Chapter 12 Chapter 10 You tickle me, and I will ride on your head

We have studied parental, sexual, and aggressive interactions between survival machines belonging to the same species.However, there seem to be some noteworthy aspects of animal interactions that are clearly not covered by the above three categories.The social habit of many animals is one aspect.Birds, insects, fish, whales, and even mammals living on the plains always gather in groups for food.Members of these collectives are usually of the same species, but there are exceptions.Zebras and wildebeests often mix together, and birds of different species can sometimes be seen gathering in flocks.

Living in groups can bring all sorts of benefits to a selfish individual.Here, I am not going to list them one by one, but I am going to talk about a few instructive examples.In it I will also recall some examples of overt altruistic behavior that I gave in Chapter 1, because I said that these examples are to be explained later.This necessarily involves a discussion of social insects; indeed, no discussion of animal altruism can be complete without mentioning social insects.Finally, in the miscellaneous content of this chapter, I will talk about the important concept of reciprocal altruistic behavior, that is, the principle of doing what is convenient for others and what is convenient for oneself.

Animals must live together because their genes benefit more from the association of living in groups and pay less for it.When hunting in groups, hyenas can catch much larger beasts than when they are alone. Although they have to divide the food after catching wild animals, it still pays off for each selfish individual participating in the group hunting.It is presumably for similar reasons that certain spiders work together to weave a vast communal web.Emperor penguins huddle tightly together for warmth.This is because after being huddled together, the exposed body surface of each penguin is much smaller than when alone.When two fish are swimming in the water, if one swims behind the other while maintaining a certain slope, it can gain a hydrodynamic benefit from the turbulence created by the fish in front.This may be part of the reason fish swim together in groups.How to use turbulence to reduce air resistance is also a trick familiar to cyclists.Birds may form a V-shape in flight for this reason as well.Since the first bird is at a disadvantage, the birds presumably compete to avoid this role.It is likely that they take turns serving as this involuntary navigator.This is a delayed mutual altruism, the form of altruism we discuss at the end of this chapter.

Many of the possible benefits of living in groups have to do with avoiding predators.Hamilton brilliantly developed this theory in a treatise entitled The Geometry of the Selfish Herd.In order not to cause misunderstanding, I want to emphasize that by the selfish herd he means a herd of selfish individuals. Let's start again with a simple pattern.Although the pattern is abstract, it can help us understand the real objective world.Imagine a group of animals of a certain species being hunted by a predator.The animal closest to the predator is often the first to be attacked.For predators, this strategy makes sense because it saves energy.But for prey animals, this strategy has an interesting consequence.That is, each of the fleeing animals is trying to avoid being in the closest position to the predator.If the animals spot the predator at a distance, they can simply run away.Even when a predator appears suddenly and silently, like a beast hiding in dense grass, each animal plays by ear and tries to avoid being in the closest position to the predator.We can imagine that there is a danger zone around every hunted animal.In this danger zone, the distance from any point to this animal is shorter than the distance from that point to any other animal.For example, if a group of animals being pursued moves in a regular geometric pattern spaced apart from each other, the danger zone for each animal (unless it happens to be on the edge) is roughly six-sided shape.If a predator happens to be lurking in the hexagonal danger zone of individual A, individual A may be eaten.Individuals on the fringes of the herd are especially vulnerable, because their danger zone is not a relatively small hexagon, but has an open end, and a wide area beyond the open end is their danger zone.

A sane individual will obviously try to minimize his danger zone.It especially tries to avoid being on the fringes of the herd.If it finds itself on the fringes, it takes immediate action and moves to the center.Unfortunately, someone has to be on the edge, and as far as each individual is concerned, that person had better not be it!Thus, as a group of animals advances, individuals on the periphery keep moving toward the center.If the group of animals were originally loose or scattered, the result of this movement toward the center of the group would quickly cause them to huddle together.Even if the pattern we speak of begins without any tendency to gather together, and the hunted animals initially scatter randomly, selfish motives will prompt each individual to try to squeeze itself among the others in order to narrow their respective danger zones. .In this way, clusters form quickly and become denser and denser.

In real life, this tendency to gather is apparently limited by various resistances, otherwise the animals would inevitably scramble and become exhausted.Still, the pattern is interesting because it illustrates how even some extremely simple hypotheses can lead to the conclusion that animals tend to clump together.Some more complex patterns were proposed.Although these models have greater practical significance, the simpler models proposed by Hamilton are not detracted from them.The latter helps us study the phenomenon of animals clumping together. The selfish herd model itself does not allow for cooperative interactions.There is no altruism here, just each individual exploiting every other individual for self-interest.But in real life it often happens that individuals seem to be actively trying to protect their fellow group members from predators.Speaking of this, I can't help but think of the alarm calls of birds.This kind of alarm call made other individuals flee for their lives, which indeed served as a warning.No one thought that the alarming individual was trying to direct the predator's fire on him.It merely lets its partners know that a predator is present, that is, alerts them.But at first glance, the behavior itself seems altruistic, since its effect is to draw the predator's attention to the caller.We can draw an indirect inference from a fact discovered by P. R. Marler.This alarm call in birds seems to have some desirable physical properties: It is often difficult for predators to detect where the call is coming from.Ask an acoustic engineer to design a sound that would be difficult for a predator to track down, and it would likely resemble the natural alarm call of many small singing birds.In nature, the formation of this alarm call must be the result of natural selection.We know what that means.This means that many individuals lost their lives because their alarm calls were not perfected.So there always seemed to be danger in sounding the alarm.The theory of the selfish gene would have to demonstrate that there is a convincing virtue to sounding the alarm that outweighs the attendant danger.

Actually it's not very difficult.In the past, it has been repeatedly pointed out that the alarm calls of birds are actually incompatible with Darwin's theory.The result is that in order to explain this phenomenon, it has become a kind of game for people to invent various reasons.So we face so many plausible explanations today that we are at a loss for what to say.Clearly, if some individuals in the flock are close relatives, the gene for the alarm call must thrive in the gene pool, since there is a high probability that some of the individuals saved will have it.Even if the individual who calls the alarm pays a high price for the altruistic behavior by attracting predators, it's worth it.

