Evolutionary Phylogeny vs Progress

Evolving portrayals of the evolution of Vertebrate classes

    Creatures in the Phylum Vertebrata (animals with backbones) are conventionally placed in seven extant and one extinct taxonomic Classes, with common names as indicated above. These classes describe eight recognizable "kinds" of creatures, based principally on dichotomies: jawless or jawed "fish-like" things, cartilaginous or bony "fish," scaleless or scaly tetrapods, and things with feathers or hair. Evolution is then popularly viewed as a progressive, linear transformation from "primitive" aquatic, fish-like forms to "advanced" terrestrial forms, with the transition from apes to humans as the endpoint of the sequence. The scale is implicitly one of increasing complexity or progress moving from bottom to top, and from left to right. This mirrors the ancient concept of the Scala Naturae or "Great Chain of Being" that dominated Western thinking through the early 17th century. This iconography creates false impressions: for example, that fish evolve into salamanders into dinosaurs into apes into humans, or that we should expect to find intermediate "links" in the chain. Implicitly, if mammals are the most complex animals, then humans are the most complex mammals, and (obviously) the end-point of the series.

    Subsequently, classical taxonomists beginning in the mid-18th century defined the seven Classes of living vertebrates by combinations of presence & absence characters that contributed to their similarity. For example, reptiles are four-legged scaly creatures, recognizing that legless snakes evolved from four-legged ancestors. Feathers and hair are structural modifications of scales that define birds and mammals, respectively. Modern Amphibia are four-legged scaleless creatures, where legless amphisbaenians evolved from four-legged ancestors, and loss of scales is a structural modification that permits dermal respiration, which may have evolved more than once.

In modern taxonomy, historical evolution is properly viewed as a branching "tree" that describes the phylogeny (relationships in time) of groups under consideration. The phylogenetic diagram below shows the phylogeny of the same set of creatures illustrated above, with a node indicating evolutionary branching of lineages, and a branch showing evolutionary change along each lineage. The horizontal scale is explicitly temporal, and shows the time of divergence of each lineage as indicated by fossil and (or) molecular evidence. Note that the order from top to bottom (Darwin's "Descent with Modification") has been arranged to show the same sequence as in the progression from bottom to top above. However, this is an artifact of laying a complex, multidimensional structure (reminiscent of a 3-D mobile) flat in two dimensions. Because every node can be rotated, many alternatives are possible. For example, the second diagram below rotates the node between Reptilia / Aves and Mammalia, which place Birds in the bottom position. Because each successive branch now descends, this would emphasize that Aves is the most recently evolved of the conventional classes, but not necessarily the most "complex", which is a subjective judgement.

    Modern phylogenetic taxonomy assigns names to these evolutionary lineages rather than similar groups, and introduces a variety of unfamiliar names. Each of the six internal nodes creates another group, with another name, many of them unfamiliar. The diagrams below do not include certain lineages, such as those leading to fish-like coelacanths and lungfish, or dinosaurs & other large, extinct scaly forms, as well as minority reptiles such as tuataras (Sphenodon), turtles, and crocodilians, which complicate the picture and naming considerably. In particular, crocodilians and birds are the most closely related living 'reptiles', as recognized by a diapsid (two-opening) skull . Mammals are most closely related to another group of extinct 'reptiles' as indicated by a synapsid (one-opening) skull. Feathers first appear in terrestrial dinosaurs, and hair in extinct 'reptilian' ancestors, but allow recognition of modern birds and mammals as distinct among living animals. As well, the Agnatha ('jawless') are recognized as an artificial group that comprises two very ancient evolutionary lineages: the hagfish lineage branched off before the evolution of jaws, and a second lineage evolved from a jawed ancestor to jawless, parasitic lampreys.