Directional Internal Selection

Internal selection is a process of natural selection in which fitness differences between organisms are caused by factors internal to the organism and are invariant across environments. Internal selection has been most successfully applied to explain conserved features such as the phylotypic stage of gnathostomes. In this contribution, we introduce a form of internal selection that can drive evolutionary transformations rather than trait conservation, i.e. directional internal selection. The basic idea is that adaptive evolutionary change of one character can induce directional selection on another trait due to constraints internal to the organism. We provide a formal definition of this concept and discuss examples of directional internal selection as well as its implications.