It has been a little over a hundred and fifty years since the publication of Charles Darwin’s revolutionary book “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life”  which changed the world forever, for believers and non-believers alike. His principal theory of Natural Selection favouring the Survival of the Fittest, albeit a topic of much dispute, both at the time of its inception and even today, continues to significantly contribute to our understanding of the mechanism of evolution. The race to perpetuate an individual’s own genes seems to have shaped the history of development of populations as small as microbes to that of giant mammals of today and yesteryears. Every species that inhabits or once inhabited this planet is involved in this unseen epic battle to survive and propagate. Darwin realized this a hundred and fifty years ago, and today, when we have come a long way in unravelling that genes are pitted against genes in this strife to survive, his theory does not seem to have lost significance at all.
There is, however, one befuddling phenomenon, widespread in the animal world that has always seemed to contradict Darwin’s theory. It is the phenomenon of Altruism or the behaviour by an individual that increases the fitness of another individual while decreasing the fitness of the actor, fitness here, referring to the success with which an individual propagates its genes. The primal motive of existence of all living creatures being survival and reproduction, it seems odd that natural selection should favour such a trait that is disadvantageous to its bearer. Yet, altruism is common in the animal kingdom instead of being a rare occurrence.
There are two theories generally accepted as logical explanations for this apparently insensible self-less behaviour. The first of the two is the Group Selection theory, first broached by Darwin (1871)  himself, which advocates that the trait for altruism, although disadvantageous to the individual bearer, may actually be advantageous at the group level, thereby getting favoured by natural selection to perpetuate. The other is the Kin Selection theory championed by Hamilton (1964)  which argues that altruism is restricted within closely related individuals so that the reduction in the individual fitness of the altruist is compensated by its enhanced inclusive fitness resulting from its helping behaviour. Of these two theories, Kin Selection has found favour with majority of the scientific community as a valid reason of prevalence of altruistic behaviour in varied animal communities.
Kin selection theory has been able to explain such perplexing animal behaviours as that seen in social insects like bees, ants, termites etc. where the altruist apparently seems to incur great losses for benefit of a related conspecific. However, there are several instances, more being discovered daily, where a self-less behaviour is directed towards an unrelated conspecific individual. Some such instances are cited below.
- Female Mule Deers (Odocoileus hemionus) have been shown to respond to recorded distress calls of fawns, both of its own species and a different species (viz. Whitetail deer – Odocoileus virginianus) even when their own fawn stood next to them. Even non-mother female deers responded in the same manner running in the direction of the source of the sound in an attempt apparently to rescue the distressed young deer. 
- A blue throated Side-blotched lizard (Uta sp.) will fight an orange-throated side-blotched lizard intruding into another blue-throated lizard’s territory. However, the two blue-throated lizards may not be even remotely related ruling out kin selection theory as an explanation of this altruistic behaviour 
- Chimpanzees (Pan troglodytes) have been shown to help a completely unfamiliar individual, either a genetically unrelated conspecific or a human being, without expecting rewards even when some effort was required .
Neither group selection nor kin selection hypothesis can explain the above observations since they involve completely unrelated, often unfamiliar individuals previously unseen. The alternative theory of reciprocal altruism (Trivers, 1971)  where a favour is done in expectation of reciprocity in future is not sufficient. Reciprocal altruism is true when the favour done does not jeopardise the chances of survival of the donor, otherwise there might not occur an opportunity for the favour to be returned. Clearly, the first and the second instances risk the personal safety of the altruist. In the third instance there is no chance of reciprocity in future since the recipient is an unfamiliar individual of the same or different species, and might not be encountered in future for the debt to be repaid.
So what can be the logic behind such behaviour which does not in any way seem to benefit the actor? Surely, natural selection will not favour any trait that does not attach with it an advantage, direct or indirect? Behavioural ecologists and social scientists have propounded two new theories as possible explanations for these perplexing phenomena.
