Bobb Schaeffer

Phylogeny and Paleontology

Paleontologists have traditionally regarded the temporal sequence of fossils as central to the concept of phylogeny. In recent years the significance of the time aspect has been questioned by a number of systematists who argue that the fossil record can offer only a very incomplete picture of phylogeny and that temporal criteria are generally less reliable than morphologic ones in working out relationships. We intend to explore these different opinions by considering both the nature of paleontological data and some methodological generalizations concerning their use.

Deuterostome Monophyly and Phylogeny

The abundant data of developmental biology, both descriptive and experimental, are scattered through many serial publications and symposium volumes. However, the diversity that has been investigated by classical and modern techniques is relatively small. This is related to the reductionist aspect of much developmental research, to a minimal interest in a comparative approach, and to the availability and suitability of laboratory animals. Most symposium reports that I have examined offer little actual synthesis, although they cover key topics such as differential gene action, embryonic induction, and pattern formation.

ADAPTIVE RADIATION OF THE FISHES AND THE FISH‐AMPHIBIAN TRANSITION

A neurologist comparing olfactory pathways in two different kinds of fishes and a molecular biologist comparing the amounts of DNA in two different kinds of mammals are both working on problems related to systematics and evolutionary biology. Comparison is basic to all considerations of relationship among organisms and to all deliberations about evolutionary change. The arbitrary subdivision of thc total integrated organism into characters or character complexes is a necessary adjunct to comparison. The more characters held in common by a group of organisms, the more closely the organisms are related. As all parts of an organism do not evolve at the same rate, the entire organism is, in effect, a mosaic of primitive, or ancestral, and derived characters. But the terms “primitive” and “derived” have a relative significance. For instance, certain characters that are primitive for the ray-finned fishes (actinopterygians) are derived in terms of the ancestral osteichthyan stock. On the other hand, the characters shared by the early members of the three major osteichthyan groups (actinopterygians, crossopterygians and dipnoans) presumably were present in their common ancestor and are therefore primitive for the class Osteichthyes (FIGURE 1 ) . All organisms have special characteristics that enable them to cope successfully with their environment. The phenomenon of adaptation, which involves a very complex interrelationship between populations of organisms and their respective environments, is a major aspect of evolution. Occupation of different major and minor habitats by a group of evolving and diversifying organisms that have a common ancestry is called adaptive radiation. The teleost fishes, which have invaded nearly every available aquatic habitat since their first appearance in the Triassic, demonstrate adaptive radiation on a grand scale. Diversification may be limited by factors such as the ancestral genotype, the canalization of morphogenesis, and selection pressure. The result is another common evolutionary phenomenon called parallelism, wherein lineages of common origin have evolved numerous, closely similar characters. A good example of parallelism is the repeated independent ossification of vertebral centra in different groups of Mesozoic ray-finned fishes. Convergence, which is the development of superficial resemblance in distantly related groups (e.g. sharks and porpoises) that occupy similar environments, is actually at the other end of the spectrum from parallelism. The meaning of similarity within this continum is therefore related mainly to the proximity of common origin. The primary purpose of this introduction is to illustrate ways of thinking phylogenetically about characteristics that can be observed in fossils or in their living descendants. Because natural or restored casts of the brain cavity (frequently called endocasts) supply only limited information about the brains of extinct fishes, the comparative neurologist must obtain most of his information

George Gaylord Simpson: His Life and Works to the Present

In 1944 George Simpson wrote that “knowing more and more about less and less may mean that relationships are lost and that the grand pattern and great processes of life are overlooked.” Awareness of this problem has had a strong influence on George’s career. Without giving up the studies that must precede sound generalizations, he has contributed to almost all aspects of evolutionary biology, and his writings have had a profound effect on modern ideas about the history of life.