How do you pass on a gene for self-sacrifice? It sounds like an oxymoron. Or the set-up for a joke. (Punch lines invited; my candidate: “Early and often.”) “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,” wrote Joan E. Strassman and David C. Queller in their September 2007 article, “Altruism Among Amoebas.” Since I’ve been working for Natural History, we’ve run what adds up, in my mind, to a series of articles on this conundrum, ranging all the way up and down the evolutionary scale—and sometimes across species boundaries—from amoebas to ants (2/09) (including honey ants (4/08), some of whose workers turn their bodies into living storage jars) to flocking birds (7-8/08) to vampire bats (11/08) to African rock hyraxes (coming in March) to blue monkeys (9/08). My fact-checking job feels like a rapid-fire education, and over time it often shapes up into “courses”—on cosmology, genetics, climate change, and in this case, on how altruistic and cooperative behavior could have evolved. So I thought I’d share with you some of the best supplemental reading I’ve found—which is not to say that I understand it myself.

Bottom line: genes are carried by individual organisms and passed along when those individuals reproduce (“vertical gene transfer”). (Even this statement has to be qualified, at least in the case of bacteria—which reproduce asexually but swap genes promiscuously, even across species, by means of viruses called phages. This “horizontal gene transfer” has rather terrifying implications for both human disease and genetic engineering, and you also have to wonder if it really is neatly confined to bacteria. But never mind [hauls self, with effort, off of mesmerizing sidetrack].) In classical evolutionary theory, natural selection happens when the phenotypic trait encoded by a gene confers a reproductive advantage on bearers of that gene: for whatever reason—because they’re hardier or cleverer or sexier, better adapted or more adaptable—they leave more offspring than competitors, and that gene spreads through the population.

A brief pause for some handy language: in the process of fact checking his current review (repeat link) of The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies, by Bert Hölldobler and E. O. Wilson, Rob Dunn told us that the gene is the unit of selection; the individual favorably outfitted by that gene is the target of selection. (This, too, will come into question as an ironclad rule before we’re through.) The first major roadblock to understanding how altruism could have evolved is simply stated by Strassman and Queller (repeat link):

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. . . . [H]ow can self-sacrifice be a successful strategy?

The second roadblock is that even if altruism conferred enough advantages to get started somehow, it should quickly be undermined by the arrival of “cheater” genes—in S & Q’s words:

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.

What’s so striking is that the same problem of the “cheater” or “free rider” confronted by single-celled “social amoebas”—which join to form first a mobile slug, then a cellulose stalk that’s fatal to its donors holding up a “fruiting body” that’s the reproductive jackpot—is also faced by such multibillion-celled organisms as vampire bats, which share blood (see video below), and blue monkeys, which form female alliances to defend territory. Putting aside for the moment the question of how and to what extent genes can direct such complex behaviors, how are “cheater” strategies, which minimize cost and maximize payoff, prevented from spreading like social cancer through these populations, driving out the altruists and cooperators who hold a society together? (Granted that once the good guys were gone, the cheaters’ free ride would end, too; but how can genes “know” that, any more than cancer cells, parasites, or pathogens do?)

Vampire bats share blood. Click arrow to begin. Video by Riziq Sayegh, www.neurohack.ca

The simplest answer to the riddle, “How do you pass on a gene for self-sacrifice?” is: “By sacrificing yourself only for those who carry some or all of the same genes as you.” This is kin selection, first elucidated in the 1960s by the British evolutionary biologist William D. Hamilton, and Strassman and Queller have shown that the behavioral basis for it—kin discrimination—occurs among social amoebas [PDF] (aka “slime molds”) of the genus Dictyostelium. When these amoebas run out of food (bacteria) and send out a chemical signal to rally the troops, they preferentially aggregate with amoebas that are more genetically similar to them. The amoebas that sacrifice themselves to form the stalk of the fruiting body, then, are still acting to pass on many of their own genes, by enabling genetic relatives to disperse as spores.

That’s one way to protect against “cheaters”: genetically speaking, it’s not such a loss if close kin reproduce instead of you. Another safeguard, explains scienceblogger Carl Zimmer of The Loom, is that the same mutations that enable “cheating” may also carry disadvantages that cancel out the edge. In “Dicty,” as the huge community of scientists studying it affectionately call the social amoeba (at that link, you’ll see that imitation is the sincerest form of tribute), one such mutation is a knockout of the gene that produces a sticky protein [PDF], helping the single cells clump together. Amoebas that don’t form the protein can easily slide from the “front lines” of stalky sacrifice to the back of the slug, where the fruiting body takes form. But they rarely get that far, because they have trouble getting into an aggregate in the first place. Their slipperiness (funny, we use just that word for those we can’t trust to “stick together”) is somehow detected and rejected.

At first glance, kin selection would also seem to explain the extraordinary sacrifice of worker ants and bees in many advanced insect societies: sterile females that forgo reproduction to serve the colony populated by their sister queen, devoting their lives to its maintenance and often their deaths to its defense. Could it be any accident, Hamilton wondered, that such advanced societies had developed almost exclusively in the order Hymenoptera, where sisters share an unusually close genetic bond? E.O. Wilson, an early convert to “Hamilton’s rule” (and a wonderfully lucid writer), explains [PDF] :

By brilliant insight, [Hamilton] connected the following two facts. First, the haplodiploid mechanism practiced by the Hymenoptera (ants, bees, and wasps), in which fertilized eggs become females and unfertilized eggs become males, causes full sisters to be more closely related to one another (by three-fourths) than are mothers and daughters (one-half). Second, almost all of the known 11 independent origins of such colonial life in nature have occurred in the Hymenoptera. . . . Hamilton concluded, quite reasonably, that kin selection is a decisive driving or at least strongly biasing force in the origin of the advanced insect colonies. . . .

The core conception by Haldane and Hamilton is expressed in what has come to be called Hamilton’s rule:

rb>c

That is, altruistic behavior will evolve if the benefit b in offspring to the recipient discounted (multiplied by) the fraction of genes shared by common descent between recipient and altruist exceeds the cost in offspring to the altruist. Hamilton’s rule, until very recently, has been the textbook encapsulation of the binding force in the origin of colonies that contain altruistic workers.

It turns out, however, that this is wrong.

NEXT: So What’s Thicker Than Blood?

(Annie Gottlieb)

Comments (add yours!)

Return to facTotem home