The general theory described below explains the origin and subsequent evolution of segmented organisms. The obvious segmentation of arthropods and the internal skeletal segmentation of vertebrates are considered as instances of the same phenomenon: structural repetition within an organism. A specific theory is described and illustrated.
This theory is primarily concerned with how our overall skeletal form evolved--head, trunk, arms, legs, digits. The morphology of musculature and organ systems is also partially explained. Segmentation is fundamental in the anatomy of vertebrates and arthropods; their evolution will never be understood without a general theory explaining the evolution of segmentation.
The proposed model is Darwinian; random variation and natural selection are basic mechanisms. It is non-Darwinian, however, in that it includes certain evolutionary leaps--sudden changes in overall form--in contrast to the gradual evolution propounded by Darwin. These abrupt mutations were simple; they arose through a well-understood mechanism; and they occured only during a brief formative period, just before the Paleozoic Era.
There is a tendency to think big when we contemplate evolution, to consider principles of physics and to philosophize. But biological knowledge is knowledge of particular systems. I will discuss what is probable and feasible in certain contexts but I make no claims about the potentialities or limitations of organic evolution in general.
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Copyright 1995 Cliff Lundberg. All rights reserved