Stochastic corrector

Here I discuss the relevance of this model to the origin of life.

Motivation for the model

It deals with the problem of how information could be transmitted to offspring - in the form of the ratio between the constituent replicators - despite the fact that these replicators are not physically joined together - thus:

• Stochastic forces present when the cell divides distributing them unevently in offspring;
• Differences in the rate of reproduction of each sort of replicator within the cell;

Explanation of the model

Organisms are composed of a small number of different types of self-replicating agents: R1 and R2.

The organisms work best if there is some specified ratio between the replicators: in the diagram the preferred ratio is 1:1.

The cells grow and eventually split into two - with the replicators being distributed randomly between the offspring.

Then selection acts - destroying any cells that deviate too far from the optimal ratio of replicators within each cell.

The result is that the information content represented by the replicators - and their proportions in the cell - is preserved between generations.

The model can still work - even if some of the replicators reproduce faster than other ones - and thus there is between-replicator selection within each cell.

Good points

Not so good points

Rewriting the model

Rather than consider the diagram as representing different sorts of replicator in a cell, consider it as representing different sorts of replicator in an ecosystem:

For example, if you have a "pool" filled with replicators, then the species in it may colonise another pool downstream.

If a key species gets wiped out - or fails to get transmitted to the new environment - then the new ecosystem will not flourish.

The idea that the proportion of replicators in each "cell" is somehow significant is abandoned in this model. That information is no longer strongly inherited.

However, the collective genomes of all the important species still get transmitted to the next generation - i.e. information about the existence of the different species is preserved.

This phrasing retains a key feature of the model - namely the possibilty of inheriting a lot more information than is present in any individual replicator.

It also still leaves open the possibility of a community of mutually-dependent organisms surviving in an environment where none of them could exist alone.

Overcoming differential reproductive rates

Szathmáry's original model invoked a kind of group selection to explain how the system could overcome this problem - suggesting the selection between the ecosystems would act to penalise ecosystems where one replicator does too well.

Selection between ecosystems is likely to have been an important factor.

Some other points may also be worth mentioning, though:

• Independent niches
  • If the different replicators do not compete significantly for resources then they may be able to co- exist peacefully - without one wiping the other out.
    Niches might be partially independent in an ecosystem - the replicators in the system:
    • ...may have different nutrient needs;
    • ...may exist in different areas of the ecosystem.
• Frequency-dependent selection
  • It's in all the replicators interests to prevent the population of any one replicator from dropping too low - since the loss of an essential player is terminal for everyone involved.
    This fact tends to favour cycles involving frequency-dependent selection - where the least-numerous critical replicator is preferentially synthesized.
    
• Mutual dependency
  • It's possible that one replicator may depend on the products of another one to successfully reproduce.
    Such dependencies may produce frequency-dependent selection that favours the scarce replicators.

The membrane-free corrector

Membranous material is unlikely to be involved in the earliest living systems - since the organic material that composes most membranes tends to form sticky messes - that are incompatible with the process of crystallisation that is likely to be responsible for the replication of the genomes of the first organisms.

Notes

References

• Eörs Szathmáry and J. Maynard Smith - The Major Transitions in Evolution. Oxford, 1995.

• Eörs Szathmáry and J. Maynard Smith - The Origins of Life, Oxford University Press, 1999;

• Eörs Szathmáry and László Demeter - Group selection of early replicators and the origin of life. Journal of Theoretical Biology 128, 463-486, 1987;