1 Systematics and the reconstruction of phylogeny
To the lay person, it might seem surprising that there is any problem with the recognition of higher taxa. The very existence of long-established vernacular names for inclusive groupings of species (e.g. finches, thrushes, parrots and hawks as distinct groups of birds) suggests that higher taxa are self-evident. Accordingly, the task of the taxonomist might seem merely to consist of recognising these groupings and assembling them in a hierarchy of increasingly inclusive categories.
Indeed, taxonomists had been at work on this task long before Darwin's time. The original intention of most earlier taxonomists had been to reveal the divine plan of Creation’: each higher taxon was seen to be united by a common basic design, and the differences between its constituent species to be derived from the specific adaptations of the design for different places in nature. The relative ‘affinities’ (similarities of basic design) between species were thus widely held to reflect a natural order before Darwin and Wallace re-interpreted them as indicating phylogenetic relationships. In the light of evolution, the hierarchy of taxa could now be seen as reflecting the phylogenetic tree of life.
If the phylogeny of life were itself known, there would be little problem in recognising higher taxa: the only arbitrary part of the exercise, of deciding where to ‘sever’ the branches to delimit the higher taxa, could simply be a matter for consensus. The problems arise because, as noted earlier, the true phylogeny is unknown, and can only be inferred from the available evidence. In practice, then, organisms are grouped according to criteria deemed to reflect relationship, and phylogeny is construed from these groupings. Conclusions may vary not only according to the characteristics of the organisms which are investigated, but also according to how they are analysed.