IntroductionHi, I'm Tim Tyler - and today I will be discussing the idea of genetic takeovers.
IntroductionA. G. Cairns-Smith has proposed a mechanism by which evolution may produce dramatic and rapid changes in the genetic substrate of organisms. His theory illuminates one aspect of how today's high-technology mechanisms of heredity are likely to have evolved.
The idea is perhaps best explained by a diagram:
Arrows within the organisms running from genotype to phenotype indicate paths of genetic expression.
A simple organism with genetic substrate
The principle of genetic continuityBefore this mechanism was clearly elucidated, it was widely believed that the principle of genetic continuity would rule out many types of dramatic changes to the genotype. It seemed that any modification would require the old genotype to still be readable by the new genetic machinery - in order to maintain a continuous line of descent of viable organisms.
Consequently, it seemed as though primitive genetic materials would be related to the modern genetic machinery in such a way that the two could be linked by a series of gradual changes.
The mechanism of genetic takeover allows for the possibility that a secondary genetic material can arise not as a modification of the primary one, but rather from molecules synthesized under its control.
Takeovers are commonThe general idea of a takeover is actually ubiquitous in evolution.
The many discontinuities in the fossil record are caused by invasions - where organisms are displaced by organisms that have evolved elsewhere. We can see similar events going on today in New Zealand and Australia.
Clay mineralsThe possibility of genetic takeovers allows for extremely dramatic changes in the genotype without violating pure Darwinian gradualism. It consequently provides a mechanism which allows much greater freedom when considering the possibile candidates for the role of the very first genetic substrate.
This resulting liberation allowed Cairns-Smith to propose that the first living organisms were in fact extremely different in their construction from the type of organism we are now familiar with.
In fact, he proposed that our ultimate ancestors were actually clay mineral crystals.
Regardless of whether his theory of Crystalline Ancestry is correct, it still seems highly likely that multiple genetic takeovers were involved in the origin of today's genetic machinery.
Evolution of the genetic substrateA genetic takeover provides a clear scenario in which an organism contains more than one type of heritable material.
Initially the second genetic material is not critical to the organism, and may thus be subject to variation and natural selection - without changes necessarily resulting in a non-viable organism.
A booster rocketA cursory look at the diagram suggests that there is no obvious mechanism which allows for information to be transferred between the two genetic substrates. The second substrate has to "start from scratch" in coming to describe the organism.
A sceptic might argue that this is just as difficult as the secondary genetic material arising from scratch out of inorganic matter - but this is not the case.
The second substrate can be a thermodynamically unlikely object, whose very existence would be implausible in the absence of an existing evolutionary process and natural selection. So, the first genetic material can play a vitally important role in the production of its sucessor.
In much the same way that a spaceship traveling into space requires a booster rocket to get it out of the atmosphere, so early life required a boost in order to get off the ground in the first place. This is because the modern genetic mechanisms are too sophisticated to plausibly form under pre-biotic conditions.
In the same way that a spaceship discards its booster rocket as dead weight, so the initial genetic machinery appears to have been completely discarded now that a superior, high-technology alternative has been developed.
Information transferWhile an immediate form of translation from the language of the primary genetic material to the language of the secondary genetic material need not exist, it is still possible for a significant quantity of information to be transmitted across the divide between them.
Essentially, the primary genome provides the environment in which the secondary genome evolves.
Since the components in the original phenotype are replaced gradually, there is likely to be a functional relationship between the original components and their replacements - since the elements are replaced one at a time.
It is important to emphasize the fact that there can be ways to transfer information from the old genetic medium to the new one - even if there is no specific apparatus that can read from the old medium and write to the new one.
The idea that the new life is necessarily the mortal enemy of its ancestors not necessarily correct - in a very real sense, some of the heritable information of the ancestors can survive in their descendants.
SignificanceGenes lie at the heart of modern organisms - and a change to its genetic substrate is one of the most dramatic events that can happen to a lineage.
We do not know exactly how many genetic takeovers there were, but it seems likely that there were several - and that they were critical milestones on the road to modern orgainsms.
Despite the pivotal nature of genetic takeovers, their significance is not recognised by most modern literature on the subject.
The modern takeoverThe idea of a genetic takeover is currently a topical one - since the first genetic takeover for over three billion years appears to be in progress.
There have not been any significant changes to the genetic substrate for a very long time - however there are currently a number of indications that a genetic takeover is imminent:
An examination of the communication of high-fidelity information between organisms and their descendants reveals that almost all the information transmitted over the history of life on earth has been stored in nucleic acids.
However very recently, various new methods of transmitting information down the generations have arisen. These have the high-fidelity of replication necessary to be able to support evolutionary processes, and are capable of transmitting large volumes of information - and this information persists via a copying process and is inherited from one generation to the next.
Until human beings came on the scene "cultural" transmission of information existed - but was much more limited. For example, a bird's offspring may inherit the songs of their parents - but probably only a small number of generations will pass before it is not possible to identify the parents from the songs of their descendants.
By contrast, human beings have brought with them the written word and - more recently - books, CDs, DVDs and other optical, electro-magnetic and electronic storage media. The result is a large volume of heritable, high-fidelity information which is not transmitted through nucleic acids.
The ultimate effect of these types of new information storage media on biological evolution seems likely to be extremely far-reaching.
The possibility of a modern genetic takeover has been pointed out by Richard Dawkins and Hans Moravec.
I have discussed the possibility of a modern takeover in more detail elsewhere - in my video/essay Memetic Takeover.
ConclusionIt appears that genetic takeovers are a fundamental - though currently little-understood - aspect of the process of evolution.
The fact that the takeovers near the start of life are very distant from us has obscured their significance as evolutionary events.
Whenever evolution develops new and superior information-storage devices there will be an immediate "pressure" for genetic information to migrate into them. This process will continue for as long as superior information-storage technologies exist.
The information storage aspect of the medium will probably become increasingly abstracted from the replication mechanism, and from the phenotypic expression of the information it carries - so in the future, upgrades to the materials that carry heritable information seem likely to be relatively painless.
In the light of the fact that we may now be facing the first genetic takeover for several billion years, some interest in the mechanisms behind them would appear to be appropriate.
ReferencesFor a page of references to Cairns-Smith's theories see here.
My original esssay on genetic takeovers;
An esssay on genetic takeover types;