If you find this notion of kin selection unconvincing, there are other theories to choose from.Trivers offers five insightful ideas for the various avenues in which an individual who calls the police on his partner can gain self-interest.But here are two of my own, which I think are more convincing. I call the first idea the Cave theory.Kaiwei is originally Latin, meaning beware.Today, elementary school students use this signal to warn other students when they see the teacher approaching.The theory applies to birds that employ a camouflage tactic, crouching motionless in undergrowth when faced with danger.Suppose there is a flock of these birds foraging in a field.At this time an eagle flew by in the distance.The eagle hadn't caught a glimpse of the flock yet, so it didn't try to fly straight.But its sharp eyes may spot the flock at any moment, when it will swoop down and strike.If a small bird in the flock finds the eagle first, the rest of the birds have not found it yet.The quick-eyed little bird could have crouched down immediately and hid in the grass.But it did him no good by doing so, for his companions were still moving about, conspicuous and noisy.Any one of them might attract the eagle's attention, and the whole flock would be in danger.From purely selfish motives, the bird that spots the hawk should immediately hiss its mates to silence them at once, to reduce the chances of them inadvertently drawing the hawk into its vicinity.This is the best strategy for this little bird.

Another idea I'm going to talk about can be called the never-leave-the-team theory.This theory applies to certain species of birds that fly away when they see a predator approaching, perhaps into a tree.Let us also imagine that one of a flock of foraging birds is the first to spot the predator.How should it act?It can fly away by itself without warning its mates.If so, it's about to become a loner, a member of a less conspicuous flock of birds.It is a well-known fact that hawks love to attack stray pigeons.Even if hawks do not have such hunting habits, there are many reasons that can be reasoned out that breaking away from the group might be a suicidal tactic.Even if its partner eventually flies away with it, the first individual to fly off the ground inevitably temporarily expands its own danger zone.Whether Hamilton's theory is correct or not, there are always some important advantages to living in groups of birds, otherwise the birds would not live in groups.Whatever these advantages may be, they are at least partially lost by the first bird to fly out of the flock.So what if the disciplined bird doesn't go AWOL?Perhaps it should rely on the cover that collective power can provide and continue its activities as if nothing had happened.But the risk is too great after all.Unobstructed and vulnerable to attack.It's much safer in the trees after all.Flying into the trees is indeed a good strategy, but make sure your partners act in unison.Only in this way will it not become a lonely bird separated from the flock, so that it will not lose the advantages provided by the collective, and at the same time it can get the benefits of flying to the trees and hiding.We see here again that what is gained by sounding the alarm is purely selfish gain.A somewhat similar theory has been proposed by E. L. Charnov and Krebs, who explicitly use the word manipulation to describe the influence that the singing bird exerts on other birds.This behavior is far from pure, selfless altruism.

On the face of it, the above theories seem to contradict the claim that individuals who sound the alarm put themselves in danger.In fact there is no contradiction in it.If it does not call the police, it will put itself in greater danger.Some individuals have been killed by the alarm calls, especially those that are easily exposed to the source of the sound.Several other instances died due to failure to call the police.Why do blackbirds make alarm calls when they are in danger?Many explanations have been proposed, Cave theory and never leaving the team theory are just two of them. How about the leaping Thomson's buck?I mentioned this phenomenon in the first chapter.This apparently altruistic suicide behavior of the deer gazelle moved Ardley to assert that only group selection theory can explain this phenomenon.This project poses a more serious challenge to the theory of the selfish gene.Birds' alarm calls are effective, but they are always careful not to signal their intentions when they signal.This is not the case with the jumping of the antelope.They are posturing to the point of being annoying.It seems that the deerbuck is sincerely trying to attract the attention of the predator, sometimes it seems to be teasing the predator.This phenomenon leads to a theory that is both intriguing and quite bold.N. Smythe first proposed the outline of this theory, but it was undoubtedly Zahavi who finally developed it logically.

