People talk about adaptation as if it were a sure pathway to success, it's actually more complicated than that:

[quoted material follows]

Not all evolutionary change can be understood in terms of adaptation. Natural selection does not lead inevitably to adaptation. The relation between adaptation and natural selection does not go both ways. Whereas greater relative adaptation leads to natural selection. natural selection does not necessarily lead to greater adaptation.

Unfortunately, in practice it is not easy to predict which of two forms will leave more offspring. A zebra having longer leg bones that enable it to run faster than other zebras will leave more offspring only if escape from predators is really the problem to be solved. if a slightly greater speed will really decrease the chance of being taken and if longer leg bones do not interfere with some other limiting physiological process. 

Lions may prey chiefly on old or injured zebras likely in any case to die soon. and it is not even clear that it is speed that limits the ability of lions to catch zebras. Greater speed may cost the zebra something in feeding efficiency. and if food rather than predation is limiting. a net selective disadvantage might result from solving the wrong problem. 

Finally. a longer bone might break more easily. or require greater developmental resources and metabolic energy to produce and maintain. or change the efficiency of the contraction of the attached muscles. In practice relative-adaptation analysis is a tricky game unless a great deal is known about the total life history of an organism. — Richard C. Lewontin 

PLB

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