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Species, Concepts Of SPECIES, CONCEPTS OF James Mallet University College London I. What are Species Concepts for? DNA bar coding A means of delimiting species via II. Statement of Bias DNA sequence clustering, usually from mi- III. Darwinian Species Criteria tochondrial DNA. IV. The Philosophization of Species, the ‘‘Inter- gene flow Movement of genes between populations, breeding’’ Concept usually via immigration and mating of whole geno- V. Alternative Species Concepts types, but sometimes single genes may undergo VI. Species Concepts Based on History horizontal gene transfer via transfection by micro- VII. Combined Species Concepts organisms. VIII. Dissent: Maybe Species Are Not Real gene pool The sum total of the genetic variation IX. The Importance of Species Concepts for Bio- within a reproductively isolated species population; diversity and Conservation this term is mostly used by supporters of the interbreeding species concept. genomic cluster A synonym for genotypic cluster. genotypic cluster In a local area, a single genotypic GLOSSARY cluster (or species) is recognized if there is a single group of individuals recognizable on the basis of cladistic A classification based entirely on mono- multiple, unlinked inherited characters or genetic phyletic taxonomic groupings within a phylogeny; markers. A pair of such genotypic clusters (or spe- taxonomic units that are paraphyletic or poly- cies) is recognizable if the frequency distribution of phyletic are rejected. A ‘‘cladist’’ is one who practices genotypes is bimodal. Within each genotypic clus- ‘‘cladistics,’’ usually in the sense of using parsimony ter in a local region, allele frequencies will conform to adjudicate between data from multiple characters to Hardy–Weinberg equilibrium, and the different in the construction of a ‘‘cladogram,’’ which is an unlinked loci will be in approximate linkage equi- estimate of the true phylogeny. librium. The presence of more than one species or cohesion The sum total of forces or systems that genotypic cluster can then be inferred if the distri- hold a species together. The term is used especially bution of genotypes is bimodal or multimodal, and in the interbreeding and cohesion species concepts. strong heterozygote deficits and linkage dis- Cohesion mechanisms include isolating mecha- equilibria are evident between the clusters. nisms in sexual species as well as ‘‘stabilizing’’ isolating mechanisms The sum total of all types of ecological selection, which may cause cohesion factors that prevent gene flow between species, in- even within asexual lineages. cluding premating mechanisms (mate choice), and disruptive selection Selection acting to preserve ex- postmating mechanisms (hybrid sterility and treme phenotypes in a population. Speciation usu- inviability). Modern authors deny that these ‘‘mech- ally involves disruptive selection, because anisms’’ have necessarily evolved to preserve the intermediates (hybrids between incipient species) species’ integrity as originally assumed, though this are disfavored (see also stabilizing selection). may sometimes be the case in reinforcement of Encyclopedia of Biodiversity Copyright & 2007 Elsevier Inc. All rights of reproduction in any form reserved. 1 2 _________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ SPECIES, CONCEPTS OF_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ premating isolation. Isolating mechanisms are a sub- will understand the author to be a nominalist who set of the factors that cause cohesion of species un- merely believes useful terms require little theoretical der the interbreeding and cohesion species concepts. underpinning; others assume the author is nonsen- monophyletic A grouping that contains all, of the sically using some definition that does not apply to descendants of a particular node in a phylogeny. actual organisms. Here, when I discuss the reality Monophyly is the state of such groupings. Compare underlying a species concept, I mean it in the first paraphyletic polyphyletic. Butterflies (Rhopaloc- sense, a hypothesis. Many authors of species con- era) and birds (Aves) are examples of two groups cepts and some philosophers of science argue that thought to be monophyletic. definitions must be underpinned by a theoretical paraphyletic A grouping that contains some, but not justification or reality. Other philosophers such as all, of the descendants of a particular node in a Wittgenstein and Popper agree that terms need no phylogeny. Paraphyly is the state of such groupings. such definition to be useful. Compare monophyletic, polyphyletic. Moths (Le- sibling species A pair of closely related, morpho- pidoptera, excluding butterflies) and reptiles (am- logically similar species (usually sister species). niotes, excluding birds and mammals) are examples speciation The evolutionary process of the origin of of two groups thought to be paraphyletic. a new species. phenetic A classification or grouping based purely specific mate recognition systems (SMRSs) Fertili- on overall similarity. Pheneticists use matrices of zation and mate recognition systems in the overall similarity rather than parsimony to con- recognition concept of species, the factors leading struct a ‘‘phenogram’’ as an estimate of the phylo- to premating compatibility within a species. See geny. Examples of phenetic methods of estimation also cohesion which is similar to SMRS, but in- include unweighted pair group analysis (UPGMA) cludes postmating compatibility as well. and neighbor joining. ‘‘Cladists’’ reject phenetic stabilizing selection Selection which favors inter- classifications on the grounds that they may result mediate phenotypes. in paraphyletic or polyphyletic groupings. taxonomic inflation The process whereby the num- phylogenetic Pertaining to the true (i.e., evolutio- bers of species in the checklist of a group increases nary) pattern of relationship, usually expressed in due to a change in species concept rather than due the form of a binary branching tree, or phylogeny. If to new discoveries of previously unkown taxa. hybridization produces new lineages, as is common in many plants and some animals, the phylogeny is said to be ‘‘reticulate.’’ Phylogenies may be esti- mated using phenetics, parsimony (‘‘cladistics’’), or SPECIES ARE CRUCIAL IN MANY BIODIVERSITY methods based on statistical likelihood. ISSUES: much of conservation, biodiversity studies, polyphyletic Groupings contain taxa with more than ecology, and legislation concerns this taxonomic level. one ancestor. ‘‘Polyphyly’’ is the state of such grou- It may therefore seem rather surprising that biologists pings. Compare paraphyletic and monophyletic. have failed to agree on a single species concept. The ‘‘Winged vertebrates’’ (including birds and bats) disagreement means that species counts could easily give an example of a polyphyletic group. differ by an order of magnitude or more when the real, reality Two tricky words found frequently in the same data are examined by different taxonomies, that species concept debate. Reality is typically used to numbers of known species are increasing due to tax- support one’s own species concept, as in: ‘‘The con- onomic inflation in spite of concerns about end- clusions set forth above y lead to a belief in the angerment, and that it becomes unclear what the study reality of species’’ (Poulton, 1904); similar examples of speciation is about. This article explores the cont- can by found in the writings of Dobzhansky, Mayr, roversy on species concepts and its implications for and especially ‘‘phylogeneticists.’’ The term reality in evolution and conservation. this sense is similar to an Aristotelian ‘‘essence,’’ a hypothetical pure, albeit obscure, truth that under- lies the messy actuality; unfortunately, in everyday language ‘‘real’’ also means ‘‘actual’’ (curiously, a re- I. WHAT ARE SPECIES CONCEPTS FOR? ality in the first sense may be ‘‘unreal’’ under the second!). By rejecting the reality of species, one can Individual organisms can usually be recognized, but therefore send very mixed messages: some readers the larger units we use to describe the diversity of life, ___________________________________________________________________________________________________________________________________________________________________________________ SPECIES, CONCEPTS OF________________________________________________________________________________________________________________________________________________________________________________ 3 such as populations, subspecies, or species, are not so international agreement among biologists; after all, if easily identifiable. Taxonomists further group species we can adopt meters and kilograms, perhaps we could into genera, families, orders, and kingdoms, while agree on units of biodiversity in a similar way. In either ecologists group species into higher structures such as case, knowledge of the full gamut of today’s competing communities and ecosystems. The justification for solutions to the species concept problem will probably these group terms is utility,
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