15. Laws of Variation II – Acclimatization and Correlation of Growth

Returning to my pocket-sized review of On the Origin of Species:

Laws of Variation: Acclimatization and Correlation of Growth

Acclimatization, is the ability of an organism to change in response to changes in the environment within a single generation. Adaptation can be thought of as acclimatization through natural selection over many generations. Often an organism seems to be restricted to a specific habitat, and we sometimes take for granted that moving it outside of that habitat would cause it to perish. However, the restricted range of an organism may be due to other reasons besides lack of adaptive ability. For instance, it may be restricted due to geographical features such as mountain ranges or water. While many organisms are indeed restricted to specific climates, others show an amazing ability for acclimatization. Darwin gives the example of the common rat or mouse, which can be found on almost every continent and in every climate. He calls this type of adaptability “flexibility of constitution”:

The rat and mouse cannot be considered as domestic animals, but they have been transported by man to many parts of the world, and now have a far wider range than any other rodent, living free under the cold climate of Faroe in the north and of the Falklands in the south, and on many islands in the torrid zones. Hence I am inclined to look at adaptation to any special climate as a quality readily grafted on an innate wide flexibility of constitution, which is common to most animals. On this view, the capacity of enduring the most different climates by man himself and by his domestic animals, and such facts as that former species of the elephant and rhinoceros were capable of enduring a glacial climate, whereas the living species are now all tropical or sub-tropical in their habits, ought not to be looked at as anomalies, but merely as examples of a very common flexibility of constitution, brought, under peculiar circumstances, into play.” (OOS, pg 141)

This innate “flexibility of constitution” – acclimatization – is what we know today as phenotypic plasticity.

Correlation of Growth is a phenomenon that Darwin observed: it occurs when changes in one part of an organism would seem to modify in a linked manner to the changes in another part. Darwin recognised that in some cases there may only be an appearance of correlated variation, due to the coincidental selection of two parts over time. Of course he did not know the genetic basis of this, now known as pleiotropy, which happens when a single gene codes for a product that affects several types of cells or has a signalling function to different targets. Darwin used the example of cats: blue eyed cats are often deaf and the tortoise-shell colouration is found only in females.

(Photo from Wikimedia Commons)


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