We can formulate Zahavi's theory in this way.The key point of this theory is that the jumping behavior of the antelope is far from a signal for other antelopes to see, but for predators to see.Of course, other kudus saw the jump, and it affected their behavior, but it was a by-product.Because this jumping behavior of the antelope was selected, mainly as a signal to predators.The general meaning of this signal is: Look!How high can I jump!I'm obviously a strong buckbuck, you can't catch me.You better be smart and catch my partner!They don't jump as high as I do.In less anthropomorphic terms, the genes that make an individual jump high and stand out are less likely to be eaten by predators, who tend to pick animals that seem easy to catch.Many mammalian predators are particularly fond of hunting down old and frail animals.An individual animal that leaps vigorously displays its youthful vigor in a boastful way.According to this theory, such boasting is not altruistic at all.We can only say that this behavior is selfish, because its purpose is to tell the predator that it should go after other animals.In a sense, it's like a high jump competition to see who can jump the highest, and the loser is the predator's chosen target. Another example I have said to explore further is the suicide of bees.It almost certainly dies when it stings a honey marauder.Bees are nothing but a highly social insect.Others are wasps, ants and termites.The object of my discussion is social insects in general, not just the death squads of bees.The track record of social insects is well known, especially for their astonishingly close coordination and apparent altruism.Their suicidal stinging mission embodies a miracle of self-restraint.In a colony of honey-pot ants there is a class of worker ants who do no other work and hang from the roof of the nest all day, motionless.Their bellies are protruding, astonishingly large, like light bulbs, filled with food.Other worker ants use them as food banks.To us humans, such worker ants no longer exist as individuals; their individuality is clearly suppressed for the collective good.The social life of ants, bees or termites embodies a higher level of individuality.The food is distributed according to such strict standards that we can even say that they share a collective stomach.They communicate with each other through chemical signals and, in the case of bees, the well-known dance.These means are so effective that the whole collective acts as if it were a unit, with its own nervous system and sense organs.They appear to be able to recognize and expel foreign invaders as selectively as the body's immune response does.Although bees are not warm-blooded, the rather high temperature inside the hive is regulated almost as precisely as the human body.Last but not least, this analogy can be extended to reproduction.In colonies of social insects, most individuals are sterile workers.The germ line is a continuous line of immortality genes running through a small number of individuals, the fertile ones.They are similar to the germ cells in our testes and ovaries.The sterile workers are similar to our liver, muscle and nerve cells. Their suicidal behavior and other forms of altruistic or cooperative behavior are less surprising once we accept the fact that workers are sterile.The reason why the body of a normal animal is manipulated is to produce offspring and to raise other individuals with the same genes to ensure the survival of its genes.Suicide for the benefit of other individuals is not compatible with producing one's own offspring in the future.Thus, suicidal self-sacrificing behaviors rarely evolve.But worker bees never produce offspring of their own.All their energies are devoted to caring for relatives who are not their own offspring so as to preserve their genes.The death of a sterile worker bee affects its own genes as a leaf falling from a tree in autumn does to the genes of a tree. Talking about social insects makes one tempted to make mysteries, but in fact there is no need for it.But it's worth looking at how the theory of the selfish gene applies to social insects, especially how it can be used to explain the evolutionary origins of the remarkable phenomenon of worker sterility.Because this phenomenon seems to cause a series of problems. A colony of social insects is a large family, all members of which are usually born to one