Term Meaning Kin selection Individual sacrifices for close genetic relatives Reciprocal altruism Individual sacrifices for another individual that is likely to reciprocate in the future Indirect reciprocity Individual gains social status by being conspicuously altruistic altruism
chromosomes (they are diploid) while males have unpaired chromosomes (they are haploid). Females in these species produce eggs by meiosis, which reduces the chromosome number by half, while males do not need to do this (see Men- delian genetics).
Social insects are famous for the tendency of worker females to sacrifice themselves to protect the hive. Consider a beehive in which all of the workers have the same mother (the queen) and the same father (a lucky drone). Since drones are haploid, the worker offspring receive not half but all of his alleles. Since the workers receive only half of the queen’s alleles, they are more closely related to their fathers than to their mothers. Their coefficient of relatedness to one another is (1 × ½) + (½ × ½) = ¾, in which the first term is their relat- edness through their father and the second is their relatedness through the queen. Because siblings in most other species are related to one another only by r = ½, a relatedness of r = ¾ indicates that sibling worker ants, bees, and wasps should be much more altruistic toward one another than siblings usu- ally are. (A queen typically mates with several drones and stores their sperm. Many of the worker daughters have differ- ent fathers. Some workers have a coefficient of relatedness r = ½ while others have r = ¾. The average relatedness among workers in most social insect colonies is therefore somewhere between one-half and three-fourths, which is still high enough to allow strong altruism to evolve.) Parents and offspring are usually related to one another by r = ½. Worker insects are more altruistic toward one another even than parents and offspring. Anyone who has experienced an attack by a swarm of bees or wasps can attest to the way the worker sisters sac- rifice themselves for their common welfare.
Since the worker insects are more closely related to one another than any of them are to their queen, the workers are in control of the nest. Even though the term queen implies rulership, queens in social insect colonies are mere egg-lay- ing machines. It would be in the best interest of the queen to produce equal numbers of male and female offspring, but the workers will not allow this to happen: The workers kill most of the drone larvae. It is also the workers that decide which female larvae should receive the “royal jelly,” which, unlike regular larval food, causes a female to develop into a queen. Workers may destroy some queens if there are too many. Haplodiploidy is not the only evolutionary precondition for the altruism of social insects. All ants, bees, and wasps have haplodiploidy, yet the only ones that have evolved soci- ality are those that have also evolved a life cycle in which the larvae are helpless grubs (see life history, evolution of) and in which nesting behavior has evolved. Solitary bees have haplodiploid genetics but do not sacrifice themselves for one another.
Kin selection may also explain why animals, including humans, tend to behave more altruistically toward their true biological offspring than toward their stepchildren. In many mammal species, such as lions, a newly arrived dominant male will kill the offspring of the previous male, as shown in the photo above. These juveniles, while perfectly good for the prosperity of the species, have a zero percent genetic related- ness to the new dominant male. If these offspring represent
any cost at all to these males, Hamilton’s rule would predict that there would be no altruism at all. The unrelated juveniles do represent a cost, because while the females are feeding and protecting them they cannot produce offspring for the new dominant male.
Kin selection helps evolutionary scientists to understand why humans are less solicitous toward stepchildren than toward biological offspring. This behavior pattern is a nearly universal feature among human societies: biologists Martin Daly and Margo Wilson call it “the truth about Cinderella.” Crime data from Canada show that, while men very seldom kill children in their families, they are 70 times as likely to kill stepchildren as biological children. Stepchildren also have higher levels of blood cortisol (an indicator of stress) than do biological children. This indicates that the fathers and step- children both behave as though altruism is often missing from the father-stepchild relationship. In blended families with both biological and adopted offspring, fathers spend more time with their biological offspring than with their step- children. But is this due to kin selection, or simply due to the fact that stepchildren are older before their stepfather first becomes acquainted with them? Researchers have found that fathers were less solicitous of stepchildren than of biological children even if the stepchildren were born after the stepfa- ther and the mother had begun living together.
