Thursday, 6 October 2011

Darwin on group/kin selection

Evolutionary biologists, who regard group selection as one big fallacy, tend to retrospectively (whiggish?) interpret Charles Darwin’s statements touching on the issue as groping towards kin selection and inclusive fitness theory. Other evolutionary biologists, however, think that group selection is multilevel selection applied to one particular tier of the natural hierarchy and therefore a correct proposal. These scholars tend to retrospectively interpret the same statements of Darwin as groping towards multilevel selection theory.

The following gives the major quotes I could find in my copies of Darwin and the contrary interpretations of them: Darwin a staunch individualist vs. Darwin the inventor of multilevel selection theory.
My own hunch is that a recent distinction between two different scenarios of multilevel selection (MLS1 and 2) could shed a light on these discrepant views. 
The Origin of Species
Defending natural selection against various competing causes of evolution in The Origin of Species, Darwin explained cases posing difficulties for his theory, like the barbed sting of bees or adaptations in sterile workers of social insects, with the good of the community or family.  

Darwin (1859/1964) on the barbed sting of bees: 
“we can perhaps understand how it is that the use of the sting should so often cause the insect’s own death: for if on the whole the power of stinging be useful to the community, it will fulfil all the requirements of natural selection, though it may cause the death of some few members.” (Darwin 1859/1964, p. 202)
 On workers and their being adapted despite being sterile:
“How the workers have been rendered sterile is a difficulty; but not much greater than that of any other striking modification of structure; for it can be shown that some insects and other articulate animals in a state of nature occasionally become sterile; and if such insects had been social, and it had been profitable to the community that a number should have been annually born capable of work, but incapable of procreation, I can see no very great difficulty in this being effected by natural selection. But I must pass over this preliminary difficulty. The great difficulty lies in the working ants differing widely from both the males and the fertile females in structure, as in the shape of the thorax and in being destitute of wings and sometimes of eyes, and in instinct.” (Darwin 1859/1964, p. 236)
“But with the working ant we have an insect differing greatly from its parents, yet absolutely sterile; so that it could never have transmitted successively acquired modifications of structure or instinct to its progeny. […] This difficulty, though appearing insuperable, is lessened, or, as I believe disappears, when it is remembered that selection may be applied to the family, as well as to the individual; and may thus gain the desired end.” (Darwin 1859/1964, p. 237)
But he also explained hybrid sterility as a side-effect of other differences not as a special endowment of species. The issue here is that, if hybrid sterility was an adaptation in the first place, it could neither benefit the parents nor the hybrids, but only the species:
“On the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences.” (Darwin 1859/1964, p. 245)
Michael Ruse's interpretation
Michael Ruse (1980) sees no group selection in Darwin's reasoning for the good of the community in social insects:
“There is no group selection here, where ‘group selection’ is understood in our above-defined sense as involving unreturned aid to non-relatives. The key to Darwin's argument is that the sterile altruists are closely related to the fertile members of the community.” (Ruse 1980, p. 619)
Furthermore, Ruse (1980) takes Darwin's conclusion that hybrid sterility is a side-effect rather than an adaptation to mean:
“But to Darwin, apart from the fact that he could not see why in nature one gets so many degrees and forms of sterility (assuming selection does cause sterility), the unambiguous group selection required to cause sterility was apparently just not a live possibility.” (Ruse 1980, p. 620)
My reading 
Reading the chapter on hybrid sterility myself, I rather got away with the impression that Darwin rejected the idea that hybrid sterility was an adaptation in the first place (for whichever level), because the evidence was too miscellaneous. As typical for Darwin, his collecting of evidence was comprehensive, but no clear patterns emerged. For example, not just hybrids between species but also between varieties or individuals of the same variety can be sterile, whereas individuals of distantly related plants can regularly be 'forced' to produce fertile hybrids. Moreover, he got a lot of evidence suggesting self-sterility (now known as inbreeding depression). In fact, Darwin draws an analogy between hybridisation and grafting. Here, the host plant also does not benefit from the grafting. Darwin argues that, here as there, the whole issue is one of something gone wrong in the natural order of things, either by accident or by human force.