mother.Workers seldom or never reproduce, and are generally divided into several distinct classes, including small workers, large workers, soldiers, and some highly specialized classes such as honeypot ants.Fertile females are called queens, and fertile males are sometimes called drones or kings.In some of the higher societies, the reproductive females do nothing else, but they do an excellent job at producing offspring.They rely on workers to provide them with food and protection, and workers are also responsible for caring for the larvae.In some species of ants or termites, the queen is literally a gigantic egg-laying factory, a body hundreds of times larger than a normal worker, barely moving, and hardly an insect in appearance.The queen is often cared for by the workers, who provide for the queen's daily needs, including providing food and transporting the queen's eggs to the collective nursery.If such an unusually large queen needs to leave the inner room, she has to ride on the backs of several groups of worker ants, and let them carry it out with dignity. In Chapter 7, I talked about the difference between procreation and parenting.I said that, in general, strategies that combine procreation and rearing can evolve.In Chapter 5, we saw that mixed, evolutionarily stable strategies can be divided into two types: either every individual in the population adopts a mixed strategy, so that individuals tend to combine reproduction and rearing judiciously, or the population Divide into two different types of individuals, which is what we originally imagined as a balanced situation between hawks and doves.It makes theoretical sense to achieve an evolutionarily stable balance between reproduction and rearing in the latter way.That is to say, the population can be divided into two parts: breeders and caregivers.But this evolutionary stability can only be maintained under the condition that the careee must be a close relative of the caregiver at least as close as the caregiver's own offspring would suppose it to be.Although in theory evolution could proceed in this direction, in practice it seems to be seen only in social insects. Individuals of social insects fall into two main groups: breeders and nurturers.Bearers are fertile males and females.The rearers are the sterile males and females of worker termites, and the sterile females of other social insects.These two groups of insects do not interfere with each other, so they can perform their tasks more efficiently.But the so-called effective here refers to who is effective?What benefits can workers get from it?This familiar question is a challenge to Darwin's theory. Someone replied: no good.They believe that the queen is supreme, bossy and domineering, manipulating the workers through chemical processes to satisfy their selfish desires, and driving them to raise their many children.We saw Alexander's theory of parental manipulation in Chapter 8, and what I said above is actually another formulation of this theory.A counter argument is that workers cultivate fertile mothers, driving the mothers to increase their fecundity in order to replicate worker genes.The survival machines made by the queen are definitely not descendants of worker insects, but they are all close relatives of worker insects.Hamilton had a unique insight that, at least in ants, bees, and wasps, workers may in fact be more closely related to larvae than queens are to larvae!Hamilton, and later Trivers and Hale, proceeded with this view as a guide and finally achieved one of the most brilliant achievements in the theory of the selfish gene.Their reasoning goes like this. A group of insects called the Hymenoptera, which includes ants, bees and wasps, has a rather peculiar system of sex determination.Termites do not belong to this group and therefore do not have this characteristic.In a typical Hymenoptera nest there is only one mature queen.It flies out to mate once when it is young, and stores the sperm in its body for use at any time for the rest of its long life, ten years or more.It distributes sperm to its own eggs year after year, fertilizing the eggs as they pass through the fallopian tubes.But not all eggs are fertilized.Unfertilized eggs become males.So the male has no father, and each cell in his body has only one set of chromosomes (all from the mother) instead of two sets of chromosomes like ours (one from the father and one from the mother).Following the analogy in Chapter 3, a male Hymenoptera has only one copy of each volume in each of its cells, instead of the usual two. The Hymenoptera female, on the other hand, is normal because she has a father and has the usual two sets of