Wednesday, March 15, 2006

 

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

Evolutionary Theory, The White Rabbit, and Genetic Assimilation

In "The evolution of adaptations", Waddington states:

The reigning modern view is that, in nature, the direction of mutational change is entirely at random, and that adaptation results solely from the natural selection of mutations which happen to give rise to individuals with suitable characteristics. I want to argue that this theory is an extremist one

and then briefly categorizes three types of adaptations: exogenous, pseudo-exogenous, and endogenous.

The next extract from the paper is necessarily extensive. Sections which are particularly relevant to the following comments have been 'colour-coded':

...It is the remaining category of adaptations, the pseudo-exogenous, which has provoked the most discussion. We are confronted here by phenomena for which an explanation could so easily be found in a direct effect of some environmental factor, were it not that further study demonstrates unequivocally that the structure concerned is determined by the heredity of the organism, and is relatively independent of the environment. The question arises whether we can bring ourselves to believe that the part which the environment can play in mimicking the condition is really irrelevant, and that the evolution of this particular adaptation has resulted from the selection of chance mutations which might have appeared and produced the phenotypes even if the environmental effects had never existed.

Some concrete examples will make the problem clearer. One of the most familiar is that of the thickened skin on the soles of our feet. This thickening is obviously an adaptation to the stresses which this region of the body has to bear; but, as Darwin pointed out, and as Semon [7] discussed in a full-length paper, the thickening already appears in the embryo, before the foot has ever borne any weight. The structure therefore cannot be a direct response to external pressure, but must be produced by the hereditary constitution independently of the specific external influence to which it is an adaptation. The situation is even more striking when similar thickenings are found on less conventional parts of the body. For instance, 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].

This may have seemed too long a train of argument to be very convincing, but a good deal of experimental evidence can now be produced to support it

Unlike the White Queen, who apparently could believe up to six impossible things before breakfast, Waddington's position where he writes:

"The question arises whether we can bring ourselves to believe that the part which the environment can play in mimicking the condition is really irrelevant."

could be described as one where he finds such a belief to be 'incredible'.

The subsequent description of the process given in the "too long a train of thought", however, eventually enables the stage to be reached where he can say:

It may then not be too difficult for a gene mutation to occur which will modify some other nearby region of the embryo.

Waddington's explanation is very readable and it certainly does reduce the gap between the 'incredible' and the credible. But, it is suggested, only from an intellectual point of view!

Waddington's solution still doesn't address the important 'discontinuity' supposed to exist in the natural world between the setting up of a "cybernetic developmental mechanism... rather like a gun set to go off when the trigger is pulled" and the fortuitous arrival of a mutation sufficient to "pull the trigger".

No such discontinuity exists when the phenomena of ostrich callosities is looked at from the perspective of the proposed internal evolutionary mechanism but further comments will have to wait until tomorrow or Friday.

John Latter

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

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