Pérdida de la conexión

The biological species concept, popularised by Ernst Mayr (1942), combined earlier ideas of Poulton and Dobzhansky. These focused on the importance of interbreeding at a local scale (Mallet 2004). Mayr varied the wording but was still vigorously defending his species concept at the age of 95. He proposes that ‘biological species [are] groups of interbreeding natural

populations that are reproductively isolated from other groups’ (Mayr 2000: 17). With the widespread adoption of the biological species concept by biologists and taxonomists, in the mid- 20th _{century, many prior-named species were demoted to subspecies, and numerous geographical} varieties or races were classified as ‘polytypic’ species. For example, the number of bird species recognised dropped by 50 per cent when ornithologists took this view (Zink 2004b). The main alternative to the biological species concept is phylogenetic species concept, which, rather than placing emphasis on whether populations could interbreed, aims to classify species according to their evolutionary relationships. With the rise of DNA analysis, these relationships could be expressed in the form of a binary branching tree or phylogeny (Mallet 2001). The phylogenetic species concept is by no means monolithic; there is a frustratingly complex spectrum of definitions (see Mishler and Theriot 2000; Wheeler and Platnick 2000; Agapow 2005; Hennig 1966), but they are based on the notion that species form when a single interbreeding population splits into two branches or lineages that do not exchange genetic material (Mallet 2001). One of

the most influential definitions of the phylogenetic species concept was proposed by Cracraft (1989: 29) where ‘[a] phylogenetic species is an irreducible (basal) cluster of organisms, diagnosably distinct from other such clusters, and within which there is a parental pattern of ancestry and descent’. Cracraft’s (1989) definition has become particularly influential in avian taxonomy where diagnostic characters have been used to reassign many taxa long thought of as subspecies to the level of full species (Mallet 2001).

There are a number of criticisms of both these concepts. Critics of the biological species concept argue, primarily, that it is not relevant to asexual species (Claridge, Dawah, and Wilson 1997), and that it is extremely difficult to demonstrate whether interbreeding is or would take place between allopatric (geographically separated) populations (Hendry 2009). As Zink (2006: 890 emphasis in original) notes, ‘what constitutes “enough” interbreeding for two taxa to be considered the same biological species varies immensely from authority to authority’. This situation is further

complicated by species that interbreed where they overlap in a hybrid zone yet the parental types remain distinct; for example, in European toads and North American oak trees (see Mallet 2001), and for North American field crickets see (Harrison 2002). Harrison (2002: 1082) observes that ‘examples of entities that interbreed to some extent, but tend to remain distinct even in the face of this interbreeding, are remarkably common, both in plants and in animals’. Estimates suggest that 25 per cent of all plant species and 10 per cent of all animal species, that are widely accepted to represent separate species, ‘hybridize successfully with at least one other species’ (Hendry 2009: 162). The ability of species to hybridize, within various phylum, also varies dramatically; thus while mammal species generally lose the ability to hybridize in less than 3 million years, and birds and frogs in less than 20 million years, sea turtles are well known for producing hybrids, despite deep lineages of 50+ million years between the species producing hybrids (Bowen and Karl 2007). So, while interbreeding might seem a logical criterion there would appear to be plenty of organisms that make it difficult to operationalise.

On the other hand, one of the main difficulties with the phylogenetic species concept is deciding how much nuclear or mitochondrial DNA divergence between putative species warrants separate species status. The phylogenetic species concept ‘provides no reprieve from the arbitrary nature of species delineation because some threshold level of difference must still be adopted’ (Hendry et al. 2000: 73). For instance, Robertson and Nunn (1998) reviewed albatross taxonomy based on complete mitochondrial DNA cytochrome b sequences and split the previously accepted 13 albatross species into 24 species. Penhallurick and Wink (2004) reanalysed the sequences and rejected species status for species with less than 2 percent divergence for any taxon pair, and argued for the previous 13 species with the other 11 populations reduced back to subspecies status. Rheindt and Austin (2005), in a reply to Penhallurick and Wink (2004), point out the

difficulty of choosing a particular percentage of divergence because of the difference in rates of molecular evolution among bird lineages. For instance, ducks can have as little as 0.1-0.5 per cent mitochondrial divergence between such recognisably distinct species as Northern Pintail (Anas acuta) and Mallard (Anas platyrhnchos). Thus, Rheindt and Austin (2005: 185) suggest therefore that ‘if Penhallurick and Wink (2004) had conducted a taxonomic revision of ducks, they might have ended up lumping the 35-40 currently recognised species of dabbling ducks (Anas) into a handful of species’. Setting an arbitrary cut-off percentage to distinguish between any taxon pair by extrapolating from divergence distances of closely related but unequivocal species then depends on what is considered ‘unequivocal’. This is complicated because the amount of difference between taxonomic species (as well as between other taxonomic levels) in various genetic and phenotypic traits is often very large (Hendry et al. 2000). In their study of 109 sister species of birds Hendry et al. (2000) found that 25 species showed less than 2 per cent sequence divergence, whereas 11 pairs showed greater than 10 per cent sequence divergence. Whether relying on the BSC or the PSC, Hendry et al. (2000: 73) suggest;

[T]he dichotomous nature of species delimitation ignores quantitative differences between species, and effectively considers all equally distinctive. Thus, any study comparing species numbers among taxa, geographical regions, or time periods obscures the fact that biological diversity is poorly quantified simply by counting the number of taxonomic species.

This suggests that it is not only difficult to decide whether or not a taxon is a species, but also there is huge variability in the quality of any given species – what we might call it speciesness. Thus another issue involved in deciding whether a population of, for instance, albatrosses or ducks warrants species status (or not) is the question of whether species are ‘real’ or are agreed upon by biologists, systematists and taxonomists. Thus, the ontological status of species is addressed in the next section, while the effects species delineation can have on conservation is explored in Section 4.5.