chromosomes in each of her body cells.Whether a female becomes a worker or a queen depends not on her genes, but on how she grows.In other words, each female has a complete set of genes for becoming a queen and a complete set of genes for becoming a worker (or in other words, there are also several sets of genes that make it a worker, soldier, etc. of various professional levels. Gene).Which set of genes plays a decisive role depends on its lifestyle, especially on the food it eats. Although the actual situation is very complicated, the basic situation is roughly the same.We don't know how this bizarre sexual reproductive system could have evolved.There is no doubt that this evolutionary phenomenon must have a cause.But we can only treat it tentatively as an incomprehensible phenomenon of the Hymenoptera, which, whatever the original reason, upsets the formula for calculating relatedness indices which we mentioned in Chapter VI. The easy way.This shows that the sperm of male insects are not different from each other like our human sperm, but are exactly the same.Males have only one set of genes per somatic cell, not two.Each sperm must therefore receive a complete set of genes, not a fraction of 50 percent, so that all sperm in a particular male are identical.Now let us calculate the kinship index between the mother and offspring of this insect. If it is known that a male has gene A in his body, what is the probability that his mother also has this gene?The answer must be 100%, because the male has no father and all its genes come from its mother.Now suppose a female is known to have gene B in her body. There is a 50 per cent chance that her son will also have this gene, since he has only received half of his mother's genes.This statement may sound like a contradiction, but it is not.Males get all their genes from their mothers, who pass on only half of their genes to their sons.The answer to this paradox is that males have only half the usual number of genes.So is the true kinship index between them one/two or one?I don't think it's necessary to worry about this problem.Indices are nothing but units of measurement that people conceive to solve problems.If its application in a particular case presents us with difficulties, we simply abandon it and reapply to the fundamental principles.From the point of view of gene A in the female, the probability that her son also has this gene is one in two.as many as its daughters.From the female's point of view, therefore, she is as closely related to her offspring as our human offspring are to their mother. But when we talk about sisters, things get complicated.Not only do siblings come from the same father, but the two sperm that impregnated their mothers are identical in every gene.Therefore, sisters are the same as identical twin sisters in terms of genes from the father.If there is gene A in a female, the gene must have come from either the father or the mother.If the gene comes from the mother, there is a fifty percent chance that its sister also has the gene.But if the gene comes from the father, the chances of its sister also having the gene are one hundred percent.Therefore, the kinship index between siblings of Hymenoptera insects is not 1/2 (normal sexual reproduction animals are 1/2) but 3/4. For this reason the Hymenoptera female is more closely related to her fellow sisters than she is to her own offspring.Hamilton saw this, even though he didn't say it so outright then.He thought it quite possible that this peculiarly close kinship prompted the female to use her mother as an efficient machine for producing her sisters.This gene, which produces sisters for females, makes copies of itself more rapidly than genes that produce their own children directly.The sterility of the worker insects is thus formed.The true gregariousness of the Hymenoptera, and the consequent sterility of the workers, seems to have evolved independently more than eleven times, while in the rest of the animal kingdom it has evolved only once in the termites.Come to think of it, this is no accident. However, there is something odd here.If the workers are to succeed in using their mother as a machine for producing sisters, they must counteract the mother's natural tendency to produce an equal number of younger brothers for her.From the worker's point of view, the chance that any of its siblings has a gene in it is only one in four.So it wouldn't necessarily be to the workers' advantage if females were able to produce the same number of fertile offspring, since then they wouldn't be able to reproduce their precious genes to the maximum. Trivers and Hare argue that workers must try to influence