杏吧原创

IT IS just about the greatest sacrifice possible, and in evolutionary terms it looks like madness. Forgoing reproduction in order to help raise another鈥檚 offspring is a supreme act of altruism. It happens routinely in hundreds of ant, bee and wasp species, where sterile workers toil for the benefit of the queen.

For the last 40 years, the evolution of such altruism has been explained by kin selection: the idea that helping your relatives 鈥� and therefore helping spread the genes you share with them 鈥� outweighs the cost of not having offspring of your own. It鈥檚 the genes that matter, not the individuals in which they reside or the groups in which those individuals live. The idea was encapsulated in Richard Dawkins鈥檚 pervasive metaphor of the selfish gene.

Now, in a move that has baffled evolutionary biologists worldwide, the father of sociobiology and the world expert on social insects, Edward O. Wilson, says insect altruism and colonial living do not after all require kin selection to evolve (BioScience, vol 58, p 17). Instead, he cheerfully says, it only requires a small 鈥渉op鈥�: 鈥淥ne small step for a wasp, one giant leap for hymenoptera.鈥�

Lightly said, but Wilson鈥檚 ideas, if proved correct, would have great consequences. Altruistic behaviour was at one time a Darwinian paradox: how could the individual struggle to survive and reproduce favour such a strategy? The solution became known as Hamilton鈥檚 rule (see 鈥淭o die for two brothers鈥�). It explains, for example, why fertile females in many species of bees and wasps cooperate in reproduction instead of nesting alone. It also explains why castes of permanently sterile workers evolved. Or does it?

Wilson, who is at Harvard University, points out that eusocial behaviour (where a mother 鈥渜ueen鈥� is assisted at the nest by non-reproducing offspring) has been discovered in a variety of organisms. Species of aphids, shrimps, beetles, naked mole rats and others have joined the club. They have many ecological traits in common 鈥� in particular a large nest that must be defended from parasites or predators 鈥� but very different genetic structures. Naked mole rats, for example, have two sets of chromosomes 鈥� making them diploid, like us 鈥� and reproduce sexually, also just like us. Aphids, on the other hand, form colonies of genetically identical clones. In aphids, therefore, one might expect altruism to be particularly common, but it is not. These and other observations led Wilson to conclude that kin selection was not the decisive factor in the evolution of sterile workers (see 鈥淜in selection: for and against鈥�).

鈥淎phids form colonies of identical clones, so you might expect altruism to be particularly common, but it is not鈥�

Flexible lifestyle

Eusociality, argues Wilson, emerges from a rare combination of life-history characteristics and pre-adaptations. In particular, it can only evolve after the insect has gone through a phase where it can be flexible in its behavioural choices. For example, females of a solitary wasp species occasionally take the role of either 鈥渜ueen鈥� or 鈥渨orker鈥� when forced to nest together. If an environmental change or a new mutation then inhibits the dispersal of their offspring, they will be forced to stay at the natal nest. The pre-adaptations enable them to cooperate efficiently. Thus, says Wilson, fully eusocial colonies emerged in one single leap, and went on evolving by means of group selection, because groups that cooperate do better than those that do not.

Wilson鈥檚 鈥渇lexibility鈥� scenario for the evolution of eusociality in insects is not new. A similar idea was proposed as an alternative to kin selection about 30 years ago by Mary Jane West-Eberhard, now at the Smithsonian Tropical Research Institute in Panama. She, however, no longer sees flexibility as incompatible with kin selection. 鈥淚n fact, [it] depends on kin selection to work,鈥� she says. 鈥淗amilton鈥檚 rule describes the precise switching point when the flexibly responsive females should stay in the groups and express costly worker aid.鈥�

Wilson鈥檚 downgrading of kin selection theory, however, goes beyond its role in eusocial evolution. Kin selection 鈥渋s not wrong, and it is internally consistent, but it is ineffective鈥�, he says. 鈥淚f you look at the literature of the theory, there are a lot of impressive-looking mathematical models but they scarcely ever come up with a real measure of anything that can be applied to nature.鈥�

Most evolutionary biologists New 杏吧原创 spoke to strongly disagree. 鈥淚 think he has rather underestimated what kin selection theory has achieved,鈥� says Andrew Bourke of the University of East Anglia, UK.

鈥淲e are finding it useful in studying microbes,鈥� says David Queller at Rice University in Houston, Texas. 鈥淚t turns out that when they are altruistic they are related too. We weren鈥檛 too surprised, because that is a prediction of kin selection theory.鈥�

Kin selection theory 鈥渕akes all kinds of predictions about social behaviour鈥�, adds Francis Ratnieks of the University of Sheffield, UK. Since many of these predictions have been supported, he says, we can be confident in it.

Wilson鈥檚 forthcoming book The Superorganism, written with Bert H枚lldobler of Arizona State University in Tempe, will further advance the case for eusociality evolving by group selection in a single leap. 鈥淚t鈥檚 essentially a new paradigm,鈥� he says. However, the two theories might turn out to be compatible.

鈥淪ociobiologists used to think that group selection was completely wrong, and some people still think that way,鈥� says Queller, 鈥渂ut a large number of us think that group selection, if you do it correctly, is just alternative language [to kin selection] for describing social evolution.鈥� For more on this, see Dawkins鈥檚 thoughts on Wilson鈥檚 paper on page 17.

All researchers agree, though, that there is more to social behaviour than just kin selection. To understand social evolution 鈥渨e have to look at phylogeny, at gene expression, at anatomy, at behaviour, at ecological context鈥�, says James Hunt of North Carolina State University in Raleigh.

In a nod to the 鈥渕odelling industry鈥� that has sprung up around Hamilton鈥檚 rule, Ratnieks concedes that sometimes models were developed in the absence of biological data. 鈥淭hese models almost had a life of their own, with underlying assumptions that in many cases were never verified.鈥�

Most sociobiologists say Wilson鈥檚 ideas don鈥檛 herald a scientific revolution. 鈥淜in selection theory has always had its detractors,鈥� says Bourke. 鈥淚ts critics have seen it as too reductionist, too genetical, perhaps even too Dawkinsian, or they just got bored with it because it became so dominant.鈥�

Wilson is not surprised. 鈥淥f course they are going to remain critical for a while,鈥� he says. 鈥淚 am used to taking the heat, and in the past I turned out to be right. If those who defend the standard theory come back in lucid terms and show, point by point, why these hypotheses are already understood, I鈥檒l be glad to concede. But I haven鈥檛 seen any evidence yet. Maybe they will now.鈥�

(see 鈥淓ditorial: Time for revolution in evolution?鈥�)

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