Concepts of Evolutionary Biology
Dollo's Rule (Doctrine of irreversibility of evolution)
- Many times during the long history of life, advanced organism have returned to ancestral habits and modes of life.
- This gives selective value to adaptation similar to those of the ancestral species, and raises the question whether the evolution is reversible?
- Study of such cases suggests that due to commonalty of environment and resultant functional adaptation, always a gross similarity between ancestral and descendent structures is achieved without any genuine reversal at all.
- Thus many reptiles and mammals have reverted to an aquatic mode of life. They have assumed a generally streamlined, fishlike form, and the limbs have become shortened, webbed and finlike.
- Yet the skeleton of such flippers is always distinctly that of the class to which animal belongs rather than that of a fish fin.
- The evidence indicates that major evolutionary steps once taken, are never reversed. This is known as Dollo's rule after Louise Dollo(1895) to whom the principal is ascribed.
- It might even expected a priori, major evolutionary steps are compound of many smaller steps, each preserved by natural selection.
- That such a sequence, occurring by chance once should by chance be exactly reversed would be a most extraordinary thing. If not impossible, it is at least most improbable for whole organisms.
- Attempts to apply Dollo's law to individual characters have failed, for these are indeed reversible by mutation.
- A very common trend in evolution, sometimes called Cope's Rule, is one forward increasing size of individuals.
- The original studies of the phenomenon were made upon vertebrates, but comparable studies have shown the same tendency in many groups of invertebrates and plants.
- A review of the paleontology of almost any group shows that its largest representatives are not its earliest ones, though not necessarily its latest ones either.
- Newll has pointed out that species now living are the largest known representatives of the vertebrates, crustaceans and annelids.
- Yet the tendency towards size increase has been by no means universal. The rise of herbs and shrubs is a recent thing, and they have been derived from trees and other large plants.
- Hooijer has pointed out that the progressive size decrease has been characteristics of many vertebrate groups during the Quaternary period, which is now in progress.
- Competition may occur between populations within an ecosystem for ny of the available resources, such as food, space, light or shelter.
- If two species occur at the same trophic level, then they are likely to compete with each other for food.
- Adaptive radiation by one or both species may than occur over a period of time with the result that they come to occupy separate niche within the trophic level, thus minimizing the extent of completion.
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