Cover Image

HARDBACK
$59.00



View/Hide Left Panel

(1963) preferred to maintain the division between pongids (divided into hylobatines and pongines) and hominids (in spite of clearly realizing this was not a natural or phylogenetic classification) mainly on the basis of notions of evolutionary grade. The human line occupied a new adaptive zone that warranted its own family, even though the pongids were a para-phyletic group. According to Simpson, this will inevitably occur when a new family emerges from an old one: “Classification cannot be based on recency of common ancestry alone.” Today some authors still use the term hominid—in the traditional way—to refer to all taxa of the human lineage after its separation from chimpanzees, whereas other authors prefer to call them hominins.

HUMAN ORIGINS AND QUANTUM EVOLUTION

Once Darwinism (in the strict selectionist sense) was returned to a central place in evolutionary theory, the neo-Darwinians could turn their attention to other matters. The paleontologist G. G. Simpson distinguished in 1944 three patterns of evolution based on the fossil record: speciation, phyletic evolution, and quantum evolution. The first was responsible for the appearance of the lower taxonomic categories and explained the enormous proliferation of species that exist in the biosphere. The second produced the intermediate-level taxonomic categories and accounted for the evolutionary tendencies that paleontologists found everywhere when organizing fossils in progressive series that seemed to reflect gradual and directional changes. The third pattern was the cause of large-scale changes in the adaptive types (biological designs or body plans) that were produced in relatively short periods of geological time and that gave rise to large evolutionary novelties in the highest-level taxonomic categories.

Quantum evolution, as its name suggests, seemed opposed to the fundamental idea of evolutionary synthesis (i.e., that natural selection governed evolution), because passing from one adaptive plateau to another implied a loss of fitness, and natural selection never favors the less adapted. Because of this, Simpson (1953) presented this mode of evolution in a more orthodox form, as a case of phyletic evolution that proceeded at a more rapid rate than normal (due to an increase in the selection pressure).

But already by 1950, at the Cold Spring Harbor Symposia on Quantitative Biology, Simpson (1950) did not use the term “quantum evolution,” but rather considered human evolution to represent a “change from one adaptive type to another.” To explain how a change from one adaptive plateau to another was possible, Simpson no longer held that a maladaptive valley had to be traversed. The intermediate forms could now enjoy the advantages of both adaptive types, the old one that was being aban-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement