Theories of Evolution of Class 12

It is change in body parts as per the need of surrounding, makes basis for evolution. After various such adaptive changes the new species is evolved.

Adaptation includes suitable changes.

Adaptive Radiation :

When organisms of common origin change in diversified direction to suit the environment. It is called adaptive radiation. (or divergent evolution)

e.g. mouthparts in insects;

Osborn’s Law of Adaptive Radiation :

Osborn (1896) described the diversification of vertebrate life after shifting to land from water. Mammal originated as terrestrial and then adapted for aquatic life (ex.cetacean, sirenians) land life (fussorial, scansorial, cursorial and arboreal) and aerial (or volant) life e.g. bat.


Genetic Basis of Adaptation

The Lederberg’s Replica Plating Experiment is a good example for the genetic basis of adaptation.

After introducing penicillin, in the culture plate it was found that some colonies were resistant to it. Bacteria acquired this ability due to mutations (preadaptive) which occurred before exposure to penicillin The new environment only selects such mutations.

Lamarckian explanation of this could be that the penicillin induced changes in some bacteria enabling them to grow in this medium. Thus these bacteria became adapted to live in penicillin medium.

The Darwinian interpretation could be that the colonies had two types of strains : Penicillin-sensitive and penicillin-resistant arisen by mutation.

Before the use of penicillin the suitable condition was for sensitive strain, hence after using penicillin they were wiped out, but was favourable for resistant strain. Thus the natural selection operates here as advantageous to the resistant strain.

Dollo’s Law : Evolution is irreversible

Cope’s Law : In general it has been found that in the course of evolution there is tendency in the organism to increase in size.

Ecogeographical Rules : Regarding color, shape and size of the animals scientists have observed certain adaptive changes propounded as following rules.

Bergman’s Law : In colder regions warm blooded animals are bigger in size as compared to animals of hotter regions.

Allen’s Law : In colder regions the extremities of bodies like pinna, tail, digits are smaller in size.

Gloger’s Rule : Warm blooded animals in hot, humid area have more melanin pigment but develop yellow-red- pigment in hot dry areas.

Rapoport’s Rule : Species adapted to colder region have wider latitudinal distribution than the species adapted to warmed climates.

Jordon’s Rule : Fishes of colder water are larger in size with more vertebra then those living in warmer water.

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