One theory suggests that sexual selection may play a crucial role in favouring genes for altruism . With increased neurological development in higher animals, the cost of raising offspring would have increased manifold. This would have necessitated the selection of sexual mates with capacity and willingness to be good parents. According to the theory of sexual selection, displays of altruism could have provided cues to potentially good parenting skills and therefore, genes relating to altruism may have been selected in the process.
The second theory has been proposed following experiments with digital evolution techniques and is a modification of the Kin Selection theory. This theory suggests that individuals help other individuals who are physically similar to themselves, even though they may be completely unrelated . The theory, based on the assumption that relatives seldom share crucial genes while complete strangers do, also suggests that given the choice, organisms that are being altruistic toward kin, will evolve to stop doing so and instead help organisms genetically similar to them. The researchers stressed on overall genetic similarity rather than conspicuous markers as means of identification of genetic relatedness.
These new theories may help in explaining why animals extend selfless help towards unrelated, unfamiliar and infrequently encountered individuals of the same or different species. However, the applicability of these theories may vary depending on the species under study. For instance, the sexual selection theory holds ground in species with extensive parental care and complex social structures, viz. in primates. It cannot, for example, answer the riddle to the altruistic traits seen in the side-blotched lizards mentioned above. The second theory, although mathematically proved, needs to be tested in the natural world with real life organisms, to be accepted as the logical explanation of altruism. In addition, neither of these two theories can explain why natural selection favours the perpetuation of weak isolates of certain strains of bacteria that thrive on the cost of the stronger isolates capable of producing antibiotic resistant substances like indole . The weaker strains cannot produce the necessary resistant molecule i.e. indole, but survive and grow under antibiotic stress using the indole molecules produced by the stronger resistant isolates of the population, the later, however, dying from the toll of such production. It seems as if natural selection is allowing a flaw in the genetic design of these bacteria by favouring weak and vulnerable isolates to survive, perhaps to be selected against and eradicated in future.
The scientific community is yet to arrive at a common consensus regarding the acceptance of a single theory that can sufficiently explain altruism in different species. The theories mentioned above are the latest addition to the already existing list of scientific speculations proposed over the years to explain one of the most intriguing phenomena occurring in the animal world. Perhaps through modifications of these and future theories yet to be proposed, we might finally have an answer to evolution’s most confounding problem that is altruism.
- Darwin, C., 1859, On the Origin of Species by Means of Natural Selection, London: John Murray
- Darwin, C., 1871, The Descent of Man and Selection in Relation to Sex, New York: Appleton
- Hamilton, W. D., 1964, ‘The Genetical Evolution of Social Behaviour I and II’, Journal of Theoretical Biology, 7: 1-16, 17-32
- Source: University of Alberta. “Mule Deer Moms Rescue Other Fawns.” ScienceDaily 31 May 2007.)
- Source: Michigan State University. “Nice Guys Do Finish First In Lizards’ Evolutionary Race.” ScienceDaily 2 May 2006.
- Warneken, F., Hare, B., Melis, A.P., Hanus, D., Tomasello, M. (2007) Spontaneous altruism by chimpanzees and young children. PLoS Biol 5(7)
- Trivers, R.L., 1971. ‘The Evolution of Reciprocal Altruism’, Quarterly Review of Biology, 46: 35-57
- Phillips, T., Ferguson, E. and Rijsdijk, F. (2010), A link between altruism and sexual selection: Genetic influence on altruistic behaviour and mate preference towards it. British Journal of Psychology, 101: 809–819.
- Clune, J., Goldsby, H. J., Ofria, C., Pennock, R. T. Selective pressures for accurate altruism targeting: evidence from digital evolution for difficult-to-test aspects of inclusive fitness theory. Proceedings of the Royal Society B: Biological Sciences, 2010.
- Lee, H. Henry., Molla, N. Michael., Cantor, R. Charles., Collins, J. James. Bacterial charity work leads to population-wide resistance. Nature, 2010; 467 (7311): 82