The Descent of Man
While Darwin’s primary concern in The Descent of Man was to establish that natural selection applies to humans as well, he also suggests differential survival of tribes as the cause of moral faculties. Moreover, some statements show that he grasped the importance of within-group and between-groups selection as opposing forces.
“When two tribes of primeval man, living in the same country, came into competition, if (other circumstances being equal) the one tribe included a great number of courageous, sympathetic and faithful members, who were always ready to warn each other of danger, to aid and defend each other, this tribe would succeed better and conquer the other. […] A tribe rich in the above qualities would spread and be victorious over other tribes: but in turn overcome by some other tribe still more highly endowed. (Darwin 1879/2004, p. 155)
Darwin already saw the vulnerability of this between-tribal selection to subversion from within:
“But it may be asked, how within the limits of the same tribe did a large number of members first become endowed with these social and moral qualities, and how was the standard of excellence raised? It is extremely doubtful whether the offspring of the more sympathetic and benevolent parents, or of those who were the most faithful to their comrades, would be reared in greater numbers than the children of selfish and treacherous parents belonging to the same tribe. He who was ready to sacrifice his life , as many a savage has been, rather than betray his comrades, would often leave no offspring to inherit his noble nature. The bravest men, who were always willing to come to the front in war, and who freely risked their lives for others, would on an average perish in larger numbers than others men. Therefore it hardly seems probable, that the number of men gifted with such virtues, or that the standard of their excellence, could be increased through natural selection, that is, by the survival of the fittest; for we are not here speaking of one tribe being victorious over another.” (Darwin 1879/2004, p. 155f)
And he pointed out that reciprocity and social punishment or reward could counteract selfishness:
 “Although the circumstances, leading to an increase in the number of those thus endowed within the same tribe, are too complex to be clearly followed out, we can trace some of the probably steps. In the first place, as the reasoning powers and foresight of the members became improved, each man would soon learn that if he aided his fellow-men, he would commonly receive aid in return. […] But another and much more powerful stimulus to the development of the social virtues, is afforded by the praise and the blame of our fellow-men. To the instinct of sympathy, as we have already seen, it is primarily due, that we habitually bestow both praise and blame on others, whilst we love the former and dread the latter when applied to ourselves; and this instinct no doubt was originally acquired, like all the other social instincts, through natural selection.” (Darwin 1879/2004, p 156).
The tribal scenario of Darwin is one of internal repression of selfishness and one tribe with a higher standard of morality supplanting tribes with a lower standard. 
“A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to aid one another, and to sacrifice themselves for the common good, would be victorious over most other tribes; and this would be natural selection. At all times throughout the world tribes have supplanted other tribes; and as morality is one important element in their success, the standard of morality and the number of well-endowed men will thus everywhere tend to rise and increase.” (Darwin 1879/2004, p. 157f; see also p. 155 and Borrello 2010, p. 13)
Michael Ruse's interpretation

Ruse's interprets Darwin's statements on human morality as the one exception where Darwin's commitment to individual selection wavered.
“Nevertheless, in dealing with man's evolution there was one point (a point incidentally noted by Wallace as inexplicable through selection) where Darwin for once did quaver in his commitment to individual selection. This was over the evolution of the human moral sense. [...] fore once he did lose sight of the individual and allow that possibly the unit of selection may have been the group, specifically the tribe.” (Ruse 1980, p. 626)
“It would seem therefore that although Darwin resolutely opposed group selection in the non-human world, when it came to our own species, although again for almost everything he was an individual selectionist, in one crucial respect of our culture - our morality - he weakened and allowed that selection must have acted a the level of the population. (Ruse 1989, p. 627)
“It should also be noted that Darwin saw many of the tribe-members as being related [...] I am not suggesting that Darwin went so far as to think that the only kind of group selection involved in promoting morality collapsed into individualistic kin selection. [...] But it does seem fair to say that his group selection was of a rather mild variety." (Ruse 1980, p. 628)
David Sloan Wilson's interpretation
David Sloan Wilson interprets the same statements of Darwin in an opposite way (see Wilson and Wilson 2007 or comment no. 4 here)

My reading
Darwin's tribal scenario seems to agree with MLS1 insofar as the traits are individual traits, but with MLS2 insofar as the differential survival of tribes suggests a component of group fitness, whereas MLS1 only considers within- and between-group components of individual fitness. (See here for details concerning the distinction of MLS1 and 2.) I'd therefore tend to distinguish Darwin's tribe selection scenario from modern MLS1 scenarios. But this is just me groping around for an understanding.

  • Darwin C. (1859/1964) On the origin of species. Facsimile of the first edition edited by Ernst Mayr. Harvard University Press.
  • Darwin C (1879/2004) The descent of man. Penguin Classics with an introduction by James Moore and Desmond Morris.
  • Ruse M (1980) Charles Darwin and group selection. Annals of Science 37: 615-630. Reprinted in: Ruse M (1989) The Darwinian Paradigm. Routledge, London.  
  • Wilson DS and Wilson EO (2007) Rethinking the theoretical foundation of sociobiology. Quarterly Review of Biology 82: 327-348.