Nurturing Life Sciences
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#XL - T Zoology
Where, though, do these hereditary variations come from? This was the great gap in Darwin’s theory, and he never filled it.
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Orthogenesis - “Straight-line evolution,” held that the variation that arises is directed toward fixed goals.
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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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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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Theory of recapitulation - Ernst Haeckel