The general context for applying the theory of arrival biases is illustrated in the figure at right. On the left are the details of mutation and development that are responsible for any tendencies in the generation of variation (varigenesis), i.e., tendencies prior to selection or drift. On the right are observable evolutionary patterns that might possibly be explained by these tendencies. The green arrow is some theory— the theory of arrival biases or some alternative theory— that specifies conditions of a cause-effect relationship linking variational tendencies to evolutionary tendencies. To apply a theory in this context is to generate evolutionary hypotheses or explanations that appeal to the internal details of mutation and development (on the left) to account for evolutionary patterns (on the right) via the conditions of causation specified by the theory. For instance, Darwin's comment that the laws of variation "bear no relation" to the structures built by selection would suggest that there are no conditions under which the internal details on the left account for the patterns on the right. The other theories all suggest that variational tendencies may influence evolution under some conditions. For instance, the theory of mutation pressure applies when mutation rates are high and unopposed by selection, thus it has a limited range of applications. The theory of evolutionary quantitative genetics can be applied very broadly to the evolution of quantitative characters, but the theory (as developed so far) does not suggest that mutation biases will have much impact.
