Branches in the lines of descent: charles darwin and

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Biological Journal of the Linnean Society, 2011, 103, 19–35. With 5 figures

Branches in the lines of descent: Charles Darwin and the evolution of the species concept
Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA
Received 23 September 2010; revised 6 December 2010; accepted for publication 7 December2010
bij_1634 19..35

Charles Darwin introduced a novel idea into the concept of species, namely that species are branches in the lines of descent (segments of population lineages). In addition to this novel evolutionary component, Darwin’s species concept also retained an older taxonomic component, namely the view that the species category is a taxonomic rank; moreover, he adopted amount ofdifference as a criterion for ranking lineages as species. Subsequent biologists retained both components of Darwin’s species concept, although they replaced Darwin’s ranking criterion with ranking criteria that either are more objectively defined or relate more directly to the biological bases of lineage separation and divergence. Numerous alternative ranking criteria were proposed, resulting in aproliferation of species definitions and a controversy concerning the concept of species. That controversy can be resolved by distinguishing more explicitly between the theoretical concept of species and the operational criteria that are used to apply the concept in practice. By viewing the various alternative ranking criteria as operational indicators of lineage separation rather than necessaryproperties of species, the conflicts among competing species concepts are eliminated, resulting in a unified concept of species. A brief examination of the history of biology reveals that an important shift related to the unified species concept has been emerging ever since Darwin reformulated the concept of species with an evolutionary basis. The species category is effectively being decoupled from thehierarchy of taxonomic ranks and transferred to the hierarchy of biological organization. Published 2011. This article is a US Government work and is in the public domain in the USA. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 19–35.

ADDITIONAL KEYWORDS: Evolutionary Synthesis – hierarchy of biological organization – incipient species – isolatingmechanisms – lineage – phylogenetic systematics – species selection – taxonomic rank.

24 November 2009 was the 150th anniversary of the publication of Charles Darwin’s (1859) On The Origin of Species by Means of Natural Selection, one of the most influential works in the history of the biological sciences. Its influence transformed almost every field of biology and extended intomany other arenas of human culture, including religion, politics, art, and literature. The 150th anniversary of such a significant publication provides an opportunity to re-examine that work in the context of subsequent developments, both to celebrate its historical influence and to highlight important similarities and differences relative to current thought.

The presentarticle examines Charles Darwin’s concept of species as described in The Origin and its influence on the subsequent evolution of the species concept. Several aspects of Darwin’s species concept have been analyzed previously, including its relationship to essentialism (Mayr, 1982), strategic elements that may have helped Darwin to communicate his revolutionary views (Beatty, 1985), and whetherDarwin viewed species as artificial or real entities (Kottler, 1978; Coyne & Orr, 2004; Stamos, 2007). Here, Darwin’s species concept is examined as a combination of two distinct components: a newer evolutionary component according to which species are conceptualized as segments of population lineages, and an older taxonomic component according to which species are conceptualized as groups of...
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