Replicators in Action

For a good explanation of the actual concrete effects of replicator-based survival on a population, examine the much-used example of bird grooming. In the standard formulation of the example, a certain species of bird is parasitized by a dangerous insect. The birds can groom themselves to remove the insects, but they cannot reach the tops of their own heads. As a result, an instinct evolves by which birds will groom each other's heads, thus keeping the entire population tick-free. Consider two alternate grooming strategies, designated by Dawkins as "sucker" and "cheat" (Selfish Gene p. 184). A sucker always grooms an outstretched head regardless of who it belongs to; a cheat never grooms anyone else but presents its own head for grooming.

In a population made up entirely of suckers, each would do quite well and the entire population would be insect-free. But if a cheat arose in the population, he would get the benefits of being groomed without the costs of grooming others. His cheating genes would enjoy a selection advantage over the sucker genes in the rest of the population. In time, the population would be dominated by cheats, and there would be few birds left who would groom. Even if the population were threatened with extinction due to the effects of the insects, the cheating genes would still have an advantage over the sucker genes.

A strategy that could prevail over both of these is a "tit-for-tat" strategy that could recognize specific individuals. "Tit-for-tat" individuals would groom those who groom them, but not those that have refused to groom in the past. Genes coding for this behavior could outcompete either of the first two strategies. (It is significant to note that this strategy could not outcompete the cheating strategy in a population consisting largely of cheats. Though "tit-for-tat" is clearly more beneficial to the organisms involved, cheating would still win out in certain situations, even if extinction was the end result.)

This is a clear example of selection taking place on the level of the replicator, not the organism or the species. Obviously, selection in this case cannot be species-centered, because the population could be driven extinct by the outcome. Similarly, an organism does not derive benefit from infestation by insects, or from the extinction of its species, so selection is not organism-centered in this case either. Clearly this example shows replicator-centered selection in action. (It should be noted that replicators, of course, cannot see their own future. Even a strategy that will ultimately lead to doom, such as cheating, can have immediate payoffs.)

Viruses: Replicating Machines

Viruses are the closest known forms of life to actual, pure replicators. They consist only of a DNA or RNA genome (the replicator part) and a protein coat (the vehicle). They require living cells' replicating machinery for their own reproduction, so they are essentially parasites very similar to the "junk DNA" in humans' and other organisms' genomes.

Virus life cycles are very simple - they invade a living cell, co-opt its replication machinery to manufacture more viral genome, use that code to make more viral protein, and then burst out of the cell in a viral torrent, seeking more cells to infect. Some viruses, called retroviruses, first use an enzyme called reverse transcriptase to convert their RNA genome into DNA with which to commandeer the cell. This provides a hint that the original replicator on earth may have been RNA, not DNA - obviously the transcription/translation system and its progress are not so immutable as once thought.

Some viruses actually insert their genome into that of the cell, causing it to manufacture new viruses some time later than the original infection. This is another possible origin of "junk DNA" - viral inserts that are now not translated into protein, but once represented ancient infections. These viruses are particularly efficient replicators because new copies are made each time the cell divides, providing many more cells which will later erupt with virus. The cell division in between infection and active replication greatly increases the potential yield of new viruses per single cell infected.

Why Viruses are not Really Replicators

Viruses are the closest living thing to being a true replicator. However, they fail to pass an essential test - viruses are not replicators because they possess vehicles, that is, their protein coats. Chemical changes in the conformational structure of the protein are not passed down to a new generation of viruses unless the basis of the change is a mutation. For this reason, viruses are not really replicators, but instead are very tiny vehicles with replicators inside.

Viruses cannot have been the first forms of proto-life either, as is sometimes implied. Viruses are parasitic and require cellular replication machinery to replicate themselves, and therefore cannot be ancestral to cellular life. However, they may resemble early proto-life in some respects - they are nearly (but not exactly) pure replicators, and their vehicle is very small and simple. Precursors to modern life may have resembled viruses, but replicated through co-opting chemicals in the environment rather than cellular machinery.

Viral Evolution

Viruses are remarkable in one respect: their extremely rapid rate of evolution. Viruses lack the proofreading machinery possessed by many cells, so any mistakes made in copying their genomes are not corrected. This extremely high mutation rate has major consequences, allowing viral evolution to keep pace with human activities. For example, viruses can easily evolve around human disease countermeasures such as vaccines; this is why new flu vaccines must be made each winter.

In fact, viral evolution is so phenomenally fast that an HIV-positive individual may be infected with a variety of different variations of HIV, yet have only one or a few just a few months later. Similarly, certain AIDS drugs sometimes have difficulties controlling the disease because the viruses evolved in response to them, rendering them ineffective after a certain period of exposure.

Links and References: Looking Further

The following resources will be helpful in studying genes and replicators.

• Books
• The Extended Phenotype by Richard Dawkins
• The Selfish Gene by Richard Dawkins
• Websites
• The World of Dawkins (unofficial)
• Sociobiology by C. George Boeree
• Tracing HIV Evolution

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