Introduction

How do replicators fit into evolution? What part do they play? The idea of replicators has drastically changed the traditional view of evolutionary theory. In this section we will explore the effects of replicator theory on evolutionary thinking, examine replicators' resolution of apparent paradoxes, and study a form of life that is little but a replicating machine: a virus.

Replicator Theory in Natural Selection

As established in the previous section Natural Selection, replicator theory implies selection at the level of the replicator, not that of the vehicle - in other words, selection operates on genes, not organisms. Of course, this cannot occur directly - natural selection operates based on phenotypes (outward appearances) rather than genotypes. But the phenotypes merely act as proxy measures for the real replicator, the gene.

This view of natural selection has major implications for the interpretation of adaptations. The traditional way of interpreting adaptations is to assume that they are for the benefit of the organism as a whole. Some go even farther to say that adaptations are for the benefit of the species, and may cause harm to individual organisms as long as they increase the overall fitness of the species as a whole.

However, replicator-based selection has a different story to tell. It implies that adaptations are for the good of the replicator, not the organism. In other words, adaptations benefit the genes first and the organism second. In most cases, the interests of the genes will coincide with those of the organisms, and the adaptation that benefits one will also benefit the other.

Despite its commonplace nature, this scenario is not always the case. Consider a simple example involving a decision on whether or not to have offspring. Suppose the animal is much more likely to live longer if it does not have any children, because it will be free to expend its resources exclusively on its own survival. If it does have children, it will live a shorter life because it will be forced to expend resources on its children as well as itself. If adaptations were exclusively for the benefit of the organism, this animal would have an instinct to preserve its own life even at the cost of an opportunity to reproduce. If adaptations were for the benefit - and survival - of the gene, this animal would have an instinct to reproduce even at the expense of its own survival, because the animal's genes would survive even if the animal did not. Since we can clearly observe animals reproducing in nature, even under adverse conditions, it seems quite evident that these adaptations are for the benefit of the genes and not the organism.

Genetic Interests: The Resolution of Paradoxes

Sexual reproduction and "junk DNA" are both considered paradoxes under the traditional view of evolution. However, both can be resolved or at least explained by the replicator-level selection.

At first glance, sexual reproduction appears to be a paradox if viewed in the organism-level interpretation. An organism that simply "buds" or otherwise reproduces asexually will pass on all of its genes to the next generation. By contrast, an organism that reproduces sexually only passes on ½ of its genes to its offspring. This once led many theorists into a group-selectionist interpretation of sex; the beneficial effects of genetic recombination on the success of the species as a whole are easily seen. However, group selection as a theory is generally lacking in supportive evidence, and no workable proposal of its mechanism has ever been introduced.

This apparent paradox is much more easily understood if viewed from the point of view of a replicator. The organism's efficiency in reproducing its own genetic makeup is irrelevant. If one views sexual and asexual reproduction as a simple genetic alternative, then the paradox recedes: sexual reproduction obviously is a benefit to a gene for sexual reproduction, and therefore sexual reproduction exists. As Richard Dawkins put it in The Selfish Gene, "A gene 'for' sexuality manipulates all the other genes for its own selfish ends." (p.44).

This argument applies equally well to other genes that appear to be detrimental to the organism's maximal efficiency of reproduction. Genes that increase the rates of copying errors made in the transcription of other genes obviously do not benefit the genes they impede, or the organism as a whole. But if their activities benefit themselves, these "mutators" will spread through the gene pool. (The same applies to genes for crossing-over during meiosis.)

This argument also works quite well in explaining the existence of introns, or "junk DNA" that is never translated into protein and thus has no effect on the phenotype of an organism. These DNA sequences are examples of replicators that take advantage of other replicators' mechanisms of survival and reproduction (organisms) to their own benefit. Again as Dawkins put it, "If the 'purpose' of DNA is to supervise the building of bodies, it is surprising to find a large quantity of DNA which does no such thing. . . . [but] the true 'purpose' of DNA is to survive, no more and no less. The simplest way to explain the surplus DNA is to suppose that it is a parasite, or at best a harmless but useless passenger, hitching a ride in the survival machines [organisms] created by other DNA." (p.45).

Reciprocal Altruism

Another apparent paradox resolved by replicator theory is the problem of reciprocal altruism - the delayed exchange of favors between two nonkin animals. Reciprocal altruism looks like a paradox because it requires an animal to voluntarily do a favor for another animal without a guarantee that the favor will be returned later. Clearly the cost of doing favors and having none returned is very great, and the benefit of never returning favors is also vast. Obviously populations with reciprocal altruism will survive better than those without it, but how could it evolve in the first place, amid groups of potential cheaters?

Reciprocal altruism is now thought to have evolved from kin selection (see Other Types of Selection). It can only evolve in populations where favors might need to be exchanged, and in animals who can recognize other individuals and recall their past behavior. In species that have reciprocal altruism, the net gain of exchanging favors with another animal generally outweighs the cost of doing favors. This consequence arises from the exchange's status as a non-zero-sum game; that is, a situation in which both parties can come out ahead and one animal's gain is not another's loss. Game theory - the discipline from which the term is derived - is often used to study reciprocal altruism and its evolution.

The classic example of reciprocal altruism is the vampire bat. Vampire bats need meals of blood in order to survive, but not all bats are successful every night at obtaining a blood meal. Moreover, a single blood meal can often be far more than one bat can eat alone. The conditions are ripe for reciprocal altruism, and that is indeed what is observed. Bats who obtain a meal of blood often share with other less successful bats, who then return the favor when the tables are turned.

Reciprocal altruism is a good example of what seems to be a paradox all around, for it does not really directly benefit replicators as kin selection does. However, replicators that build vehicles that recognize one another and exchange favors are more likely to survive, on average, than replicators that build stingy, selfish vehicles.

Reciprocal altruism in primates has led to a number of effective adaptations ensuring honest trades. The emotions of indignation at being cheated and satisfaction with a good deal both arise from reciprocal altruism in primate species. Indeed, the human sense of justice probably has its roots in ensuring profitable and honest reciprocal altruism among early human tribes.

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