In this chapter, we will attempt to address several interrelated questions about species and species formation. First we ask what, if anything, is a species? As we shall see, while most scientists are happy to agree on the essentials, the answer to this question is far from straightforward. We then briefly discuss the range of ways new species can evolve, and provide evidence for these different pathways. Finally, following from our opening quotations, we ask a somewhat more abstract and philosophical question that brings together many of the separate threads we have introduced: why is life not composed of a single species? . . . What is a species? . . . The classification of organisms into species is so familiar that it is easy to accept without much critical thought. On reading ‘Tiger, tiger burning bright’, or headlines such as ‘Man bites Dog’, we have no problem envisaging who the main protagonists are. Mention a tiger, and one immediately thinks of a large cat with stripes. To most people, species are simply a collection of organisms with a given set of physical traits. All classification systems include elements of personal preference as to how one chooses to classify any group of objects (e.g. by shape, size, or colour). However, there is evidence that ‘species’ represent categories that are more consistent between observers than the various ways of sorting out one’s stamp collection. The Fore, a highland people of New Guinea, are perhaps best known in the western world for the devastating prion-based disease ‘Kuru’ that afflicted their population as a result of ritualized consumption of dead family members. However, the people have close links to their natural environment and a remarkably detailed system of classifying the larger animals they see around them. In an early study to test the degree to which species assignations are consistent among peoples with different backgrounds, Jared Diamond compared the Fore nomenclature with that developed by European taxonomists. Birds found regularly in the Fore territory were divided by the Fore into 110 distinct types, and by zoologists into 120 types, with an almost exact one-to-one correspondence between Fore ‘species’ and taxonomists’ ‘species’.
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