R. Norman answers John Sutherland's question:

>>So in the context of evolution, what is adaptation?  
>
>Adaptation is a very general term for a change in a cell, an organism,
>or a population (species) over time to adjust or modify some aspect of
>its behavior or function to the environment.

The best definition of "adaptation" is the word itself. It literally means
"towards aptness," which is often written as "towards
fitness," although I much
prefer the phrase "towards appropriateness." This latter phrase
captures in two
words the crux of what Darwin meant by the word.

Everything about the physical process of evolutionary adaptation is a movement
across an adaptive topography towards a condition of increasingly greater
behavioral appropriateness in the statistically expected phenotypes produced by
an evolving lineage.



>There is sensory adaptation where a sensory neuron ceases to respond
>(or responds more weakly) to a sustained stimulus.  The time scale may
>be on the order of seconds.  There is physiological adaptation to
>environmental circumstances that occurs on the time scale of seconds
>or minutes to days and months.  There is evolutionary adaptation that
>occurs over generations and is expressed as changes in the genetic
>information. 

That's certainly true enough, although many people try not to use the word
"adaptation" in this context much any more simply to reduce
confusion. If the
word is used at all in physiology, it's now commonly used as the phrase,
"ontogenetic adaptation." The more common term for this
phenomenon nowadays is
"habituation," which is defined to be an actively mediated
feedback process of
quelling the response to a common stimulus. This definition is offered to make
it distinct from a more simple sensory fatigue response -- which will always
occur if stimulation goes on long enough..

Wirt Atmar
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