Why do animals cooperate and help each other? Why do ants live in societies where they all seem to cooperate collectively? Surely, if the ultimate goal for any animal was to reproduce effectively, why would they risk their lives helping others? An altruistic act, that is acts that are beneficial to others but comes with a cost to oneself, was a fundamental problem with Darwin’s theory of evolution. In this article I will discuss the evidence for debate concerning two approaches that explains this: inclusive fitness and group selection.
This article will employ some evolutionary terms that the lay reader may not be used to, so if you are uncertain about terms such as “natural selection” or “trait” then refer to my definitions of them in the table below.
|Fitness||Fitness refers to an organism’s ability to reproduce effectively. This includes the ability to survive and adapt to the environment. An organism that is unlikely to survive and therefore unlikely to pass on its genes to its children will have low fitness.|
|Natural selection||This term refers to how nature selects organisms with the highest fitness. In an environment of limited space and resources, only a limited amount of animals can survive, and so whichever animal has the highest fitness, or survivability, will be selected to live and pass on its genes. Thus, evolution occurs because if one animal has an ability that makes it more likely to survive than another animal, then this ability will remain in the population. In very simple terms, imagine two antelopes that are identical except one is slow and one is fast. The slow one will soon be eaten by predators, and its “slow speed genes” will not be passed onto the next generation, while the fast antelope will survive, and over several generations “high speed genes” will become common in all antelopes (because only the fast ones survive).|
|Trait||A trait is any ability or property of an animal. For instance, a cheetah has the trait of speed, and spotted dots on its fur that provides camouflage in the savannah environment. Traits also refer to behaviours, so being aggressive is a trait, and helping others is a trait. If a trait improves the fitness of an animal then it is likely to survive through generations and remain in the population as a whole.|
Up until the 1960s there were no straight answers to the questions of why animals help each other, but then Hamilton (1964) proposed that helping is due to genetic relatedness. Simply put, if the genetic benefit of helping others outweigh the cost, then altruism occurs. For instance, you share 50% of your genes with your siblings, so in mathematical terms, should you save one of your brothers from dying at the expense of your life, then you have lost 100% of your genes and saved 50%, and so saving your brother is actually a loss of genes. By contrast, should you save three of your brothers from dying at the cost of your own life, then you have secured more copies of your genes than are lost:
(.5+.5+.5) – 1 = .5
In effect, saving your three brothers ensures you that three (or 150%) copies of your genes survive, while if you did nothing only 100% survives. See the math? In simplified terms, this is the essence of what is called inclusive fitness, or kin selection. It was made famous by Richard Dawkins’ book “The Selfish Gene” (Dawkins, 1976), where he proposed that genes act as replicators, whose only interest is to make copies of itself. This process also involves recognising copies of itself inside relatives, and cooperative behaviour occurs because the gene is acting to preserve these copies.
The theory of inclusive fitness sorted out the question of cooperation, and several behaviours and evolutionary strategies could now be explained through the perspective of genes acting as replicators. However, there was one alternative theory that could also explain the evolution of many traits in animals: group selection.
The essence of group selection involves natural selection acting on groups, where a set of individuals are the unit of selection, and not the individual as proposed by Hamilton. Simply put, if one group has a trait that makes it stronger than another group, then natural selection will act on this trait even if it is disadvantageous to the individual. For instance, some female lions will act as guards to defend the territory of their pride (Packer & Heinsohn, 1996). Being a defender has no benefits; in fact it only has costs such as injury or loss of life. Yet this is an altruistic act that has clearly evolved. If you think about it, inclusive fitness can explain this: the pride may contain relatives and so acting as a defender has an overall genetic advantage, but altruistic behaviours can be found in groups without relatives as well (Dugatkin & Mesterton-Gibbons, 1996). Why? Group selection proposes that lions with defenders have a higher fitness than lions without defenders and is more likely to survive. Thus, natural selection acts on the group!
The concept of group selection is highly controversial and numerous evolutionary biologists are opposed to it (Abbot et al., 2011), including Dawkins (2012; Dicks, 2000). Some scientists recognise the possibility of group selection, but feels that it doesn’t offer anything new compared to inclusive fitness, which is considered the preferred approach (Maynard Smith, 1976; Williams, 1966). The debate of group selection vs. inclusive fitness is in actuality an illusion: in reality the group selection and inclusive fitness approach overlap almost completely, though they have different ways of reaching the same result (Gardner, West, & Wild, 2011). They are sometimes regarded as simply two different languages talking about the same thing (Wilson, 2012a). Some regard group selection the most appropriate explaining the evolution of certain behaviours, and inclusive fitness most appropriate for others (van Veelen, 2009; Wilson, 2012b). This leads us to yet a third approach: multilevel selection theory.
Multilevel selection theory proposes that natural selection can occur at different biological levels (Wilson, 1997). The theory recognises that neither group selection nor inclusive fitness can account for the evolution of every single trait. Therefore, a trait must be evaluated on a case-by-case basis as there is no general theory that can account for all traits. As multilevel selection incorporates group selection within its framework it is not surprising many also oppose it.