the sex ratio in favor of females.They applied Fisher's method of calculating optimality ratios (which we discussed in the previous chapter) to the special case of Hymenoptera and recalculated.It turns out that, for the mother, the optimal investment ratio is, as usual, one:one, but for the sisters, the optimum ratio is three:one, favoring sisters over brothers.If you are a Hymenoptera female, the most efficient way for you to reproduce your own genes is not to reproduce yourself, but to have your mother produce reproductively viable sisters and brothers for you in a ratio of three to one.But if you must reproduce yourself, it's in your genes' best interest to have an equal number of fertile sons and daughters. We have seen above that the difference between queens and workers is not genetic.Genetically speaking, a female embryo can be either a worker or a queen, with the former wanting a sex ratio of three:1 and the latter wanting a sex ratio of one:one.What exactly does hope mean?It means that the genes in the queen reproduce themselves best if they have an equal proportion of fertile sons and daughters.But the same gene in a worker reproduces itself best if it influences the worker's mother to have more daughters.Note that there is no contradiction in this statement.For the gene must make the most of all the forces at its disposal.If this gene can affect the growth process of an individual that is sure to become a queen in the future, its best strategy to use this control power is a situation; and if it can affect the growth process of a worker individual, it uses that power The optimal strategy is a different story. This means that how to use this reproductive machine has caused a conflict of interest for both parties.The queen strives to have an equal proportion of males and females.The workers work to influence the sex ratios of these fertile offspring to a ratio of three females to one male.If our assumption about workers using the queen as a reproductive machine is correct, workers should be able to achieve a three-to-one male-to-female ratio.Otherwise, if the queen really has all the power and the workers are nothing more than the queen's slaves and docile royal nursery nannies, then we should be looking at a one:one ratio, because that's what the queen would love to achieve a ratio of .In such a peculiar struggle between generations, which side can win?This problem can be proved by experiments.Trivers and Hare have done this experiment with a large number of ant species. The sex ratio of interest is the ratio of reproductive males to females.They are large, winged ants.At regular intervals, they fly out of the anthills in groups to mate.Afterwards, the young queen may have to form another colony.To estimate sex ratios it is necessary to count these winged individuals.Be aware that in many species the reproductive males and females vary in size.This situation compounded the problem.As we have seen in the previous chapter, Fisher's method of calculating the ratio of optimum fitness can only be applied strictly to the amount of investment in males and females, but not to the number of males and females. .Trivers and Hale took this situation into account, so they weighed the ants during the experiment.They used twenty different ant species and calculated sex ratios based on the investment in fertile males and females.They found that the male-to-female ratio was convincingly close to a three-to-one ratio, thus confirming the theory that workers actually run everything for their own benefit. Thus, among the species of ants studied, the worker ants seem to win this conflict of interests.This situation is not surprising, because the worker individual, as the guardian of the larvae, naturally enjoys more real power than the queen individual.Genes that try to manipulate the swarm through individual queens are no match for genes that manipulate the swarm through individual workers.Interestingly, under what special circumstances can the queen enjoy greater real power than the workers?Trivers and Hale found that it was possible to rigorously test the theory in a special case. We know that certain species of ants keep slaves.The worker ants of these slave-serving species either don't do any day-to-day work, or if they do, they're clumsy.They are good at hunting around for slaves.This kind of situation where the two armies confront each other and kill each other is only seen in humans and social insects.Among many ant species there is a special class called soldier ants.They have particularly hard and well-developed upper and lower jaws, which are sharp weapons for fighting.They attack other ant colonies