The panselectionist viewpoint is the theory that natural selection acting on genetic variation is virtually the only acceptable mechanism of evolutionary change. Darwin did not support panselectionism. Though he considered natural selection to be the most important mechanism of evolution, he held a pluralistic viewpoint in which he insisted his theory did not rely on natural selection alone. In contrast, Wallace did ascribe to panselectionism, believing that all variation must be in some way adaptive. The authors of the article The Spandrels of San Marco and the Panglossian Paradigm hold Darwin’s view that, though natural selection is perhaps the most important instrument of evolution, it is certainly not the only one. One alternative mechanism the authors mention is selection without adaptation. They offer a hypothetical example in which a mutation that significantly increases the fertility of individuals occurs. It would proliferate quickly within a population, but unless it also caused a change in efficiency of resource utilization, the individuals will just lay twice as many eggs, with the excess dying because of limited resources. If a predator that eats eggs is led to switch to the species now that the eggs are more plentiful, the population size may actually decrease as a consequence, yet natural selection at all times will favor individuals with higher fecundity, despite the organism not being any better adapted. Another example is genetic drift. Due to limitation on population size, the groups that form the first step in the speciation process are often founded by only a small number of individuals. As a result, frequencies of alleles change by random genetic drift, and populations will become genetically differentiated despite the absence of any selective force.
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