Introduction

This brings us to the central theoretical problem of socio-biology: how can altruism, which by definition reduces personal fitness, possibly evolve by natural selection? (Wilson, 1975, p. 3)

Altruistic behaviors, which reduce the personal reproduction of an actor and benefit another individual (Figure 1), are found in a diverse set of organisms, ranging from microbes, through social insects, to higher vertebrates and humans (Figure 2). Altruism presents a conundrum for evolutionary thinking because Darwin's theory of natural selection appears to suggest that selfish and competitive strategies are favored over evolutionary time. Why would natural selection select for a behavior that reduces personal reproduction?

As we will see, altruism can evolve when the actor and recipient carry the same genes, at one or more loci - the actor can then increase copies of their genes through the recipient's reproduction. This explanation, which comes from what is called inclusive fitness (or kin selection) thinking, remains the key solution for the problem of altruism, as originally defined in the evolutionary literature. However, more than one usage of altruism has developed in behavioral ecology and with alternative definitions came other explanations, which will be discussed.

Care with definitions becomes even more important when one looks outside of biology. In common parlance,

Effect on recipient

Mutualism

Altruism

Selfishness Spite

Cooperation Competition

Figure 1 The four types of social action based on their effect on the direct fitness (lifetime personal reproduction) of the actor and recipient. Altruism and spite can either have no or a negative fitness effect on the actor.

altruism is often taken to indicate an actor's psychological 'intention' to act selflessly. The biologist's focus on 'outcome' and evolutionary fitness (Figure 1 ), therefore, can contradict the mainstream meaning of altruism in at least two ways. First, it allows the possibility of altruism in simple organisms, like microbes, that lack conscious intention. In addition, a gene for altruism will only be selected when the action increases its carrier's fitness -genes cannot be selected to produce behaviors that decrease their frequency. Evolutionary discussions of

Prairie Dog Altruism

Figure 2 Species that display altruistic behaviors. (a) Prairie dogs live in family groups in communal burrows or 'towns'. When danger approaches, guard individuals will bark and warn others, at apparent cost to themselves. They also display cooperative brood care. (b) Fruiting bodies of the slime mold Dictyostelium discoideum. Thousands of cells aggregate together in these groups and many die altruistically to form a stalk that holds the others aloft as dispersal spores. (c) The gall-dwelling aphid Pemphigus obesinymphae. When disturbed, soldier aphids emerge and attack intruders. (d) The yellow-jacket eusocial wasp, Dolichovespula saxonica. Workers both altruistically work and lay eggs (shown) in this species. The level of worker reproduction, however, is kept low by both genetic relatedness and policing behaviors (Figure 4). (c) Photo used with kind permission of Patrick Abbot.

Figure 2 Species that display altruistic behaviors. (a) Prairie dogs live in family groups in communal burrows or 'towns'. When danger approaches, guard individuals will bark and warn others, at apparent cost to themselves. They also display cooperative brood care. (b) Fruiting bodies of the slime mold Dictyostelium discoideum. Thousands of cells aggregate together in these groups and many die altruistically to form a stalk that holds the others aloft as dispersal spores. (c) The gall-dwelling aphid Pemphigus obesinymphae. When disturbed, soldier aphids emerge and attack intruders. (d) The yellow-jacket eusocial wasp, Dolichovespula saxonica. Workers both altruistically work and lay eggs (shown) in this species. The level of worker reproduction, however, is kept low by both genetic relatedness and policing behaviors (Figure 4). (c) Photo used with kind permission of Patrick Abbot.

altruism, therefore, typically involve hidden genetic benefits, which can be troublesome for those that require altruism to be truly selfless.

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