It is proposed by Lamarck. It states that evolution of life forms occurred by the inheritance of acquired characters.
Acquired characters are developed by use & disuse of organs.
o Evolution by use of organs: E.g. Long neck of giraffe is due to continuous elongation to forage leaves on trees. This acquired character was inherited to succeeding generations.
o Evolution by disuse: E.g. Disappearance of limbs in snakes.
This theory was eliminated out because it is proved that the characters are inherited only through genes.
- Proposed by Charles Darwin.
- It was based on observations during a sea voyage in a sail ship called H.M.S. Beagle.
- Alfred Wallace (a naturalist worked in Malay Archepelago) had also come to similar conclusions.
- Work of Thomas Malthus on populations influenced Darwin.
Darwinism is based on 2 key concepts:
o Branching descent: It explains that all organisms are modified descendants of previous life forms.
o Natural selection: Consider a bacterial colony A growing on a given medium. If the medium composition is changed, only a part of the population can survive under new condition. This variant population (B) outgrows the others and appears as new species, i.e. B is better than A under new condition. Thus, nature selects for fitness.
Natural selection is based on the following facts:
· Heritable minor variations: It is either beneficial or harmful to the organisms.
· Overproduction: Population size grows exponentially due to maximum reproduction (E.g. bacterial population).
· Limited natural resources: Resources are not increased in accordance with the population size.
· Struggle for existence: It is the competition among organisms for resources so that population size is limited.
· Survival of the fittest: In struggle for existence, organisms with beneficial variations can utilize resources better. Hence, they survive and reproduce. This is called Survival of the fittest. It leads to a change in population characteristics and new forms appear.
Darwin ignored about origin of variation and mechanism of evolution or speciation.
- Hugo de Vries proposed Mutation Theory of evolution.
- He conducted experiments on Oenothera lamarckiana
(evening primrose) and believed that evolution takes place through mutation and not by minor variation.
- Darwinian variation is minor, slow and directional. It results in gradual evolution.
- Mutational variation is sudden, random & directionless. Here, speciation is by saltation (single step, large mutation).
- Mutation is the origin of variation for evolution.