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Preadaptation and Multiple Evolutionary Pathways

Walter J. Bock
Evolution
Vol. 13, No. 2 (Jun., 1959), pp. 194-211
DOI: 10.2307/2405873
Stable URL: http://www.jstor.org/stable/2405873
Page Count: 18
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Preadaptation and Multiple Evolutionary Pathways
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Abstract

1) The basitemporal articulation of birds forms the hinge between the bones of the medial brace of the mandible. This brace consists of the medial process of the mandible abutting against the basitemporal plate and serves to prevent the disarticulation of the quadrate hinge because of strong forces on the mandible when the bill is opened. 2) The evolution of the medial brace is outlined. It is shown that the medial process of the mandible increases in length in connection with increasing strength of the jaw muscles. In some groups, the medial process finally abuts against the base of the brain case with the basitemporal articulation developing where the two bones came into contact. Increase in the length of the medial process of the mandible and the subsequent development of the medial brace occurred in many independent lines of birds. 3) The evolution of the medial brace is compared to the evolution of the mammalian jaw articulation. Again, a process developed on the mandible of the mammal-like reptiles in response to a selection force for stronger jaw muscles. The process finally touched the brain case and first formed a brace for the mandible and then assumed the role of jaw suspension. Analogy from the evolution of medial brace argues in favor of the hypothesis that the mammalian jaw hinge evolved several independent times. 4) The concepts of preadaptation and multiple pathways of evolution are discussed. It is suggested that the evolution of most new structures involves old structures which are preadapted for the new function. The preadapted level is a threshold at which there is a functional shift, not a morphological shift. If the selection force for a new function acts on a group of organisms, the resulting adaptations do not have to be similar, but may vary in structure. Because the morphological differences between these several adaptive answers are nonadaptive, these differences must not be explained on functional grounds. 5) A model of macroevolutionary change is presented. The major conclusion drawn from this model is that if the functional significance of structures are properly analyzed and correct historical sequences were established, then the evolution of most, if not all, new structures can be explained on the present concept of gradual evolutionary change.

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