In elongation zone, cells elongate rapidly and undergo a final round of divisions to produce a central ring of cells called the endodermis

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#XL - T Zoology

Orthogenesis - “Straight-line evolution,” held that the variation that arises is directed toward fixed goals.

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#XL - T Zoology

 Mutationist - Theories were advanced by some geneticists who observed that discretely different new phenotypes can arise by a process of mutation.

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#XL - T Zoology

Ronald A. Fisher, John B. S. Haldane and Sewall Wright- which showed that mutation and natural selection together cause adaptive evolution.

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#XL - T Zoology

Neutral theory of molecular evolution- Developed by Motoo Kimura, who argued that most of the evolution of DNA sequences occurs by chance (genetic drift) rather than by natural selection. 

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#XL - T Zoology

Theory of recapitulation - Ernst Haeckel

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#XL - T Zoology

He proposed that the embryonic development of an individual organism (its ontogeny) followed the same path as the evolutionary history of its species (its phylogeny).