Evolution by the Numbers. James Wynn

intend dividing the varieties into two classes, as Asa Gray and Henslow give the materials, and, further, A. Gray and H.C. Watson have marked for me the forms, which they consider real species, but yet are very close to others; and it will be curious to compare results. If it will all hold good it is very important for me; for it explains, as I think, all classification, i.e. the quasi-branching and sub-branching of forms, as if from one root, big genera increasing and splitting up, etc., as you will perceive. But then comes in, also, what I call a principle of divergence, which I think I can explain. (Darwin to Hooker, August 22, 1857)

      This shift of attention towards the importance of the breadth of variation to evolutionary success inspired Darwin to think more carefully about why variation might matter so much in the process of selection. These investigations also led him to develop his principle of divergence of character, which responds to a question he and his critics considered a major obstacle to any theory of variation: “How do the small differences that are observable between populations of closely related species and varieties grow into the large differences that we see between genera, families, etc.?” (Browne 74).

      Because the calculated ratios showed that greater variety was a hallmark of species and genera with larger populations and ranges, Darwin felt encouraged to explain evolutionary success in terms of variation. He reasoned that when a species reaches equilibrium between its numbers and resources, the only way it could continue to grow is through diversification. Evidence from The Origin of the Species suggests this line of thinking:

      Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural powers of increase be allowed to act, it can succeed in increasing . . . only by its varying descendants seizing on places at the present occupied by other animals: some of them, for instance, being able to feed on new kinds of prey . . . some inhabiting new stations. . . . The more diversified in habits and structure the descendents of our carnivorous animal became, the more places they would be enabled to occupy. (93)

      Using the hypothetical case of the “carnivorous quadruped,” Darwin argues that a species’ survival and replication can be improved if some of its members can expand, through variation, into ecological niches where there is less competition for resources with other conspecifics. In a separate passage, he extends this reasoning, concluding that because larger genera and species produce greater degrees of variation, they will be better off in the struggle for existence.

      In a large genus it is probable that more than one species would vary. . . . In each [large] genus, the species, which are already extremely different in character, will generally tend to produce the greatest number of modified descendents; for these will have the best chance of filling new and widely different places in the polity of nature. (99)

      In addition to making the case that bigger is better, Darwin argues by implication that the breadth of diversity of larger genera and species promotes further diversity and population expansion. This means that larger genera and species will most likely continue to grow and outstrip their smaller counterparts, unless checked by some unforeseen circumstances. The supposed correlation between size and variation assumed in this passage suggests that, for Darwin, bigger genera and species are bigger in the first place because they are more diverse. Thus, diversity itself becomes a crucial factor in evolutionary success.