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Macroevolution for the People

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Evolution is about genetic and developmental continuity across generations, the same ones that join you with your children, parents, and grandparents. Broad consensus exists among scientists that variation in this continuity from one generation to the next, extrapolated over geological time, comprises a (and probably the) fundamental mechanism behind the diversity of life. One popular objection to biological evolution involves a dichotomy: micro- vs. macroevolution. Creationists are fine with "microevolution," an undeniable phenomenon which explains the appearance of (for example) drug-resistant bacteria within the scale of a human lifetime. "Macroevolution" is something utterly different, or so it is claimed.

Nonsense. When terms like micro and macro are used to bolster the case for creationism or a "designer", the dichotomy is a ruse, along with implications that scientific debate on macroevolution supports assertions of supernatural design. To explain why, I'd like to resurrect the bleeding edge of paleobiology circa 1972, and by the way I'm doing so partly in response to a recent review by UC Berkeley biologist Brian Swartz of my book, Evolution and Belief. In it, Swartz wrote some nice things and raised some points with which I very much agree. On the other hand, I think his portrayal of certain aspects of macroevolution are incorrect. More importantly, even when two biologists disagree about some aspect of evolutionary theory, they have far more in common with each other than either does with advocates of creationism or so-called "Intelligent Design" (ID).

Terms like micro- vs. macroevolution have been used in mainstream biology for several decades to grapple with the relative contributions of distinct, natural mechanisms to evolution. Genuine scientific debate exists about how one natural influence on biological descent (e.g., adaptation) might predominate over another (e.g., group selection), but none of this entails an attribution of "design" to understand biodiversity. This is for the simple reason that a major thrust of science is to elucidate mechanisms, or how things work. Evolution through natural selection is one such mechanism which---even if it were inaccurate or incomplete---cannot be replaced by assertions of "design" that are completely bereft of a mechanism.

Books sympathetic to ID & creationism, for example the Discovery Institute's 2007 Explore Evolution, have never articulated a tangible mechanism that could compete with selection or any other evolutionary process, much less comprise (in Swartz's words) a "strong attack" against it. This is a book which tells a school-age audience one thing (e.g., stratigraphy does not match phylogeny), while citing experts who argue precisely the opposite. This is a book whose supporters bizarrely deny that it advocates ID, when its authors and recycled arguments have been doing exactly that for years. Explore Evolution draws on the vocabulary of micro vs. macroevolution for the purpose of pretending there's debate about natural vs. non-natural selection; it is less interested in the genuine debate about (for example) selective hierarchies or biological constraint.

Fifty years ago, the consensus of evolutionary biology was---arguably---that "evolution by natural selection" meant adaptive changes in gene frequencies in single lineages over time, or microevolution, the cumulative effect of which yields common ancestry among biologically diverse species. Although many equate "evolution" with an endeavor to explain animal form in terms of adaptation, evolution is really a broader phenomenon, encompassing topics like genetic drift, adaptation, selection on individuals and perhaps also groups, all mediated by development and constraint. Resulting from this process are patterns like common ancestry and shared features of anatomy, genetics, and geography. I distinguished these concepts in my book, and gave examples of constraint, group selection, adaptation, development, and molecular mechanisms behind some of these phenomena. Swartz's review rightfully emphasized that---both in my book and elsewhere---distinctions between pattern and process are worth repeating, along with the understanding that evolution comprises more than random mutation and selection thereof.

So where do Swartz and I part ways? Consider a question he posed in his review: "do higher-level processes diminish the importance of conventional selection when evolution is viewed in the long run?" Although he didn't come right out and say it, Swartz implied a "yes", whereas for me the answer is "not at all". We might also differ in our understanding of "conventional selection"; I'm willing to entertain a broader definition that entails mechanisms that are natural, but not necessarily adaptive or individual. In my view, any implication that macroevolution is not dependent on selection operating on genetic and developmental continuity across generations is misleading.

Take "punctuated equilibrium" for example. This idea, first enumerated by Niles Eldredge and Stephen Jay Gould in 1972, asked how the fossil record might appear given a particular theory of speciation. Small, geographically isolated founder populations (for which the terms like "allopatric" or "peripatric" apply) are less likely than large populations to leave behind fossils, and compared to large populations may exhibit more rapid fixation of novel characters. Eldredge and Gould lamented an expectation that, they said, paleontologists had about gaps in the fossil record as artifacts of geology and deposition, rather than anything biological. "Gradualists", they wrote, held that "new species arise from the slow and steady transformation of entire populations" and budding paleontologists were led into this expectation by muddled textbooks and uncritical colleagues.

In contrast, Eldredge and Gould argued that gaps in the record were not artifacts but resulted from a positive, biological signal. "If new species arise in very rapidly in small, peripherally isolated populations, then the great expectation of insensibly graded fossil sequences is a chimaera". In other words, the appearance of a species, without a complete series of intermediate forms linking it to a close relative, comprises positive evidence for the occurrence of a particular model of speciation: allopatry. When a paleontologist confronted with such a pattern infers geological gaps, rather than a model of biological speciation, s/he misses the forest due to the empty spaces between the trees.

Much has happened since 1972, including a general appreciation that while macroevolutionary thinking has benefitted evolutionary research, "punctuated equilibrium" did not fundamentally depart from the evolutionary mechanisms already appreciated by most 20th century biologists (see summaries by Leroi and Futuyma). The critical point to remember---whether one is countering claims that SJ Gould "didn't believe in evolution" or distilling the subtleties of group selection to undergrads---is that microevolution is at the core of Eldredge and Gould's idea. Gould liked hyperbole and flamboyant writing, but even he underscored the conventional, Darwinian basis of punctuated equilibrium many times, for example in 1985 (p. 3), 1987 (pp. 2-3), 1993 (p. 223), and 2002 (p. 986).

Whether you're a creationist or science writer, if you think that macroevolution really does exceed mechanisms such as inheritance, development, mutation, constraint, drift, and selection, then the burden is on your shoulders to better articulate just what it is you're talking about. Perhaps more importantly, to present "design" as an alternative to evolution by natural selection is to restate the problem without answering it, much like claiming that "Rolex" answers the question "how does this watch work"? It doesn't. Even when we disagree on aspects of macroevolutionary theory, mainstream evolutionary biologists generally realize this. Our task is to try and understand the mechanisms by which "from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."