This much is generally understood. However, it is important to realise that Darwin also argues that evolution has no purpose (individuals are only concerned to increase the passing on of their genes to their progeny) and no direction (organisms simply become better adapted to their local environment rather than 'improving' or 'developing').
It is also important to realise that evolution, powerful though it is, is not alone a sufficient explanation for evolutionary change. The standard misapplication of evolutionary theory "assumes that biological explanation may be equated with devising accounts ... about the adaptive value of any given feature in its original environment" [Gould, 1994, p. 63]. Other factors may well be powerful influences. Darwin focused on organisms, but there are factors that operate both below and above the level of the organism. Research into genetic biology has demonstrated that change in the DNA of individual organisms is essentially random. At the level of species or higher groupings, mass extinctions or environmental change can wipe out entire categories of organisms in similarly random ways, unrelated to their fitness for a particular environment.
The fossil record reveals some of this change. It also reveals that history is quite unpredictable. As Stephen Jay Gould persuasively argues:
History can be explained, with satisfying rigor if evidence be adequate, after a sequence of results unfolds, but it cannot be predicted with any precision beforehand ... History includes too much chaos, or extremely sensitive dependence on minute and unmeasurable differences in initial conditions, leading to massively divergent outcomes based on tiny and unknowable disparities in starting points [Gould, 1994, p. 63].
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