Oaks exemplify problems with the reproductive species concept which motivate a reconsideration of the use and nature of species. Ecology is important in the reconsideration. The species level is usually overemphasized in evolutionary thought; selection acts on phenotypes and any mutualistic units. Standard definitions tend to inhibit free conceptual progress. Multispecies, sets of broadly sympatric species that exchange genes, may occur among animals as well as plants and may conceivably bridge kingdoms. This phenomenon can be adaptively important. There may be (...) taxa without species. The degree of modality of adaptive zones can be investigated empirically. (shrink)

It is important to distinguish adaptation per se (adaptedness, or being adapted) from the more specific concept of adaptation for some function. Commonly used criteria for adaptation in either sense have limited applicability. There are, however, a number of widely applicable criteria for adaptation per se, such as several kinds of cost, low variation, the maintenance of integration, and the fitness distribution of mutations. Application of these criteria leads to the conclusion that adaptation is overwhelmingly prevalent for features of organisms. (...) Neither the presence nor the absence of adaptation has a privileged status in inference. Effectively neutral evolution can occur on adaptive buttes while maintaining the same degree of adaptation, but it is likely to be relatively minor. (shrink)