For some tiny Australian insects, the willingness to die for one's home is a relative matter.
A whistle-blower risks her job by speaking out about threats to the environment, a soldier gives his life in defense of his homeland, a New York City firefighter dies in the collapse of the World Trade Center towers as he struggles to save the lives of strangers. Heroic actions like these fill us with deep emotion and pride in the altruistic possibilities of humanity. Yet we are not the only altruists. Termites rush to a breach in their nest and clamp their jaws onto the snout of a marauding anteater, almost guaranteeing their own death. A worker honeybee that stings us to defend her mother and other family members in the hive is doomed, for she cannot extract her barbed stinger from her victim without ripping out her innards in the process. Unlike the human examples, these animal altruists do not perhaps deserve to be called heroic, and they are acting only in defense of their own kin. But both pose a dilemma for evolutionary biology: how can selfsacrifice have evolved if the altruistic individuals so often bring about their own destruction?
For Charles Darwin, this paradox could be resolved by the idea that natural selection operates not only at the level of individuals but also at the level of families. He reckoned that just as farmers can retain a favored trait of domestic animals (such as well-marbled cattle flesh) by breeding relatives of the superior individual, natural selection can preserve and promote seemingly selfless traits of wild animals-as long as genetic relatives benefit from the act. One hundred years after the Origin of Species, the late biologist W.D. Hamilton formalized Darwin's conjecture, developing what is now called kin selection theory. This theory predicts that, all else being equal, the degree of an individual's altruism toward family members should depend on how closely related they are genetically. Most of the time, the real world acts just as Hamilton predicted. But in many species, close relatives in social groups behave selfishly toward one another, and in some species in which groups are not made up of close relatives, altruistic actions are common. To understand how social behavior evolves, we must consider not only genetic relatedness but also aspects of habitat and ecology that may select for altruism.
With this goal in mind, I decided to study thrips, a group of punctuation-sized insects (in the order Thysanoptera) most commonly encountered as pests of houseplants. Among the approximately 5,000 described species are thrips ranging in size from 0.5mm motes to 15-mm giants. All have sucking mouthparts with which they feed on green plant tissue, pollen, spores, or fungal mycelia. I chose to study these insects for two reasons. The first is that in thrips, as in the order Hymenoptera (ants, bees, and wasps), females develop from fertilized eggs and are diploid (having two sets of chromosomes, one from each parent), while males develop from unfertilized eggs and are haploid (having a single set, inherited from the mother). This haplodiploidy, as Hamilton was the first to point out, has important implications for the evolution of altruism. In haplodiploid species, females share 75 percent of their genes with their full sisters: 50 percent via their father (in these species, all the sperm produced by a male are genetically identical) and 25 percent via their mother (she has two alleles, or forms, for each gene, halving the probability of passing on any particular allele). Females can thus pass on their own genes more effectively by helping to rear and protect full sisters (to which they are three-quarters identical) than by leaving home and producing their own offspring (only one-half identical).
My second reason for focusing on thrips had to do with ecology. About twenty species in the deserts of Australia occupy galls, plant tissues that have been modified by feeding insects to form a hollow cavity. …