MACROEVOLUTION


Were Darwin's extrapolations justified? Judging from the conclusions of many of the scientists attending one of the most important conferences in evolutionary biology in the past forty years, the answer is probably not.

"The central question of the Chicago conference was whether the mechanisms underlying microevolution can be extrapolated to explain the phenomena of macroevolution. At the risk of doing violence to the positions of some of the people at the meeting, the answer can be given as a clear, No.

... Evolution, according to the Modern Synthesis, moves at a stately pace, with small changes accumulating over periods of many millions of years yielding a long heritage of steadily advancing lineages as revealed in the fossil record. However, the problem is that according to most paleontologists the principle feature of individual species within the fossil record is stasis, not change...

In a generous admission Francisco Ayala, a major figure in propounding the Modern Synthesis in the United States, said "We would not have predicted stasis from population genetics, but I am now convinced from what the paleontologists say that small changes do not accumulate."

"Feathers are features unique to birds, and there are no known intermediate structures between reptilian scales and feathers. Notwithstanding speculations on the nature of the elongated scales found on such forms as Longisquama ... as being featherlike structures, there is simply no demonstrable evidence that they in fact are. They are very interesting, highly modified and elongated reptilian scales, and are not incipient feathers."

"The Modern Synthesis is a remarkable achievement. However, starting in the 1970s, many biologists began questioning its adequacy in explaining evolution. Genetics might be adequate for explaining microevolution, but microevolutionary changes in gene frequency were not seen as able to turn a reptile into a mammal or to convert a fish into an amphibian. Microevolution looks at adaptations that concern only the survival of the fittest, not the arrival of the fittest. As Goodwin (1995) points out, "the origin of species -- Darwin's problem -- remains unsolved."

This theme is developed at much greater length, and with considerable insight, in Rudy Raff's new book, The Shape of Life: Genes, Development, and the Evolution of Animal Form, University of Chicago Press, 1996 (520 pages, $29.95 in paperback).

"The facts of microevolution do not suffice for an understanding of macroevolution."

The Meanings of Diversity and Disparity

"I must introduce at this point an important distinction that should allay a classic source of confusion. Biologists use the vernacular term diversity in several different technical senses. They may talk about "diversity" as number of distinct species in a group: among mammals, rodent diversity is high, more than 1,500 separate species; horse diversity is low, since zebras, donkeys, and true horses come in fewer than ten species. But biologists also speak of "diversity" as difference in body plans. Three blind mice of differing species do not make a diverse fauna, but an elephant, a tree, and an ant do -- even though each assemblage contains just three species.

The revision of the Burgess Shale rests upon its diversity in this second sense of disparity in anatomical plans. Measured as number of species, Burgess diversity is not high. This fact embodies a central paradox of early life. How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species? -- for the two are correlated, more or less in lockstep, by the iconography of the cone (see figure 1.16). ...

Several of my colleagues (Jaanusson, 1981; Runnegar, 1987) have siggested that we eliminate the confusion about diversity by restricting this vernacular term to the first sense -- number of species. The second sense--- difference in body plansshould then be called disparity. Using this terminology, we may acknowledge a central and surprising fact of life's history -- marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs."

"We have had enough of the Darwinian fallacy. It is time that we cry: 'The emperor has not clothes.'"

Lynn Margulis says that history will ultimately judge neo-Darwinism as "a minor twentieth-century religious sect within the sprawling religious persuasion of Anglo-Saxon biology."

"Micro-evolution involves mainly changes within potentially continuous populations, and there is little doubt that its materials are those revealed by genetic experimentation. Macro-evolution involves the rise and divergence of discontinuous groups, and it is still debatable whether it differs in kind or only in degree from microevolution. If the two proved to be basically different, the innumerable studies of micro-evolution would become relatively unimportant and would have minor value in the study of evolution as a whole."

"[T]he origin of no innovation of large evolutionary significance is known."


"[L]arge evolutionary innovations are not well understood. None has ever been observed, and we have no idea whether any may be in progress. There is no good fossil record of any."