monophyletic too, but their shared traits were present in the ancestors of all songbirds already. It is the loss of these ancestral traits rather than their presence that signifies which songbirds are more closely related to each other than to other songbirds. However, the requirement that taxa be monophyletic – rather than paraphyletic as in the case of the Corvida – is itself part of the cladistic view of Taxonomy, not necessarily followed to an absolute degree by other schools. The two methodologies are not mutually exclusive. There is no reason why, e.g., species identified using phenetics cannot subsequently be subjected to cladistic analysis, to determine their evolutionary relationships. Phenetic methods can also be superior to cladistics when only the distinctness of related taxa is important, as the computational requirements are lower. The history of pheneticism and cladism as rival taxonomic systems is analysed in David Hull's 1988 book Science as a Process. Today Traditionally there was a great deal of heated debate between pheneticists and cladists, as both methods were initially proposed to resolve evolutionary relationships. Perhaps the "high-water mark" of phenetics were the DNA-DNA hybridization studies by Charles G. Sibley, Jon E. Ahlquist and Burt L. Monroe Jr., from which resulted the 1990 Sibley-Ahlquist taxonomy for birds. Highly controversial at its time, some of its findings (e.g. the Galloanserae) have been vindicated, while others (e.g. the all-inclusive "Ciconiiformes" or the "Corvida") have been rejected. However, with computers growing increasingly powerful and widespread, more refined cladistic algorithms became available and could put the suggestions of Willi Hennig to the test. The results of cladistic analyses turned out to be superior to those of phenetic methods – at least when it came to resolving phylogenies. Many systematists continue to use phenetic methods, particularly in addressing species-level questions. While a major goal of taxonomy remains describing the 'tree of life' – the evolutionary path connecting all species – in fieldwork one needs to be able to separate one taxon from another. Classifying diverse groups of closely related organisms that differ very subtly is difficult using a cladistic approach. Phenetics provides numerical tools for examining overall patterns of variation, allowing researchers to identify discrete groups that can be classified as species. Modern applications of phenetics are common in botany, and some examples can be found in most issues of the journal Systematic Botany. Indeed, due to the effects of horizontal gene transfer,