e Explanation for the Appearance of Temporal Openings_

Bone has three primary functions: support, protection and participation
in calcium metabolism. Let us assume that the requirements of calcium
metabolism affect the mass of bone that is selected for, but do not
grossly affect the morphology of the bones of that mass. Then selection
operates to meet the needs for support within the limits that are set
by the necessity to provide the protection for vital organs. After the
needs for protection are satisfied, the remaining variable and the one
most effective in determining the morphology of bones is selection for
increased efficiency in meeting stress.

Let us also assume that bone increases in size and/or compactness in
response to selection for meeting demands of increased stress, but is
selected against when requirements for support are reduced or absent.
Selection against bone could only be effective within the limits
prescribed by the requirements for protection and calcium metabolism.

We may therefore assume that there is conservation in selection against
characters having multiple functions. Since bone is an organ system
that plays a multiple role in the vertebrate organism, a change in the
selective pressures that affect one of the roles of bone can only be
effective within the limits set by the other roles. For example,
selection against bone that is no longer essential for support can
occur only so long as the metabolic and protective needs of the
organism provided by that character are not compromised. If a character
no longer has a positive survival value and is not linked with a
character that does have a positive survival value, then the metabolic
demands for the development and maintenance of that character no longer
have a positive survival value. A useless burden of metabolic demands
is placed upon the organism because the character no longer aids the
survival of the organism. If selection caused, for example, muscles to
migrate away from the center of the cheek, the bone that had previously
provided support for these muscles would have lost one of its
functions. If in a population of such individuals, variation in the
thickness of the bone of the cheek occurred, those with thinner bone in
the cheek would be selected for, because less metabolic activity was
diverted to building and maintaining what is now a character of reduced
functional significance. A continuation of the process would eliminate
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