Reciprocal Altruism
Evolutionary biologist Robert Trivers pointed out another way in which altruism could be favored by natural selection. An animal might perform some costly act of help to another animal if the recipient was likely to return the favor at some time in the future. Because the recipient may reciprocate in the future, this behavior is called reciprocal altruism. Recip- rocal altruism helps to explain altruism toward individuals to
This male lion has just killed a lion cub. When male lions take a new mate, they often kill the female’s previous offspring, thus making the resources of the female lion available for raising the new male’s offspring. (Courtesy of George Schaller)
which the animal is not closely related. Reciprocal altruism will not work unless there is a minimal level of intelligence. This is because reciprocal altruism is susceptible to cheaters. There must be some punishment for the individual that keeps all its resources while accepting the help of others. The other animals need to have enough intelligence to remember who the cheaters are. This could be one of the major contribut- ing factors in the evolution of human language and human intelligence. Evolutionary biologist Robin Dunbar suggests that language evolved largely because it allowed humans to keep track of the intricacies of social structure, which would include the ostracism of cheaters. In animal species with strong social hierarchies, the sub- ordinate males receive no benefit for being altruistic toward the dominant males. Altruism between social classes conveys no benefit in those species. The males may, however, benefit greatly from carrying out acts of reciprocal altruism that win allies from their social peers. Because of the need both for paying back the altruism and for punishing cheaters, recipro- cal altruism works best in animal species that are intelligent, social, and long-lived.
Indirect Reciprocity
Neither kin selection nor reciprocal altruism can explain altruism toward individuals who are unlikely or unable to reciprocate. While such altruism is rare in nonhuman species, it is very common among humans. Evolutionary biologist Geoffrey Miller points out that, in modern human charities, the recipient is often indigent and unable to reciprocate, and the recipient seldom knows who the donor is anyway. The donors often are not interested in the efficiency of resource transfer to the recipient. It would be much more efficient if a rich person continued to earn money, then donated that money, rather than working the equivalent number of hours in a soup kitchen. What, then, could be an evolutionary explanation for this kind of altruism?
The key to this kind of altruism may be whether or not another animal is observing it. The altruist can obtain greater social stature by being altruistic toward individuals that are unrelated or that cannot repay. Mathematicians Karl Sig- mund and Martin Nowak have produced calculations that demonstrate this advantage. Human donations of time and/or money to charities, says Geoffrey Miller, more closely resem- ble a display of wealth than a calculated plot to get recipro- cal benefits. Altruism, like conspicuous consumption, may constitute a message to the population at large. Conspicuous charity proclaims, “I am rich enough to give away some of my resources. This tells you that I am not only rich but also that I am a good person.” Conspicuous consumption tells the observers only the first of those two things. The reputation of being a good person might yield enough social benefits to compensate for the cost of the altruism.
In particular, the altruist may gain advantages in mate choice. sexual selection could favor a conspicuous display of altruism, whether through charity or through a heroic deed to benefit the community. It is usually the males that display and the females that choose. Although among humans sexual selection has been more mutual, it is still the males who show off, and the females who choose, more often than the other way around. Conspicuous altruism is not merely showing off; it is actually useful information to the individual (usu- ally the woman) making the choice of a mate: Such a man must have good resources and must be a good person who will be good to her. Displays of altruism need to be exces- sive, or prolonged, or both, in order for the woman to know the man is not faking it. Geoffrey Miller uses the example of Ebenezer Scrooge, the character in a novel by British writer Charles Dickens. Before his transformation, Scrooge not only did not participate in kin selection (he was not generous to his nephew) or in reciprocal altruism (he was not generous to Bob Cratchett), but also it is no surprise that he was single. Sexual selection can, and routinely does, produce adaptations that are costly to the individual, whether it is human altruism or the tail of a bird of paradise. Miller uses sexual selection as an explanation not only of the peculiarly human excesses of altruism but all aspects of human intelligence (see intel- ligence, evolution of).
