Species problem

Abstract: The issue of species delimitation has long been confused with that of species conceptualization, leading to a half century of controversy concerning both the definition of the species category and methods for inferring the boundaries and numbers of species. Alternative species concepts agree in treating existence as a separately evolving metapopulation lineage as the primary defining property of the species category, but they disagree in adopting different properties acquired by lineages during the course of divergence (e.g., intrinsic reproductive isolation, diagnosability, monophyly) as secondary defining properties (secondary species criteria). A unified species concept can be achieved by treating existence as a separately evolving metapopulation lineage as the only necessary property of species and the former secondary species criteria as different lines of evidence (operational criteria) relevant to assessing lineage separation. This unified concept of species has several consequences for species delimitation, including the following: First, the issues of species conceptualization and species delimitation are clearly separated; the former secondary species criteria are no longer considered relevant to species conceptualization but only to species delimitation. Second, all of the properties formerly treated as secondary species criteria are relevant to species delimitation to the extent that they provide evidence of lineage separation. Third, the presence of any one of the properties (if appropriately interpreted) is evidence for the existence of a species, though more properties and thus more lines of evidence are associated with a higher degree of corroboration. Fourth, and perhaps most significantly, a unified species concept shifts emphasis away from the traditional species criteria, encouraging biologists to develop new methods of species delimitation that are not tied to those properties.

Abstract: Ghiselin, M. T . (Department of Zoology and Bodega Marine Laboratory, University of California, Berkeley, P. 0.Box 247, Bodega Bay, California 94923). 1975. A radical solution to the species problem. Syst. 2001.23:536-544.-Traditionally, species (like other taxa) have been treated as classes (universals). In fact they may be considered individuals (particular things). The logical term "individual" has been confused with a biological synonym for "organism." If species are individuals, then: 1) their names are proper, 2 ) there cannot be instances of them, 3 ) they do not have defining properties (intensions), 4 ) their constituent organisms are parts, not members. "Species" may be defined as the most extensive units in the natural economy such that reproductive competition occurs among their parts. Species are to evolutionary theory as firms are to economic theory: this analogy resolves many issues, such as the problems of "reality" and the ontological status of nomenclatorial types. [Biological species; systematics; philosophy.] It would appear that the philosophy of taxonomy is about to undergo a major upheaval. Symptomatic is its Gordian knot, the species problem. Some years ago (Ghiselin, 1966a) I attempted to cut it with the sword, casually remarking that, in the logical sense, species are individuals, not classes. So entrenched had the habit become of treating species as universals rather than as particular things, and their names as general rather than proper, that it took some time before this pronouncement was taken seriously. The position in question is hardly new. It goes back at least to Buffon, but I believe this was the first explicit use of it against what has been misleadingly called the nominalistic species concept. Even Ruse ( 1973), a philosopher who cites the foregoing work, passes the notion over in silence. On the other hand Hull (1974) has lately endorsed the idea that, from the point of view of evolutionary theory, biological species and monophyletic taxa are individuals. And Mayr (1969a), while not going so far, strongly emphasizes the point that species are more than just nominal classes. It would appear that the subject deserves further analysis and comment.

Abstract: One hundred and thirty-six years since On the Origin of Species 3., biologists might be expected to have an accepted theory of speciation. Instead, there is, if anything, more disagreement about speciation than ever before. Even more surprisingly, 60 years after the biological species concept, in which species were considered to be reproductive communities isolated from other such communities, we still do not all accept a common definition of what a species is. And yet, if speciation is to be any different from ordinary evolution, we must have a clear definition of species. The emerging solution to the species problem is an updated, genetic version of Darwin's own definition. This definition is useful and is already being used in taxonomy, in biodiversity studies and in evolution.

Abstract: Summary 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.