r, is affected by organismal size and generation time.

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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. The problem was serious because, according to the prevailing belief in blending inheritance, variation should decrease, not increase.

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

So that a species evolves in a predetermined direction by some kind of internal drive, without the aid of natural selection.

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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

They supposed that such mutant forms constituted new species and thus believed that natural selection was not necessary to account for the origin of species. The last influential mutationist was Richard Goldschmidt

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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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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.