Thursday, March 16, 2006

 

Re: The evolution of adaptations (Waddington) (2)

Preamble (for the benefit of new/future visitors): The concept of an internal evolutionary mechanism originates from the 'anomalies' referred to in " A Model of an Internal Evolutionary Mechanism and reflects a known single state.

There is a hesitation in extrapolating beyond this 'start condition', partially because the current mathematical model can only represent a single trajectory (see An Internal Evolutionary Mechanism and 'Direction in Evolution': Preliminary Notes) irrespective of how it is applied, but also because further development requires insight and research as much as 'logic'. For this reason the proposed mechanism will continue to be described as a 'localized area of equilibrium at the apex of an homeostatic hierarchy' (rather than, for example, any assignation along the lines of 'organs, tissues, and cells' to progressively lower levels within the hierarchy).

Main Content:

Today's extract from "The evolution of adaptations" is slightly shorter than yesterday's: the emphasis this time is on Waddington's description of the process whereby the callosities of ostriches become hereditary (rather than the intellectual strategy whereby the 'incredible' was reduced to the 'credible').

Having said that, however, I wonder if Waddington said "This may have seemed too long a train of argument to be very convincing" because of his own lingering doubts as much as any likely to arise in a reader.

After all, reducing the incredible to the credible was done within the framework of the 'modern view' of his day and its continuing dependency upon the discontinuity contained within setting up a "cybernetic developmental mechanism... rather like a gun set to go off when the trigger is pulled" and then subsequently waiting for the fortuitous arrival of a mutation sufficient to "pull the trigger". The human mind may be able to jump such gaps but perhaps the natural world is unable to.

...the ostrich squats down in such a way that the under surface of the body comes into contact with the ground at its two ends, fore and aft. In just these places a considerable callosity develops in the skin (figure 2), and Duerden [8] showed that these thickenings make their appearance in the embryo before hatching.

...It certainly seems very far-fetched to attempt to explain such phenomena without bringing in the fact that the environment might be expected to produce similar effects. Let us consider, therefore, what might happen to an ostrich in which the appropriate callosities were not hereditarily determined. Presumably its skin, like that of most other animals, would react directly to external pressure and rubbing by becoming thicker. Now the point which seems to have been overlooked in previous discussions of the matter is that this capacity to react must itself be dependent on genes. Since populations of animals are never quite uniform in any character, we must expect that the ostrich ancestors varied in their capacity to produce the most suitable callosities; and there could he effective natural selection for those which performed the most satisfactory exogenous adaptation. A race would evolve in which the stresses set up by squatting in a particular way would call forth the development of appropriate adaptive thickenings of the skin.

At this stage, the thickenings would still not be hereditary and independent of the pressure and rubbing; they would still be acquired characters in the conventional sense. We can find a hypothesis of how they might come to be hereditarily fixed if we turn to consider another aspect of the matter. The callosities are the results of developmental processes. Now, one of the main characteristics of animal development is that it tends to be canalized or buffered, so that the optimum end result is produced even if there are minor variations from the normal conditions while the process is going on [11]. Natural selection, in fact, does not merely ensure that only those animals survive which have something near the optimum characteristics, but favours those genotypes which tend to produce such animals under any conditions. It gradually builds up efficient cybernetic mechanisms, to use a fashionable phrase. Thus we may expect to reach a stage in which our ostriches nearly always develop callosities of just the right size and position, even in those individuals which, to put it crudely, sit down very seldom or those which loll about the whole time.

Once such a cybernetic developmental mechanism has been built up, it will be rather like a gun set to go off when the trigger is pulled. The development of the callosities will proceed quite autonomously, once the process can be started. The initial stimulus, which may be a greater or lesser amount of external pressure, has become a relatively minor factor in the whole situation. It may then not be too difficult for a gene mutation to occur which will modify some other nearby region of the embryo in such a way that it takes over the function of the external pressure, interacting with the skin so as to 'pull the trigger' and set off the development of the callosities [12].

Before embarking upon a description of how callosities can become hereditary from the perspective of an internal evolutionary mechanism, it may be worth making the the following points:

1) Commentaries such as this one are not being offered as 'convincing arguments' in themselves. The primary purpose, in this instance for example, is not to argue that Waddington was somehow 'wrong' in his interpretation. The proposed mechanism is testable: these notes are more intended to show how natural phenomena can be looked at in a way that augments the argument for such testing to be done.

2) The ensuing references to "existing thresholds being exceeded" can not be more specific unless - for any organism - one or more instances of such an occurrence are already known. See Info wanted on two intriguing 'Lamarckian' experiments for possible examples of this.

3) Anyone who actually reads this post may reach the end and say "Thanks very much, but I'll stick with what I know!". There will be a third post in this mini-series (appearing either tomorrow or Saturday) addressing an interesting aspect which Waddington doesn't mention:

Curiously, it [the ostrich] also has calluses on its ankles which are of no use to it, as it turns its foot sideways when sitting [More]

As an additional 'enticement', commenting on the above will allow the proposed mechanism's relationship with the "Law of Use and Disuse" to be put into context.

How callosities may become hereditary


It is lunchtime and I am sitting in a Social Club busily scribbling away on the back of unwanted print-outs. From experience I know that if I continue this activity long enough today then the skin on the outside of my little finger will start to feel sore. Persevering for several days will eventually cause a callus with form.

The fact I consciously experience distress from the affected area indicates there will be a corresponding effect on the 'localized area of equilibrium at the apex of an homeostatic hierarchy' located within the 'older' (in evolutionary terms) structures of the brain.

Providing I do nothing further to aggravate the constant pressure on the outside of my little finger then formation of the callus will have three effects:

1) Tissue damage ceases at the site of the callus
2) Pain is no longer felt
3) The 'localized area of equilibrium', although 'offset' by the presence of the callus, is no longer affected by transient 'inputs' from the affected area - a state of equilibrium has been re-established.

If I persisted with the activity of writing for long enough within my own lifetime, and my (and other people's) descendants did so in theirs, then eventually the existing thresholds of the localized area of equilibrium would be exceded. Conceptually, this would trigger 'top-down' changes within the homeostatic hierarchy of the genome until the initial state of equilibrium has been restored (ie the 'offset' mentioned above would be gone). This would take 'n' generations, initially 'fast' but then tapering off, and it wouldn't matter if every pencil in the world disappeared once the process began. The 'size and shape' of the callus (as much as location and any other characteristic) would reflect the point at which equilibrium was restored relative to when no callus existed.

As usual I've finished far later than anticipated and any editing will have to wait until tomorrow (usually I see flaws the following day but move on regardless - after all the proposal is still 'under construction'!).

John Latter

This is the second of three posts on Waddington's "The evolution of adaptations". The other two are at:

Re: The evolution of adaptations (Waddington) (1)
Re: The evolution of adaptations (Waddington) (3)

The title link of all three posts goes to the Main Blog entry for Waddington's paper and this link goes to the paper itself.

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