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Lecture notes Adaptive features of organisms Darwin's two theses together explain the features of organisms:
- Descent with modification implies that a feature results from a modification of a pre-existing form. Ancestry will constrain the range of variation available to natural selection.
- Natural selection results in features that, under particular environmental conditions, bestow an advantage in the competition to survive and reproduce. (Such features are called adaptations.) Each successive step in the evolution of an adaptive feature must itself be adaptive. If a form or feature requires any particular step to be disadvantageous, such a form will not exist under constant selection. That is, adaptations are not necessarily predicted to be "perfect". The probability of a reversal of a complex series of changes is very small (Dollo's "law").
The next pages present:
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