exclusively for the benefit of their own group.Such raids aimed at capturing slaves were but a special form of their war effort.They attack the nest of another species, attempting to kill the defending worker or soldier ants of the other species, and eventually take away the unhatched larvae of the other species, which hatch in the predator's nest.They do not know that they have become slaves.They start to work according to the inherent neural program, and perform their duties exactly as if they were in their own caves.These slaves stay in the ant nest and take care of various daily tasks such as managing the ant nest, cleaning, collecting food, and caring for the larvae, while the worker ants or soldier ants who specialize in catching slaves continue to go out to capture more slaves. It is a good thing that these slaves are of course unaware that they are not related at all to the Queen and the larvae they tend.They unknowingly raise batch after batch of newly captured slave soldier ants.自然選擇在影響奴隸物種的基因時，無疑有利於各種反奴隸制度的適應能力。不過，這些適應能力顯然並不是十分有效的，因為奴隸制度是一種普遍現象。 從我們目前論題的觀點來看，奴隸制度產生了一種有趣的後果。在捕捉奴隸的物種中，女王現在可以使性比率朝它喜歡的方向發展。這是因為它自己所生的子女，即那些專門捕捉奴隸的螞蟻不再享有管理托兒所的實權。這種實權現在操在奴隸手中。這些奴隸以為它們在照顧自己的骨肉兄弟或姐妹。它們所做的大抵無異於它們本來在自己穴裡也同樣要做的一切，以實現它們希望達到的有利於姐妹的三：一比例。但專門擄掠奴隸的物種的女王能夠採取種種反措施，成功地扭轉這種趨勢。對奴隸起作用的自然選擇不能抵消這些反措施，因為這些奴隸同幼蟲並無親緣關係。 讓我們舉個例子來說明這種情況。假定在任何一個螞蟻物種中，女王試圖把雄性卵子加以偽裝，使其聞起來像雌性的卵子。在正常情況下，自然選擇對職蟻識破這種偽裝的任何傾向都是有利的。我們可以設想一場進化上的鬥爭的情景，女王為實現其目的不斷改變其密碼，而職蟻不斷進行破譯。在這場鬥爭中，惟能通過有生殖能力的個體把自己的基因傳遞到後代體內的數量越多，誰就能取勝。我們在上面已經看到，在正常情況下，職蟻總是取勝的一方。但在一個豢養奴隸的物種中，女王可以改變其密碼，而奴隸職蟻卻不能發展破譯的任何能力。這是因為在奴隸職蟻體內的任何一個有破譯能力的基因並不存在於任何有生殖能力的個體體內，因此不能遺傳下去。有生殖能力的個體全都是屬於豢養奴隸的物種，它們同女王而不是同奴隸有親緣關係。即使奴隸的基因有可能進入任何有生殖能力的個體體內，這些個體也是來自那些被擄掠的奴隸的老家。因此，這些奴隸最多只能忙於對另一套密碼進行破譯！由於這個緣故，在一個豢養奴隸的物種中，女王因為可以隨心所欲地變更其密碼而穩操左券，絕對沒有讓任何有破譯能力的基因進入下一代的風險。 從上面這段比較複雜的論證得出的結論是，我們應該估計到在豢養奴隸的物種中，繁殖有生殖能力的雌蟲和雄蟲的比率是一：一而不是三：一。只有在這種特殊情況下女王能夠如願以償。這就是特里弗斯和黑爾得出的結論，儘管他們僅僅觀察過兩個豢養奴隸的物種。 我必須強調指出，我在上面是按照理想的方式進行敘述的。實際生活並非如此簡單。譬如說，最為人所熟知的群居昆蟲物種蜜蜂似乎是完全違反常情的。雄蜂的數量大大超過雌蜂，無論從職蜂或從蜂后的觀點來看，這種現象都難以解釋。漢密爾頓為了揭開這個謎，他提出了一個可能的答案。他指出，當一隻女王飛離蜂房時，它總要帶走一大群隨從的職蜂，它們幫這只女王建立一個新的群體。這些職蜂從此不再返回老家，因此撫養這些職蜂的代價應該算是繁殖成本的一部分。這就是說，從蜂房每飛走一隻女王就必須培育許多額外的職蜂來補缺。對這些額外職蜂所進行的投資應算作對有生殖能力的雌蜂的投資額的部分。在計算性比率的時候，這些額外的職蜂也應在天平上稱份量，以求出雌蜂對雄蜂的比例。如果我們這樣理解問題的話，這個理論畢竟還是站得住腳的。 這個精巧的理論還有另外一個更加棘手的問題需要解決。在一些物種中，年輕的女王飛出去交配時，與之交配的雄蜂可能不止一隻。這意味著女王所生育的女兒之間的親緣關係平均指數小於三／四，在一些極端的例子裡，甚至可能接近一／四。有人把這種現象解釋為女王藉以打擊職蜂的一種巧妙的手段！不過這種看法似乎不合邏輯。附帶說一句，這似乎意味著女王飛出去交配時，職蜂應伴隨在側，只讓女王交配一次。但這樣做對這些職蜂本身的基因並沒有任何好處只有對下一代職蜂的基因有好處。每一隻職蜂所念念不忘的是它自身的基因。有些職蜂本來是願意伴隨其母親的，但它們沒有這樣的機會，因為它們當時還沒有出生。一隻飛出去交配的年輕女王是這一代職蜂的姐妹，不是它們的母親。因此，這一代職蜂是站在女王這一邊而不是站在下一代職蜂那一邊的。下一代的職蜂是她們的侄女輩。好了，說到這裡，我開始感到有點暈頭轉向。是結束這個話題的時候了。 我在描述膜翅目職蟲對其母親的行為時使用了耕耘的比喻。這塊田地就是基因田。職蟲利用它們的母親來生產它們自身的基因的拷貝，因為這樣比職蟲自己從事這項工作更富有成效。源源不斷的基因從這條生產流水線上生產出來，包裝這些基因的就是稱為有生殖能力的個體。這個耕耘的比喻不應與群居昆蟲的另外一種可以稱為耕耘的行為混為一談。群居昆蟲早就發現，在固定的地方耕種糧食作物比狩獵或搜集糧食有效得多。而人類在很久之後才發現這個真理。 譬如說，在美洲有好幾個螞蟻物種以及與這些物種完全無關的非洲白蟻都培植菌類植物園。最有名的是南美洲的陽傘蟻（parasol ants）。這種蟻的繁殖能力特別強。有人發現有的群體其個體竟超過兩百萬個之多。它們築穴於地下，複雜的甬道和迴廊四通八達，深達十英尺以上，挖出的泥土多達四十噸。地下室內設有菌類種植園地。這種螞蟻有意識地播種一種特殊品種的菌類。它們把樹葉嚼碎，作為特殊的混合肥料進行施肥。這樣，它們的職蟻不必直接搜尋糧食，只要搜集製肥用的樹葉就行了。這種群體的陽傘蟻吃樹葉的胃口大得驚人。這樣它們就成為一種主要的經濟作物害蟲。但樹葉不是它們的食糧，而是它們的菌類的食糧。菌類成熟後它們收穫食用，並用以飼養幼蟲。菌類比螞蟻的胃更能有效地消化吸收樹葉裡的物質。因此螞蟻就是通過這樣的過程而受益。菌類雖然被吃掉，但它們本身可能也得到好處，因為螞蟻促使它們增殖，比它們自己的孢子分散機制更有效。而這些螞蟻也為植物園除草，悉心照料，不讓其他品種的菌類混跡其間。由於沒有其他菌類與之競爭，螞蟻自己培植的菌類得以繁殖。我們可以說，在螞蟻和菌類之間存在某種利他行為的相互關係。值得注意的是，在與這些螞蟻完全無關的一些白蟻物種中，獨立地形成了一種非常相似的培植菌類的制度。 螞蟻有其自己的家畜和自己的農作物。蚜蟲綠蚜蟲和類似的昆蟲善於吮吸植物中的汁液。它們非常靈巧地把葉脈中的汁液吮吸乾淨，但消化這種汁液的效率卻遠沒有吸吮這種汁液的效率高，因此它們排泄出仍含有部分營養價值的液體。一滴一滴含糖豐富的蜜汁從蚜蟲的後部分泌出來，速度非常之快，有時每個蟲在一小時內就能分泌出超過其自身體重的蜜汁。在一般情況下，蜜汁像雨點一樣灑落在地面上，簡直和《舊約全書》裡提到的天賜靈糧一樣。但有好幾個物種的螞蟻會等在那裡，準備截獲蚜蟲排出的食糧。有些螞蟻會用觸角或腿撫摩蚜蟲的臀部來擠奶。蚜蟲也作出積極的反應，有時故意不排出汁液，等到螞蟻撫摩時才讓汁液滴下。如果那隻螞蟻還沒有準備好接受它的話，有時甚至把一滴汁液縮回體內。有人認為，一些蚜蟲為了更好地吸引螞蟻，其臀部經過演化已取得與螞蟻臉部相像的外形，撫摩起來的感覺也和撫摩螞蟻的臉部一樣。蚜蟲從這種關係中得到的好處顯然是，保證安全，不受其天然敵人的攻擊。像我們牧場裡的乳牛一樣，它們過著一種受到庇護的生活。由於蚜蟲經常受到蟻群的照料。它已喪失其正常的自衛手段。有的螞蟻把蚜蟲的卵子帶回地下蟻穴，妥為照顧，並飼養蚜蟲的幼蟲。最後，幼蟲長大後又輕輕地把它們送到地面上受到蟻群保護的放牧場地。 不同物種成員之間的互利關係叫做共生現象。不同物種的成員往往能相互提供許多幫助，因為它們可以利用各自不同的技能為合作關係作出貢獻。這種基本上的不對稱性能夠導致相互合作的進化上的穩定策略。蚜蟲天生一副適宜於吮吸植物汁液的口器結構，但這種口器結構不利於自衛。螞蟻不善於吮吸植物的汁液，但它們卻善於戰鬥。照料和庇護蚜蟲的螞蟻基因在基因庫中一貫處於有利地位。在蚜蟲的基因庫中，促進蚜蟲與螞蟻合作的基因也一貫處於有利地位。 互利的共生關係在動植物界中是一種普遍現象。地衣在表面上看起來同任何其他的植物個體一樣。而事實上它卻是在菌類和綠海藻之間的，而且相互關係密切的共生體。兩者相依為命，棄他就不能生存。要是它們之間的共生關係再稍微密切那麼一點的話，我們就不能再說地衣是由兩種有機體組成的了。也許世界上存在一些我們還沒有辨認出來的，由兩個或多個有機體組成的共生體。說不定我們自己就是吧！ 我們體內的每個細胞裡有許多稱為線粒體的微粒。這些線粒體是化學工廠，負責提供我們所需的大部分能量。如果沒有了線粒體，要不了幾秒鐘我們就要死亡。最近有人提出這樣的觀點，認為線粒體原來是共生微生物，在進化的早期同我們這種類型的細胞就結合在一起。對我們體內細胞中的其他一些微粒，有人也提出了類似的看法。對諸如此類的革命性論點人們需要有一段認識的過程，但現在已到了認真考慮這種論點的時候了。我估計我們終將接受這樣一個更加激進的論點：我們的每一個基因都是一個共生單位。我們自己就是龐大的共生基因的群體。當然現在還談不上證實這種論點的證據，但正如我在上面幾章中已試圖說明的那樣，我們對有性物種中基因如何活動的看法，本身其實就支持了這種論點。這個論點的另一個說法是：病毒可能就是脫離了像我們這種群體的基因。病毒純由ＤＮＡ（或與之相似的自我複製分子）所組成，外面裹著一層蛋白質。它們都是寄生的。