13. Natural Selection VII – Divergence of Character

Continuing with my bite sized synopsis of On the Origin of Species: Divergence of Character
Darwin thought of varieties as species in the process of formation. The question then arises: at what point do the small differences shown by varieties become large enough to be considered a separate species within the same genus? This can occur in nature…

“…from the simple circumstance that the more diversified
the descendants from any one species become in structure, constitution, and
habits, by so much will they be better enabled to seize on many and widely
diversified places in the polity of nature(i.e., the environment), and so be
enabled to increase in numbers.”
(OOS pg 112)

He goes on to say that once a species numbers have reached equilibrium within its environment, the only ways it can increase its numbers is the descendant varieties being able to exploit other food groups, supplant other species with similar needs or by inhabiting different or unoccupied niches. Thus the more variations a species can produce, the greater success it will have in occupying new habitats.
This is where Darwin includes the one and only illustration that was in the original On the Origin of Species. The capitalized letters on the lower line represent well defined species within a genus. The lines across, marked with Roman numerals, represent about 1000 generations each (They could also be seen as a million or 10 million generations). The lower case letters represent varieties that are “well marked”, or distinguishable varieties. We see that over thousands of generations, varieties are naturally selected, and produce still more varieties. a1 varies more than A, and a2 has still more variations than A. Branches with no letters signify extinction of that variety. In ten thousand generations, A has diversified into a10, f10 and m10. The varieties that are most distinct from each other are likely to be most successful, because they utilize their environment in different ways than the parent species. Given enough time, varieties will become distinct as species, and further diversification over time can lead to the formation of new genera, families and orders.

“Hence, the struggle for the production of new and modified
descendants, will mainly lie between the larger groups, which are all trying to
increase in number. One large group will slowly conquer another large group,
reduce its numbers, and thus lessen its chance of further variation and
improvement. Within the same large group, the later and more highly perfected
sub-groups, from branching out and seizing on many new places in the polity of
Nature, will constantly tend to supplant and destroy the earlier and less
improved sub-groups. Small and broken groups and sub-groups will finally tend to
(OOS, pg 126)

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