In the first decade of twentieth century, Hugo de Vries based on his work on evening primrose brought forth the idea of mutations. Large difference arose suddenly in a population. He believed that it is a mutation which causes evolution and not the minor variations that Darwin talked about. Mutations are random inheritable and directionless, while Darwinian variations are small and directional. Evolution for Darwin was gradual, while de Vries believed mutation caused speciation and hence called it saltation. Studies in population genetics, later, brought out some clarity.
Branching descent and natural selection are the two key concepts of Darwinian theory of evolution. He gave the examples of giraffes, which in an attempt to forage leaves on tall trees had to adapt by elongation of their necks. As they passed on this acquired character of elongated necks to succeeding generations, giraffes, slowly, over the years/ came to acquire long necks. Nobody believes this conjecture any more. Natural selection is based on certain observations such as stability in population size except for seasonal fluctuation, members of a population vary in characteristics even though they look superficially similar, most of variations are inherited, etc.
The fact is that population sizes in reality are limited, means that there had been competition for resources. Only some survived and grew at the cost of others that could not flourish. The novelty and brilliant insight of Darwin was this : he asserted that variations which are inheritable and which make resource utilization better for few will enable only those to reproduce and leave more progeny. Hence, for a period of time, over many generations, survivors will leave more progeny, and there would be a change in population characteristics and hence new forms appear to arise.
| S. No. |
Darwin's theory |
Hugo de Vries |
| (i) |
Directional |
Directionless |
| (ii) |
Gradual / slow |
Sudden |
| (iii) |
Continuous variation |
Random |
| (iv) |
Variations appear automatically |
Variation appear due to change in genetic make up |
| (v) |
Evolution is a continuous process |
Evolution is a jerky or sudden process |
| (vi) |
The theory is based on Origin and selection of continuous variation. |
This theory is based on discontinuous variations. |
| (vii) |
Formation of new species is a slow process that takes a long time. |
Formation of new species can takes place immediately. |
