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Since the dawn of taxonomic science, its practicioners had arranged groups via the emphasis of certain characters. The group we know as birds were delimited because they all had feathers, and when Linnaeus was writing, were toothless. Invertebrates lacked a notochord, vertebrates possesed one. And while groups appeared to be nested within in each other, this was seen as just another part of God's special creation.
With and modern evolution biology, however, scientists, including Darwin himself, began to understand that: "Our classifications will come to be, as far as they can be so made, genealogies; and will then truly give what may be called the plan of creation." Such genealogies are more usually called "phylogenies", using a word invented by , a dedicated investigator of such arrangements. And the idea that taxa are to represent groupings defined by evolution has been an integral part of biology ever since.
, invented by entomologist in the 1950's, is the sort of rigorous application of the concept of evolution to taxonomy that Darwin envisioned. Phylogenies are established by what distinctive features their members share to the exclusion of more distantly related organisms. Thus, if one wants to identify some subgroup of insects, features that all insects have in common, like six legs and segmented bodies, are useless. One has to use features or combinations of features that only that subgroup has, like front wings becoming hard wing covers for (beetles) or scaly wings for (butterflies and moths). One constructs phylogenies with this technique by trying to find the family tree that involves the fewest feature changes (steps), and thus the smallest amount of . Like phenetics, cladistics is almost always done by computer.
Groups are then delimited on this basis. Unlike the taxonomists of yore, phylogenetic taxonomists only recognize groups; a group derived from a single common ancestor that contains all descendents of that ancestor. Thus, any definition of invertebrate that contains all forms without a notochord but not their descendents, i.e., those with a notochord, is seen as artificial and unbiological.
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