is: (i) potentially reciprocal; (ii) potentially profitable to altruists natural selection, group selection, kin-group selection, and al- as well as to recipients; (id) environmentally determined, usually truism itself. by position of individuals in group or environmental situations; and (iv) a net-gain lottery. These generalizations are illustrated Selection of sets and groups by four idealized cases; the difficulty of applying them to real cases is illustrated by alarm-calling in groups of birds. Although Selection-In-Principle is most simply defined as differential altruism is a group phenomenon, it evolves by individual se- elimination of preformed sets, or of sets that have formed lection, by processes equivalent to co-evolutions. Its evolution themselves by action of their members (1). This is a multilevel is: (i) opposed by competition; (ill) costly, complex, and slow, and definition, applicable to selection among molecules (sets of tending to produce an imprecise flexible altruism rather than atoms), genes (sets of molecules), individual organisms (sets of a precisely detailed one; and (iii) supplemented by group se- of lection (differential extinction of groups). That altruism in genes), and groups (sets) individuals. human beings conforms to these generalizations is a good Darwinian Natural Selection (as distinguished from se- working hypothesis. However, analysis does not "take the al- lection directed by humans) is defined in different ways by truism out of(human) altruism." Humans do not calculate it, but different biologists (1). I prefer to define it simply as differential behave altruistically because they have human altruistic elimination of individual organisms, or as set selection at the emotions. level of living individuals (sets of genes). Two misconceptions exist about natural selection. One is that This paper is one of a series of which the purpose is to look at it not only eliminates but also preserves or creates. This has-been important evolutionary problems in new ways: by treating implied or asserted by many evolutionists from Darwin (as evolution consistently as a multilevel process; by stressing both noted in ref. 1) to Gilpin (ref. 2, p. 9, with references to Wright the simple principles that hold at all levels and the literally and others). However, in the absence of a guiding intelligence, inconceivable complexity of their manifestations at living levels which evolutionists do not detect, natural selection cannot make in the real world; by refining definitions where necessary; and positive choices; it cannot directly favor, preserve, or create by using nonmathematical rather than mathematical models, anything. It can only determine what kinds of genes or indi- although the conclusions of mathematical evolutionists are viduals shall not survive. It must allow other kinds, including noted. novelties, to survive if evolution is to continue, but allowing The present paper is concerned with altruism, with its bio- survival of novelties is not the same as creating them. For fur- logical characteristics and evolution. This subject is indeed ther discussion of what natural selection can and cannot do and important both in evolutionary theory and in practical under- of the source of the energy of evolution, with a comparison of standing of human beings. Biologists now treat it confusingly, the force of selection to the force of an hydrolic ram, see again inconsistently, and sometimes unrealistically, so that a new look ref. 1. at it should be useful. The second misconception is that natural selection rapidly and precisely "maximizes" adaptations, including altruistic Biological altruism behaviors. However, evolution by selection is so costly and slow, Altruism is defined by biologists in several different ways. I details under selection are so numerous and diverse, and envi- prefer now to define it broadly, as occurring in all interactions ronments are so complex and change so rapidly, that most in which some individuals benefit others at cost to themselves. species most of the time are probably far from precisely adapted (It might be defined more precisely in terms of fitness transfers, to their environments but are just a little better than their but this would be too technical here.) It may be indirect or di- competitors, for the time being (3). rect, involuntary or voluntary, and one-way or reciprocal, and Group Selection too can be defined in several different ways. costs may be paid by acts or in other ways. This definition ex- I prefer to define it simply and literally as differential elimi- cludes concepts of volition or benevolence, and includes much nation of preformed groups of individual organisms, or of that is not altruism in the everyday sense of the word; the nar- groups that have formed themselves by action of the individuals concerned (4, 5). As thus defined, it is set selection at the sets- The costs of publication of this article were defrayed in part by the of-individuals level. It occurs among simple groups of two or payment of page charges. This article must therefore be hereby marked more individuals, special groups within populations, demes, "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate whole species, and (in the broadest sense) groups of individuals this fact. of different species. 385 Downloaded by guest on September 29, 2021 386 Evolution: Darlington Proc. Natl. Acad. Sci. USA 75 (1978) Some evolutionists have thought of group selection as a reconcile the presumption of selfishness with the fact of al- creative process in which "the individual is subordinated to the truism. In it, individuals make altruistic sacrifices to their kin, group as a whole." This concept was at least adumbrated by paying costs as individuals, but profiting genetically. Writers Darwin, Haldane, and Wright; has been emphasized and am- who have treated it recently, with historical resumes, include plified by Wynne-Edwards; and is still accepted, as a definition, Wilson (8), Eberhard (9), and Dawkins (6). by some evolutionists. However, the concept is unrealistic, or Two aspects of kin-group selection-its theoretical arithmetic at best a misleading oversimplification. Like selection at other and its actual occurrence-should be clearly separated. The levels, group selection cannot be creative; it can make nothing arithmetic is primarily a matter of simple fractions determined new, but can act only on groups that have made themselves (by by proportions of shared genes. In ordinary, sexually repro- action of their member individuals); and it can act only by ducing, Mendelian populations, two nonidentical sibs inherit elimination. It is not a process in which the individual is sacri- one-half of the same genes; an uncle or aunt and a nephew or ficed to the group as a whole, but one in which some individuals niece, one-fourth; two first cousins, one-eighth; etc. These genes are sacrificed to the benefit of other individuals, which then are identical, derived by replication of the genes of most-recent constitute the group. It can always be translated into terms of common ancestors. From these fractions it is commonly cal- selection of individuals in environments in which the fitnesses culated that an altruist will profit genetically-will "increase of some or all individuals in a group are increased by the pres- the representation of its genes in the next generation"-if it ence of other individuals. sacrifices itself for more than 2 sibs, more than 4 nephews or Genes and individuals, selfish and altruistic nieces, or more than 8 first cousins; or if it makes a partial sac- rifice of < one-half (any amount less than one-half) of its Genes and the individuals carrying them must be selfish. Any "personal fitness" for a sib, < one-fourth for a nephew or niece, kind of gene that consistently sacrificed itself to the benefit of etc. other kinds, with no chance of any return, would eliminate itself However, these calculations greatly exaggerate the probable from its gene pool. Unselfish, one-way "genes for altruism" actual effectiveness of kin altruism and kin-group selection, for cannot exist, although they have been invoked many times by several reasons. many biologists. Dawkins (6) makes this point at book-length First, all individuals in a Mendelian population share some in The Selfish Gene, and other persons have said the same thing genes, and the selective advantage of kin (as against non-kin) in fewer words. It follows that individuals that are unselfishly altruism is reduced in proportion, or part of the advantage is altruistic, that do not receive or "expect" any kind of return, spread through the whole population. cannot be genetically differentiated within populations of Then, kin altruism yields the calculated genetic profits only selfish nonaltruists.