1.4 Grades and clades
If species are grouped together because they show a similar extent of accumulated anagenetic change with respect to their ancestors, then the taxa so formed constitute grades. In Figure 1, morphological change is represented along the horizontal axis. The three columns show grades of anagenetic modification, with parts of the phylogeny occupying each grade. Grades are easy to recognise, because they are based upon raw similarities between species, but they may be misleading as far as the reconstruction of phylogeny is concerned.
Why may some grades contain more than one branch of a phylogenetic tree (as in the central column of Figure 1)?
Convergent features may have evolved independently in separate lineages. A grade grouping of such species based on these features (as in the central column of Figure 1) would thus exclude their latest common ancestor (which remains in the left-hand column in Figure 1). Thus the central grade grouping in Figure 1 does not comprise a single branch from a phylogenetic tree. A grouping which assembles species with independently evolved similarities is said to be polyphyletic. Some early Victorian naturalists, for example, grouped elephants and rhinoceroses together as ‘pachyderms’, because of their thick skins (which is what the name means in Greek). However, it has long since been recognised that this feature is convergent in these animals, and so the name is no longer used for systematic purposes. Modern taxonomists attempt to avoid using polyphyletic taxa because they are misleading in phylogenetic reconstruction. To continue to talk about ‘pachyderms’, for example, might create the false impression that elephants and rhinoceroses are more closely related to each other than either is to, say, horses. Numerous other lines of evidence indicate. rather, that horses and rhinoceroses are the more closely related pair.
Not all grades are polyphyletic, however, and grade groupings which do include their common ancestor (as in the left and right columns in Figure 1) make up modem classifications. In Figure 1, for example, birds and mammals represent groupings considered to differ sufficiently from their reptilian ancestors to be recognised as distinct grades.
If, alternatively, the pattern of cladogenesis (i.e. shared ancestry) is taken as the sole criterion for recognising higher taxa, then all the descendants of a common ancestral species must be grouped together (along with the ancestral species itself), to form a clade (Figure 1: various clades are enclosed by different shades of green). A clade represents a single whole branch from a phylogeny, and, because it is derived from a single common ancestor, it is said to be a monophyletic grouping.