The essence of multilevel selection is the following: selfish behaviour that maximise fitness will beat altruistic behaviour at the individual level, but at the group level altruistic behaviour will beat selfish behaviour. For instance, in chickens it is favourable to be aggressive rather than docile at the individual level, so you might expect all chickens to evolve to be aggressive. However, at a group level docility is preferred, so docility evolved even though at the individual level this is disadvantageous (Wade, Bijma, Ester, & Muir, 2010). Don’t worry; if you are confused just think of this: sometimes in evolution what is best for the group outweighs what is best for the individual.
Personally I regard multilevel selection as an umbrella term that incorporates both inclusive fitness and group selection. I believe this is a good way of uniting both of the approaches. Perhaps it should be more accurately called multilevel natural selection theory. According to Wilson and Wilson (2007) there is a historical resistance towards multilevel selection because its concept is often confused with what is called naïve group selection. Before Hamilton proposed inclusive fitness, natural selection was regarded to operate on any level, including at the individual, group, or species level (hence the incorrect term “for the good of the species”). This approach was labelled “naïve” because the mechanisms of how this selection operates were not understood. Once inclusive fitness was proposed, multilevel selection theory was pushed aside.
The debate regarding inclusive fitness and group selection often centres on the evolution of eusociality. This typically refers to colonies of insects such as ants or bees, where the colony is run by a queen and her caste of workers. In ant colonies all of the ants work together as a unified team, protecting the colony and providing food and cooperative brood care. In most ant colonies it is also only the queen that reproduces. Inclusive fitness can explain how: sister ants in the colony share on average 75% of their genes. This is referred to as a haplodiploid species. This is often confusing, but I will attempt to explain:
In haplodiploid species a queen mates only once with a single male. She can continuously lay eggs, and unfertilised eggs will become males, while fertilised eggs will become females. The males will therefore have 100% of the genes of the queens (because no genes from the father are transferred). The females will have 50% of the queen’s genes and 50% of the father’s genes. However, since the queen only mates once, all the sisters will share the same father genes. Thus, the 50% they get from the father is identical in ALL the sisters! In addition, they get 50% genes from the mother, but these genes will vary since she continuously lays eggs. Think of it as if one sperm inseminated 100 eggs from the same female: the mother will pass on 50% of her genes, but exactly which genes vary every time. The father will always pass on the exact same genes every time. Thus, every sister ant will share between 50% and 100% of their genes, or 75% on average. If you think about it, it is actually in the sister ant’s interest for the queen to create more sisters. After all, should the female workers reproduce on their own they will get offspring that only share 50% of their genes, compared to 75% that they share with their sisters!
Inclusive fitness suggests that altruism in ants occur because they are all so closely related. However, some researchers believe that this relatedness between workers is actually a consequence of the evolution of eusociality, and not the cause. Instead, group selection proposes that the ant colony should be viewed as one unit, or superorganism, and its fitness is relative to that of other ant colonies (Nowak, Tarnita, & Wilson, 2010). Basically, the strongest ant colony will reproduce and survive, and so natural selection will regard the ant colony as a single organism.
Wilson (2012a) proposes that the paradigm crisis that evolutionary biology is in now can be rectified by scientists becoming “bilingual”, meaning they understand both inclusive fitness and group selection and are not biased towards one or the other. I regard myself as a bilingual, and thus by extension a supporter of multilevel selection theory. I believe that group selection and inclusive fitness are both correct approaches, and the evolution of any trait must be evaluated on a case-by-case basis. Sometimes inclusive fitness will be the best explanation, sometimes it won’t.
One problem with this “bilingual approach” is that being bilingual means that you recognise the validity of multilevel selection theory because it incorporates both inclusive fitness and group selection. In other words, you are not really bilingual, but a multilevel selectionist.
The issue of group selection is a huge topic and I could continue writing for ages as there are so many studies, books, opinions, models, and definitions available. In all honestly I was myself a strong advocate for inclusive fitness theory until I was assigned to review a multilevel selection paper (Wilson & Wilson, 2007). The essay that followed included the biggest literature review I had done, and it convinced me of the viability of group selection, but kept my belief in inclusive fitness intact. Thus I consider myself a “bilingual”. While I think evidence is currently strongest for inclusive fitness, I nonetheless believe multilevel selection to be a viable theory and that any behavioural trait must be evaluated independently.
Abbot, P. et al. (2011). Inclusive fitness theory and eusociality. Nature, 471, E1-E4.
Dawkins, R. (1976). The Selfish Gene (30th Anniversary ed.). Oxford: Oxford University Press.
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Dicks, L. (2000). All for one! New Scientist, 167(2246), 3030.
Dugatkin, L. A., & Mesterton-Gibbons, M. (1996). Cooperation among unrelated individuals: reciprocal altruism, by-product mutualism and group selection in fishes. BioSystems, 37, 19-30.
Gardner, A., West, S. A., & Wild, G. (2011). The genetical theory of kin selection. Journal of Evolutionary Biology, 24, 1020-1043.
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Nowak, M. A., Tarnita, C. E., & Wilson, E. O. (2010). The evolution of eusociality. Nature, 466(7310), 1057-1062.
Packer, C., & Heinsohn, R. (1996). Lioness leadership. Science, 271(1215-1216).
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Wade, M. J., Bijma, P., Ester, E. D., & Muir, W. M. (2010). Group selection and social evolution in the domesticated animals. Evolutionary Applications, 3(5-6), 453-465.
Williams, G. C. (1966). Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought. New Jersey: Princeton University Press.
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