A meeting at the gene

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Abstract
Modern biology has many triumphs to celebrate, but a generally applicable species definition is not among them. This is no small debt, because classification of the differentness of organisms is a basic and undeniable human need inherited from our non‐human ancestors. If for no other purpose than simple communication, we need words to describe the levels of differentness among the organisms that we observe, from the highest Linnéan ranks down to the things that taxonomists call species. But biologists have found it impossible to agree upon where, exactly, to position the upper and lower boundaries of species in the spectrum of variation observable for any given group of organisms. It is perhaps biology's most grotesque concession that 140 years after the publication of The Origin of Species (Darwin, 1859), we still do not know exactly what those things are whose origin the theory of evolution explains. We know that species reside somewhere within a continuum of genetic diversity extending from the individual to the kingdom, but we are wholly unable to pinpoint them. This circumstance has a far‐reaching impact upon efforts to quantify biological diversity, because if species as units of diversity are an outdated concept (Bachmann, 1998), we are faced with a serious problem: how to measure biodiversity? To illustrate the matter, consider the following estimates provided in a recent review: 14 million total contemporary species distributed across some 1–6 billion global populations with extinction rates surmised to encompass at least 27 000 species per year (Purvis and Hector, 2000). These values, like all current measures of biodiversity, involve in one way or another the concept of species numbers or richness. But how can we rely upon them when the unit of count is an undefined quantity? Conservationists are fully aware of the seemingly insurmountable difficulties posed by defining …