這種說法認為，病毒是由逃離群體的叛逆基因演化而來，它們在今天通過空氣直接從一個個體轉到另一個個體，而不是借助於更尋常的載運工具精子和卵子。假設這種論點是正確的，我們完全可以把自己看成是病毒的群體！有些病毒是共生的，它們相互合作，通過精子和卵子從一個個體轉到另一個個體。這些都是普通的基因。其他一些是寄生的，它們通過一切可能的途徑從一個個體轉到另一個個體。如果寄生的ＤＮＡ通過精子和卵子轉到另一個個體，它也許就是我在第三章裡提到的那種屬於佯謬性質的多餘的ＤＮＡ。如果寄生的ＤＮＡ通過空氣或其他直接途徑轉到另一個個體，它就是我們通常所說的病毒。 但這些都是我們在以後要思考的問題。目前我們正在探討的問題是發生在更高一級關係上的共生現象，即多細胞有機體之間的而不是它們內部的共生現象。共生現象這個字眼按照傳統用法是指屬不同物種的個體之間的聯繫關係（as sociations）。不過，我們既然已經避開了物種利益的進化觀點，我們就沒有理由認為屬不同物種的個體之間的聯繫和屬同一物種的個體之間的聯繫有什麼不同。一般地說，如果各方從聯繫關係中獲得的東西比付出的東西多，這種互利的聯繫關係是能夠進化的。不管我們說的是同一群鬣狗中的個體，或者是完全不同的生物如螞蟻和蚜蟲，或者是蜜蜂和花朵，這一原則都普遍適用。事實上，要把確實是雙向的互利關係和純粹是單方面的利用區別開來可能是困難的。 如果聯繫的雙方，如結合成地衣的兩方，在提供有利於對方的東西的同時接受對方提供的有利於自身的東西，那我們對於這種互利的聯繫關係的進化在理論上就很容易想像了。但如果一方施惠於對方之後，對方卻遲遲不報答，那就要發生問題。這是因為對方在接受恩惠之後可能會變卦，到時拒不報答。這個問題的解決辦法是耐人尋味的，值得我們詳細探討。我認為，用一個假設的例子來說明問題是最好的辦法。 假設有一種非常令人厭惡的蜱寄生在某一物種的小鳥身上，而這種蜱又帶有某種危險的病菌。必須儘早消滅這些蜱。一般說來，小鳥用嘴梳理自己的羽毛時能夠把蜱剔除掉。可是有一個鳥嘴達不到的地方它的頭頂。對我們人類來說這個問題很容易解決。一個個體可能接觸不到自己的頭頂，但請朋友代勞一下是毫不費事的。如果這個朋友以後也受到寄生蟲的折磨，這時他就可以以德報德。事實上，在鳥類和哺乳類動物中，相互梳理整飾羽毛的行為是十分普遍的。 這種情況立刻產生一種直觀的意義。個體之間作出相互方便的安排是一種明智的辦法。任何具有自覺預見能力的人都能看到這一點。但我們已經學會，要對那些憑直覺看起來是明智的現象保持警覺。基因沒有預見能力。對於相互幫助行為，或相互利他行為中、做好事與報答之間相隔一段時間這種現象，自私基因的理論能夠解釋嗎？威廉斯在他一九六六年出版的書中扼要地討論過這個問題，我在前面已經提到。他得出的結論和達爾文的一樣，即延遲的相互利他行為在其個體能夠相互識別並記憶的物種中是可以進化的。特里弗斯在一九七一年對這個問題作了進一步的探討。但當他進行有關這方面的寫作時，他還沒有看到史密斯提出的有關進化上穩定策略的概念。如果他那時已經看到的話，我估計他是會加以利用的，因為這個概念很自然地表達了他的思想。他提到俘虜的窘境博弈論中一個人們特別喜愛的難題，這說明他當時的思路和史密斯的已不謀而合。 假設B頭上有一隻寄生蟲。A為它剔除掉。不久以後，A頭上也有了寄生蟲。A當然去找B，希望B也為它剔除掉，作為報答。結果B嗤之以鼻，掉頭就走。B是個騙子。這種騙子接受了別人的恩惠，但不感恩圖報，或者即使有所報答，但做得也很不夠。和不分青紅皂白的利他行為者相比，騙子的收穫要大，因為它不花任何代價。當然，別人為我剔除掉危險的寄生蟲是件大好事，而我為別人梳理整飾一下頭部只不過是小事一樁，但畢竟也要付出一些代價，還是要花費一些寶貴的精力和時間。 假設種群中的個體採取兩種策略中的任何一種。和史密斯所做的分析一樣，我們所說的策略不是指有意識的策略，而是指由基因安排的無意識的行為程序。我們姑且把這兩種策略分別稱為傻瓜和騙子。傻瓜為任何人梳理整飾頭部，不問對象只要對方需要。騙子接受傻瓜的利他行為，但卻不為別人梳理整飾頭部，即使別人以前為它整飾過也不報答。像鷹和鴿的例子那樣，我們隨意決定一些計算得失的分數。至於準確的價值是多少，那是無關緊要的，只要被整飾者得到的好處大於整飾者花費的代價就行。在寄生蟲猖獗的情況下，一個傻瓜種群中的任何一個傻瓜都可以指望別人為它整飾的次數和它為別人整飾的次數大約相等。因此，在傻瓜種群中，任何一個傻瓜的平均得分是正數。事實上，這些傻瓜都幹得很出色，傻瓜這個稱號看來似乎對它們不太適合。現在假設種群中出現了一個騙子。由於它是唯一的騙子手，它可以指望別人都為它效勞，而它從不報答別人給它的好處。它的平均得分因而比任何一個傻瓜都高。騙子基因在種群中開始擴散開來。傻瓜基因很快就要被擠掉。這是因為騙子總歸勝過傻瓜，不管它們在種群中的比例如何。譬如說，種群裡傻瓜和騙子各佔一半，在這樣的種群裡，傻瓜和騙子的平均得分都低於全部由傻瓜組成的種群裡任何一個個體。不過，騙子的境遇還是比傻瓜好些，因為騙子只管撈好處而從不付出任何代價，所不同的只是這些好處有時多些，有時少些而已。當種群中騙子所佔的比例達到百分之九十時，所有個體的平均得分變得很低：不管騙子也好，傻瓜也好，它們很多因患蜱所帶來的傳染病而死亡。即使是這樣，騙子還是比傻瓜合算。那怕整個種群瀕於滅絕，傻瓜的情況永遠不會比騙子好。因此，如果我們考慮的只限於這兩種策略，沒有什麼東西能夠阻止傻瓜的滅絕，而且整個種群大概也難逃覆滅的厄運。 現在讓我們假設還有第三種稱為斤斤計較者的策略。斤斤計較者願意為沒有打過交道的個體整飾。而且為它整飾過的個體，它更不忘記報答。可是哪個騙了它，它就要牢記在心，以後不肯再為這個騙子服務。在由斤斤計較者和傻瓜組成的種群中，前者和後者混在一起，難以分辨。兩者都為別人做好事，兩者的平均得分都同樣高。在一個騙子佔多數的種群中，一個孤單的斤斤計較者不能取得多大的成功。它會化掉很大的精力去為它遇到的大多數個體整飾一番由於它願意為從未打過交道的個體服務，它要等到它為每一個個體都服務過一次才能罷休。因為除它以外都是騙子，因此沒有誰願意為它服務，它也不會上第二次當。如果斤斤計較者少於騙子，斤斤計較者的基因就要滅絕。可是，斤斤計較者一旦能夠使自己的隊伍擴大到一定的比例，它們遇到自己人的機會就越來越大，甚至足以抵消它們為騙子效勞而浪費掉的精力。在達到這個臨界比例之後，它們的平均得分就比騙子高，從而加速騙子的滅亡。當騙子尚未全部滅絕之前，它們滅亡的速度會緩慢下來，在一個相當長的時期內成為少數派。因為對已經為數很少的騙子來說，它們再度碰上同一個斤斤計較者的機會很小。因此，這個種群中對某一個騙子懷恨在心的個體是不多的。 我在描述這幾種策略時好像給人以這樣的印象：憑直覺就可以預見到情況會如何發展。其實，這一切並不是如此顯而易見。為了避免出差錯，我在計算機上模擬了整個事物發展的過程，證實這種直覺是正確的。斤斤計較的策略證明是一種進化上穩定的策略，斤斤計較者優越於騙子或傻瓜，因為在斤斤計較者佔多數的種群中，騙子或傻瓜都難以逞強。不過騙子也是ESS，因為在騙子佔多數的種群中，斤斤計較者或傻瓜也難以逞強。一個種群可以處於這兩個ESS中的任何一個狀態。在較長的一個時期內，種群中的這兩個ESS可能交替取得優勢。按照得分的確切價值用於模擬的假定價值當然是隨意決定的這兩種穩定狀態中的一種具有一個較大的引力區，因此這種穩定狀態因而易於實現。值得注意的是，儘管一個騙子的種群可能比一個斤斤計較者的種群更易於滅絕，但這並不影響前者作為ESS所處的地位。如果一個種群所處的ESS地位最終還是驅使它走上滅絕的道路，那麼抱歉得很，它捨此別無他途。 觀看計算機進行模擬是很有意思的。模擬開始時傻瓜佔大多數，斤斤計較者佔少數，但正好在臨界頻率之上；騙子也屬少數，與斤斤計較者的比例相仿。騙子對傻瓜進行的無情剝削首先在傻瓜種群中觸發了劇烈的崩潰。騙子激增，隨著最後一個傻瓜的死去而達到高峰。但騙子還要應付斤斤計較者。在傻瓜急劇減少時，斤斤計較者在日益取得優勢的騙子的打擊下也緩慢地減少，但仍能勉強地維持下去。在最後一個傻瓜死去之後。騙子不再能夠跟以前一樣那麼隨心所欲地進行自私的剝削。斤斤計較者在抗拒騙子剝削的情況下開始緩慢地增加，並逐漸取得穩步上升的勢頭。接著斤斤計較者突然激增，騙子從此處於劣勢並逐漸接近滅絕的邊緣。由於處於少數派的有利地位同時因而受到斤斤計較者懷恨的機會相對地減少，騙子這時得以苟延殘喘。不過，騙子的覆滅是不可挽回的。它們最終慢慢地相繼死去，留下斤斤計較者獨佔整個種群。說起來似乎有點自相矛盾，在最初階段，傻瓜的存在實際上威脅到斤斤計較者的生存，因為傻瓜的存在帶來了騙子的短暫的繁榮。 附帶說一句，我在假設的例子中提到的不相互整飾的危險性並不是虛構的。處於隔離狀態的老鼠往往在舌頭舔不到的頭部長出瘡來。有一次試驗表明，群居的老鼠沒有這種毛病，因為它們相互舔對方的頭部。為了證實相互利他行為的理論是正確的，我們可以進行有趣的試驗，而老鼠又似乎是適合於這種試驗的對象。 特里弗斯討論過清潔工魚（cleaner fish）的奇怪的共生現象。已知有五十個物種，其中包括小魚和小蝦，靠為其他物種的大魚清除身上的寄生蟲來維持生活。大魚顯然因為有人代勞，為它們做清潔工作而得到好處，而做清潔工的魚蝦同時可以從中獲得大量食物。這樣的關係就是共生關係。在許多情況下，大魚張大嘴巴，讓清潔工游入嘴內，為它們剔牙，然後通過魚鰓游出，順便把魚鰓也打掃乾淨。有人認為，狡猾的大魚完全可以等清潔工打掃完畢之後把它吞掉。不過在一般情況下，大魚總是讓清潔工游出，碰都不碰它一下。這顯然是一種難能可貴的利他行為。因為大魚平日吞食的小魚小蝦就和清潔工一樣大小。 清潔工魚具有特殊的條紋和特殊的舞姿，作為清潔工魚的標記。大魚往往不吃具有這種條紋的小魚，也不吃以這樣的舞姿接近它們的小魚。相反，它們一動不動，像進入了昏睡狀態一樣，讓清潔工無拘無束地打掃它們的外部和內部。出於自私基因的稟性，不擇手段的騙子總是乘虛而入。有些物種的小魚活像清潔工，也學會了清潔工的舞姿以便安全地接近大魚。當大魚進入它們預期的昏睡狀態之後，騙子不是為大魚清除寄生蟲，而是咬掉一大塊魚鰭，掉頭溜之大吉。但儘管騙子乘機搗亂，清潔工魚和它們為之服務的大魚之間的關係，一般地說，還是融洽的，穩定的。清潔工魚的活動在珊瑚礁群落的日常生活中起著重要的作用。每一條清潔工魚有其自己的領地。有人看見過一些大魚像理髮店裡排隊等候理髮的顧客一樣排著隊伍，等候清潔工依次為它們搞清潔工作。這種堅持在固定地點活動的習性可能就是延遲的相互利他行為形成的原因。大魚能夠一再惠顧同一所理髮店而不必每次都要尋找新的清潔工，因此，大魚肯定感覺到這樣做要比吃掉清潔工好處大。清潔工魚本來都是些小魚，因此這種情況是不難理解的。當然，模仿清潔工的騙子可能間接地危害到真正的清潔工的利益，因為這種欺騙行為產生了一些壓力，迫使大魚吃掉一些帶有條紋的、具有清潔工那種舞姿的小魚。真正的清潔工魚堅持在固定地點營業，這樣，它們的顧客就能找上門來，同時又可以避開騙子了。 當我們把相互利他行為的概念運用於我們自己的物種時，我們對這種概念可能產生的各種後果可以進行無窮無盡的耐人尋味的猜測。儘管我也很想談談自己的看法，可是我的想像力並不比你們強。我想還是讓讀者自己以此自娛吧！