B

function also arises in a modular way and not simply by accretion. The molecules are occasionally modified when copied, and the copies can subsequently accumulate variation and modified or new function. This process can be termed duplication with variation and, as we will see, is a particular but important aspect of Darwin's major principle of descent with modification.

Duplication Above the Level of the Cell

Modular organization is related to sequestration as is seen by the important fundamental step of the evolution of cells, the modular units of which organisms are

Adrenal Steroid Hormones

Cortisol

Corticosterone

Aldosterone

Cortisol

Corticosterone

Aldosterone

Gonadal Steroid Hormones

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