Complexity_ A Guided Tour - Melanie Mitchell [42]
Stephen Jay Gould, 1941–2002. (Jon Chase/Harvard News Office, © 1997 President and Fellows of Harvard College, reproduced by permission.)
Niles Eldredge (Courtesy of Niles Eldredge.)
Thus Gould and his collaborators asserted that the “gradualism” pillar of the Modern Synthesis is wrong. They also believed that the other two pillars—the primacy of natural selection and small gene variations to explain the history of life—were not backed up by evidence.
Although Gould agreed that natural selection is an important mechanism of evolutionary change, he asserted that the roles of historical contingency and biological constraints are at least as important as that of natural selection.
Historical contingency refers to all the random accidents, large and small, that have contributed to the shaping of organisms. One example is the impact of large meteors wiping out habitats and causing extinction of groups of species, thus allowing other species to emerge. Other examples are the unknown quirks of fate that gave carnivorous mammals an advantage over the carnivorous birds that once rivaled them in numbers but which are now extinct.
Gould’s metaphor for the role of contingency is an imaginary “tape of life”—a kind of time-lapse movie covering all of evolution since the beginning of life on Earth. Gould asks, What would happen if the tape were restarted with slightly different initial conditions? Would we see anything like the array of organisms that evolved during the first playing of the tape? The answer of the Modern Synthesis would presumably be “yes”—natural selection would again shape organisms to be optimally adapted to the environment, so they would look much the same as what we have now. Gould’s answer is that the role played by historical contingency would make the replayed tape much different.
Biological constraints refer to the limitations on what natural selection can create. Clearly natural selection can’t defy the laws of physics—it can’t create a flying creature that does not obey the laws of gravity or a perpetual-motion animal that needs no food. Gould and many others have argued that there are biological constraints as well as physical constraints that limit what kind of organisms can evolve.
This view naturally leads to the conclusion that not all traits of organisms are explainable as “adaptations.” Clearly traits such as hunger and sex drive lead us to survival and reproduction. But some traits may have arisen by accident, or as side effects of adaptive traits or developmental constraints. Gould has been quite critical of evolutionists he calls “strict adaptationists”—those who insist that natural selection is the only possible explanation for complex organization in biology.
Furthermore, Gould and his colleagues attacked the third pillar of the Synthesis by proposing that some of the large-scale phenomena of evolution cannot be explained in terms of the microscopic process of gene variation and natural selection, but instead require natural selection to work on levels higher than genes and individuals—perhaps entire species.
Some evidence for Gould’s doubts about the Modern Synthesis came from work in molecular evolution. In the 1960s, Motoo Kimura proposed a theory of “neutral evolution,” based on observations of protein evolution, that challenged the central role of natural selection in evolutionary change. In the 1970s, chemists Manfred Eigen and Peter Schuster observed behavior analogous to punctuated equilibria in evolution of viruses made up of RNA, and developed an explanatory theory in which the unit of evolution was not an individual virus, but a collective of viruses—a quasi-species—that consisted of mutated copies of an original virus.
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