Casting an Eye on Complexity

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In the previous issue, I offered some thoughts on how to think about evolution (Eldredge 2008). It turns out that how “we” collectively think about evolution, starting with the most basic, fundamental conceptualization about what evolution is, has differed dramatically depending on when a given evolutionist was active, what her subdiscipline is— and even to some degree on what country she was born in. For example, a systematist or paleontologist might prefer a one-sentence definition of “evolution” as “the idea that all species that have ever lived on earth are descended from a single common ancestor.” In contrast, a geneticist might prefer to define evolution as “any permanent change in genetic information.” In addition, while both definitions may suffice for some systematists or geneticists, others (such as myself) would insist on adding “and the causal processes underlying such patterns of descent and/or change in genetic information.” And while there may be still other definitions of evolution, these two seem to constitute a dichotomy that has pervaded biological evolutionary theory at least since Darwin. One of them is about the origin and fates of species—and, by extension, higher taxa such as genera, families etc. The other focuses on the origin and further modification of genetically based properties of organisms (morphology, physiology, behavior). And although such nondeterministic causes of genetic change as Sewall Wright’s “genetic drift” (see Eldredge 2008) are in operation, basically theories of genetic change (i.e., over and above mutational and related sorts of changes in individual organisms) are theories of adaptation through natural selection—and have been so throughout the history of evolutionary thought. What’s more, the distinction between these two fundamentally different ways of defining evolution—the “taxic” vs. the “transformationist” approach (as I initially called them in Eldredge 1979) are seldom acknowledged. The tendency (probably ever since Darwin published the Origin in 1859) is to see the “origin of species” as the simple and direct consequence of the ongoing transformation of the heritable properties of organisms via adaptation through natural selection. The thought goes: If natural selection modifies a species enough, over the course of time a new species will inevitably emerge. But, as I will explore in detail in the next issue (dedicated to Charles Robert Darwin on the occasion of his 200th birthday), Darwin himself initially saw evolution as a “taxic” phenomenon: ancestral species give rise to descendant species, much as mothers give birth to infant children. He was thinking in these terms even on the Beagle and continued to do so after he returned home in 1836. Later, he essentially dropped this line of thought—but only after he had come up with the idea of natural selection—and switched over to thinking of evolution as essentially the simple modification of adaptations through natural selection. Today, we have theories of “speciation”—how new species are “born” from ancestral species (see Thanukos 2008, for a useful review in these pages), and we also have theories of adaptation through natural selection. And it is clear that the two fundamentally different ways of thinking about evolution I have sketched out above are intimately related to one another: This is not an either/or dichotomy, but rather two equally valid ways of looking at evolution that resonate with one another in more of a dialectical than a dichotomous way. For though it is possible to have a descendant species arise with hardly much genetic change Evo Edu Outreach (2008) 1:355–357 DOI 10.1007/s12052-008-0070-7