Sarcomere

Within each skeletal muscle cell are many myofibrils consisting of a repeating array of a specialized structure

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 Natural selection was Darwin’s brilliant hypothesis, independently conceived by Wallace, that accounts for adaptations, features that appear “designed” to fit organisms to their environment.

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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.