Altruism among Amoebas

Article excerpt

A person who dies so that others can escape starvation is a hero. But how can evolution explain the same behavior in a nonhuman organism whose genes are "selfish"?

Can you think of a species, other than our own, in which some individuals sacrifice their own interests for the sake of others? If you're like many other nature lovers, you probably thought of the social insects, such as ants and wasps. In those species, worker females devote their long, complex lives to the service of their queen and her young. But another group takes altruism to a whole new level: the social amoebas. In a single act of self-sacrifice, certain individuals give up their lives so that other amoebas can survive and later multiply.

Why should that be puzzling? If selfsacrifice is a characteristic that persists within our own species, wouldn't you expect to find its roots deeper in nature? Actually, all the way up and down the evolutionary scale, from single-celled amoebas to human beings, the persistence of a tendency to help others at one's own expense is a conundrum for natural selection. After all, natural selection normally acts on the genetic endowments of individuals, one by one, not on groups as a whole. If an individual does not pass on its genes to offspring, for whatever reason, those genes will be that much scarcer in the next generation. The process is blind, ruthless, and competitive, and it would seem to shut the door on genes for altruism. In particular, genes that tend to produce freeloaders-individuals that take advantage of altruism in others without sharing the cost-should survive and quickly crowd out any genes for altruism. Such "cheater" genes ought to be favored by natural selection, and spread through any population. So how can self-sacrifice be a successful strategy?

Our curiosity about that question led us to the Appalachian Mountains of Virginia, where social amoebas of the species Dictyostelium discoideum had been collected before. Dictyostelium amoebas feed on bacteria, so we asked ourselves where bacteria might be most abundant. And sure enough, we discovered a "fruiting body" of social amoebas on the very first pile of not-so-fresh deer pellets we examined under a field microscope. A tiny golden orb, held up by a slender white stalk, seemed to float a millimeter or so above its circular base, glued onto the dung. The light from our microscope made it gleam. The sight was both exotic and commonplace: Hundreds of biologists around the world work on this social amoeba in the laboratory. But we were apparently the first to see a D. discoideum fruiting body in its natural habitat [see photographs on this and opposite pages].

Our discovery marked both a departure and a continuity in our careers as biologists. Early on, we each developed a deep interest in biological altruism, inspired by the work of the English evolutionary biologist William D. Hamilton. In the 1960s Hamilton argued that altruistic behavior could evolve if the genes responsible for that behavior benefited relatives that shared copies of the same altruistic genes. (Relatives are more or less likely to share a gene depending on how closely or distantly the individuals are related.)

Hamilton pointed out that an individual can pass on altruism genes even if it has no offspring-by helping a relative pass on copies of genes they share. If that helping, or altruistic, behavior is more effective at passing on the individual's genes than some alternate behavior, Hamilton reasoned, the genes for altruism are likely to propagate, through a process called kin selection. He argued, for instance, that the unusual three-quarters relatedness among ant sisters could help explain their altruism. Richard Dawkins, an evolutionary biologist at the University of Oxford, later popularized Hamilton's idea in his best seller, The Selfish Gene.

With our mutual interest in altruism, it was natural for us to collaborate. We spent a quarter century studying the social behavior of wasps in places such as the olive groves of Tuscany and the rain forests of Venezuela. …