"index.html" >

Home Biological Evolution Cultural Evolution Languages Philosophy History of Evolution Interactivities Evolution FAQ's Replicators Forum Guestbook About Us Replicators List (disabled by ThinkQuest) subscribe unsubscribe Sample article

Introduction: What is a Replicator?

In his famous books The Selfish Gene and The Extended Phenotype, Richard Dawkins presented his theory of replicators. He defines replicators as "anything in the universe of which copies are made" (This edition of The Extended Phenotype, p. 83). DNA sequences are the most immediately obvious examples of replicators, since they are copied many times as they are expressed and as cells undergo division. Replicators have two cross-sectional classifications: they may be passive or active, and dead-end or germ-line.

Passive and Active Replicators

The first distinction between types of replicators is the distinction between passive and active replicators. Passive replicators are those whose particular content has no effect on their likelihood of being copied. Conversely, active replicators are those whose content or message affects their likelihood of being copied (and those copies' likelihoods of being copied). Examples might be DNA sequences that are never transcribed or translated into protein (though this is still under debate).

Dead-end and Germ-line Replicators

The second distinction between types of replicators is the distinction between dead-end and germ-line replicators. Dead-end replicators are those which may be copied a finite number of times, but have no expectation of a (potentially) infinite line of descendants. Conversely, germ-line replicators are those which have the potential to give rise to an infinitely long line of descent. Germ-line replicators are not guaranteed infinite descent, but the potential does exist. If it is a passive replicator, its realization of that potential is mainly based on chance; however, if it is an active replicator, then its content affects its likelihood of realizing that potential (more on this later).

Biography of a Successful Replicator

Dawkins defines a successful replicator as having the following three qualities: longevity, fecundity, and fidelity (This edition of The Extended Phenotype, p. 84). These three characteristics are defined as follows:

• Longevity:
  A physical replicator does not need to last forever, but it must exist long enough to be replicated. The longer a replicator exists, the more likely it is to be copied and the more time it has to make more copies.
• Fecundity:
  A replicator needs to make a certain number of copies to persist as a replicator. The more copies made by a replicator, the more successful it will be in out-replicating others.
• Fidelity:
  A replicator needs to have a certain degree of accuracy in its copies. The more accurate the copies, the more likely the replicator to be successful. However, it is copying errors (slight breakdowns in fidelity, i.e., mutations) that drive evolution.

Of course, there are tradeoffs between these three attributes of a successful replicator. For example, often a replicator must "choose" between greater fecundity (more copies) and greater longevity (longer survival). Obviously more copies works to the replicator's advantage, but longer survival means more time in which to make additional copies. Stable balances between these three attributes must be found by replicators in nature, or else the replicators will become extinct.

The Replicator and the Vehicle

Some replicators could simply self-propagate given certain chemical environments. For example, a DNA molecule immersed in the proper chemicals (nucleotides and enzymes) can make copies of itself; this is the basis behind modern genetics techniques such as PCR. However, the world is not always kind enough to provide the appropriate chemical environments to replicators. Therefore, replicators began to create vehicles, or entities built by replicators to promote their propagation. Organisms are vehicles - they are built by teams of replicators (genes) to promote the replicators' survival and reproduction. Therefore, adaptations assist in the survival of the organism not for the survival of the organism itself, but rather for the survival of its replicators.

The Optimon

In The Extended Phenotype, Dawkins defines the term optimon as an active germ-line replicator. It is this entity, he argues, that is the unit of natural selection and the primary beneficiary of all adaptations. As stated above, replicators build vehicles - individual organisms - to further their propagation. Since the organism is built for the benefit of its replicators, clearly any adaptation is also for the benefit of the replicators. An adaptation enhancing an organism's survival and reproduction also enhances the survival and reproduction of the replicators.

The optimon, or active germ-line replicator, is also the unit of natural selection in Dawkins' theory (William Hamilton and George Williams both also upheld this principle, but developed it in different directions). Natural selection obviously cannot choose directly among replicators, so it uses phenotypic manifestations of optimons (i.e., adaptations) as criteria. Natural selection uses phenotypes as proxy measures for selection among optimons, for whose benefit adaptations really exist.

Looking Further: Links and References

The following links and references will be helpful in the study of genes as replicators and natural selection of replicators.

• Books
  • The Extended Phenotype by Richard Dawkins
  • The Selfish Gene by Richard Dawkins

• Websites
  • The World of Dawkins (unofficial)
  • Replicator Characteristics

Biological Evolution | Cultural Evolution | Languages Philosophy | History of Evolution | Interactivities FAQ | Forum | Guestbook | References | About Us Home | Search | Index | Features | Newsflash! Site News | Our Awards | New!  Featured Links

	Copyright © 2000 Team C004367