Because altruism can provide fitness benefits, natural selection has also favored the evolution of emotions that reinforce altruism. Altruism feels good, in a number of ways, including feelings of satisfaction for being altruistic, gratitude toward donors, and rage toward cheaters. Neurobiologists have measured brain activity in human subjects involved in simulated situations of cooperation. They found that altruis- tic cooperation activated the same brain regions (such as the anteroventral striatum, also known as the pleasure center) as cocaine, beautiful faces, good food, and other pleasures. They also found this response when the subjects participated in sweet revenge against cheaters. The idea that the enjoy- ment of altruism has a natural basis is not new. American president Thomas Jefferson wrote in a letter to John Law in 1814, “These good acts give pleasure, but how it happens that they give us pleasure? Because nature hath implanted in our breasts a love of others, a sense of duty to them, a moral instinct, in short, which prompts us irresistibly to feel and to succor their distresses.”
Evolution can therefore explain the tendency toward altruism in three ways: kin selection, reciprocal altruism, and indirect reciprocity. Since it is the proclivity, rather than the act itself, which evolution explains, humans can perform indi- vidual acts of self-sacrifice that yield no fitness benefit. But if such acts were common, the tendency to perform them would be selected against. A person can sacrifice herself or himself in a totally unselfish fashion—and there are numerous exam- ples of such saints and heroes—because the behavior pattern evolved in the human species as a result of people sacrificing themselves in a selfish fashion. Evolutionary altruism has also influenced the evolution of ethical systems (see evolutionary ethics). Humans not only behave altruistically but believe that this is the right way to act. Evolutionary ethicist Michael Shermer indicates that, dur- ing the course of human evolution, feelings of affiliation with others and affection for others have evolved as reinforcements of altruism, first through kin selection within extended fami- lies and then through reciprocal altruism within communities. These feelings, being the product of natural selection, have a altruism
genetic basis. About 35,000 years ago, at a time Shermer calls the bio-cultural transition, cultural evolution became more important than biological evolution. The feelings of altru- ism that had already evolved were now applied beyond the community, to include society as a whole (altruism toward people who could not reciprocate), the entire human species, and even the entire biosphere of species. Today many humans choose to extend altruism to the whole world. The behavioral and emotional basis of this altruism evolved by means of kin selection, reciprocal altruism, and sexual selection. Further Reading Axelrod, Robert, and William D. Hamilton. “The evolution of coop- eration.” Science 211 (1981): 1,390–1,396.
Daly, Martin, and Margo Wilson. The Truth about Cinderella: A Darwinian View of Parental Love. New Haven, Conn.: Yale Uni- versity Press, 1999.
DeWaal, Frans B. M. “How animals do business.” Scientific Ameri- can, April 2005, 72–79.
Dugatkin, Lee Alan. The Altruism Equation: Seven Scientists Search for the Origins of Goodness. Princeton, N.J.: Princeton University Press, 2006.
Hamilton, William D. “Altruism and related phenomena, mainly in the social insects.” Annual Review of Ecology and Systematics 3 (1972): 193–232.
Miller, Geoffrey. The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature. New York: Doubleday, 2000. Nowak, Martin A. “Five rules for the evolution of cooperation.” Sci-
ence 314 (2006): 1560–1563.
Sherman, P. W. “Nepotism and the evolution of alarm calls.” Science 197 (1977): 1,246–1,253.
Shermer, Michael. “The soul of science.” American Scientist 93 (2005): 101–103.
Singer, Peter. A Darwinian Left: Politics, Evolution, and Coopera- tion. New Haven, Conn.: Yale University Press, 1999.
Trivers, Robert L. “The evolution of reciprocal altruism.” Quarterly Review of Biology 46 (1971): 35–37.
