Some features of phenotypic evolution

Pattern of evolution as exemplified by the history of vertebrates

Before we treat evolutionary rates at the molecular level, I shall briefly mention some features of phenotypic evolution as revealed by fossil records. This will help to show that the pattern of molecular evolution is quite different from that of phenotypic evolution.

Through paleontological studies in the past hundred or so years, much valuable information has been obtained on the evolutionary history since the Cambrian period (600–500 million years ago) when abundant fossil records became available. Particularly noteworthy is the elucidation of the history of vertebrates, a group to which we ourselves belong. (For authoritative accounts of this subject, see Romer, 1967, 1968 and McFarland et al., 1979.) The history of vertebrates is also relevant when we discuss the evolutionary rate of globins in the next section. So I shall summarize some relevant aspects of vertebrate history.

The oldest known true vertebrates were jawless fish, the agnatha, whose fossils are found in the Ordovician period (500–440 million years ago; for geological periods, see Fig. 4.2). Bony scales of such animals are known from late in the Cambrian period. They ranged in size from several to fifty centimeters long, and had curious appearances, with unpaired fins.