Warneken, Felix, and Michael Tomasello. “Altruistic helping in human infants and young chimpanzees.” Science 311 (2006): 1,301–1,303.
amphibians, evolution of
Amphibians are vertebrates that usually have an aquatic juvenile and terrestrial adult stage. Modern amphibians include animals such as frogs, salaman- ders, and caecilians. Frogs have an aquatic juvenile form (the tadpole) that swims with fins and a tail and breathes with gills, while the adult frog has no tail, has legs, and breathes with lungs. Most newts and salamanders, which are long and tailed, also have aquatic juvenile and terrestrial adult forms, although in some, such as the axolotl, aquatic juveniles become sexually mature (see neoteny). Caecilians, often mis- taken for snakes, are legless and live in burrows. The term amphibian refers to the fact that most of them live (bio-) both (amphi-) on land and in water. Genetic analyses suggest that all amphibians share a common evolutionary ancestor that lived on the earth during the Devonian period. Amphibians were the first tetrapods, or animals that walked upon four (tetra-) feet (-pod).The common evolutionary ancestor of all modern amphibians has not been identified, and it certainly was not the only amphibian alive at the time. Nine genera of Devo- nian amphibians have been found, spanning a 20-million- year period. Many of these fish-amphibian animals lived at the same time, and scientists cannot determine with certainty which if any of them was the ancestor of all modern amphibi- ans. There is no doubt of the evolutionary transition from fish (see fishes, evolution of) to amphibian, as it was occurring simultaneously in many different lineages. Animals intermedi- ate between fishes and modern amphibians included: • Eusthenopteron foordi lived in the late Devonian period. It had all the same fins that modern fishes still possess, rather than hands or feet. However, at the bases of the fins, it had bones analogous to the arm and leg bones of terrestrial ver- tebrates. Eusthenopteron had no bones that corresponded to the digits of modern tetrapods. This organism looked very much like an ordinary fish, and it probably spent nearly all of its time in shallow water.
• Panderichthys rhombolepis also lived during the late Devo- nian period. It lacked some of the fish fins and had thicker ribs than fishes possess. The thicker ribs were important in supporting the weight of the body when on land and away from the buoyancy of water. However, lacking legs, this species must have spent nearly all of its time in water. • Ichthyostega stensioei and Acanthostega gunnari looked like
fishes with legs. Their skulls and skeletons looked fishlike, but they had hands and feet and ribs even thicker than those of Panderichthys. Ichthyostega probably spent more time on land than Acanthostega. Ichthyostega moved like a seal, dragging itself by its forelimbs. It was the first vertebrate to have a non-swimming locomotion. In fishes, the hyomandib- ular bone helps to support the gills; this bone corresponds to the stapes, the ear bone of tetrapods (innermost ear bone of mammals). The stapes of Acanthostega has been found, and it resembles a fish hyomandibular bone. The stapes, how- ever, was not free to vibrate and therefore could not have functioned in hearing on land. Later amphibians, in the Car- boniferous period, had stapes that functioned in hearing.
In April 2006, paleontologist Neil Shubin announced the discovery of a new transitional form between fishes and amphibians. It was named Tiktaalik roseae from a word in the language of the Nunavat, the Canadian First Nations community that owns the fossil. This animal had a skull, neck, ribs, elbows, and wrists that resemble those of later amphibians, but had fishlike fins.
By the early Carboniferous period, there were many dif- ferent amphibian lineages. All have become extinct except for two: the branch that led to modern amphibians, and the branch that led to reptiles (see reptiles, evolution of).
There has been debate regarding the reasons for the evo- lution of legs. Suggestions include: • One early proposal was that fishes had to walk on land to get from one pond to another if their home pond began to dry up. There are fishes today that crawl on land and even climb trees, entirely without legs.
• Modern salamanders have legs yet many of them live underwater. They use their legs for walking underwater on rock surfaces against the current. It is possible that the first legged amphibians evolved in rushing water.
• Early amphibians may have used their legs to drag them- selves around in shallow water, where they would be safe from deep water predators.
• The shallow water in which early amphibians lived may have been deficient in oxygen due to decomposition of leaf litter. If the amphibians lifted themselves up and breathed air, they could overcome this problem. • Evolutionary biologist Robert A. Martin suggests that legs assisted in clasping during sexual reproduction, a function they still possess in many modern amphibians.
It is likely that legs proved useful for several different functions over a long period of time. Whatever combination of advantages may have selected for the evolution of legs, it had to be something that worked in a primitive condition. The earliest amphibians with legs could scarcely lift them-