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The Definitions of "Evolution"

The National Center for Science Education (NCSE) alleges that Explore Evolution (EE) uses "erroneous" and "irrelevant" definitions of the term "evolution" and employs "a false distinction between microevolution and macroevolution." These errors are said to flow from a "modern creationist strategy" of misrepresenting the definitions of science and evolution. Evolution, according to the NCSE, is a "single concept." Really?

Beginning with Darwin himself, evolutionary biologists have distinguished the theory that evolution (i.e., descent with modification) occurred from hypotheses about how evolution occurred. Indeed, it would be impossible to understand post-Darwinian controversies about, for instance, the relative importance of natural selection, if "evolution" were a single, or unitary concept, or if logically and evidentially distinct notions referred to as "evolution"--such as change over time, universal common descent, and natural selection--were "just different consequences of the same idea."

Universal common descent, for example, a thesis about historical pattern, could not be merely a different "connotation" of evolution, such that the same word, "evolution," simultaneously connoted the process of natural selection, or any other evolutionary mechanism. If "evolution" were such a unitary concept, the intense debates that occupied evolutionary biology throughout the late 19th and early 20th centuries, and that continue today, would never have occurred.

The history of evolutionary theory plainly indicates that biologists distinguish multiple meanings of "evolution," some of which they find reasonable and well-supported, and others not. Surveying the post-Darwinian reception by the scientific community of the arguments in the Origin of Species, historian of biology Peter Bowler (2003, 181) writes,

The fact that evolution was accepted while selection remained under suspicion shows that we must evaluate developments in the scientific community at different levels....There was no simple conversion to a monolithic theory brought about by the weight of evidence.

Neo-Darwinian theorist Ernst Mayr (1982) distinguishes "five major components" of Darwin's theory of evolution: (1)"evolution as such," or what EE calls "change over time," (2) universal common descent, (3) speciation, (4) gradualness, and (5) natural selection. Mayr observes that

Although the biologists accepted evolution and common descent almost unanimously, most of them had reservations with respect to natural selection....None of the opposition to Darwin was as serious as that coming from his own profession. In the 80 years after 1859, the non-Darwinian biologists were decidedly in the majority. (1982, 120)

Most biologists in the period Mayr describes accepted "evolution," if that word denoted universal common descent, but rejected "evolution" if that word denoted Darwin's favored role for natural selection. This could only be possible, however, if "evolution" possesses multiple logically and evidentially distinct meanings.

In short, "evolution" is not, and never has been, a single concept. EE correctly separates the multiple meanings of the word, and explains that accepting one meaning of evolution does not entail accepting others.

The distinction between "microevolution" and "macroevolution."

The NCSE alleges that EE misrepresents the distinction between "microevolution" and "macroevolution." In their fullest discussion of this charge, they write:

In reality, anyone who denies the logical link between genetic changes within a population ("microevolution") and speciation ("macroevolution") is similar to someone who watches the sun come up in the east and move west across the sky, but denies that it will set in the west. The only difference between genetic changes within a population and generation of a new species from that population is time. Given enough time, the sun will set in the west.

Yes--the sun will set, but if understanding macroevolution were as easy as agreeing on the daily inevitability and compass direction of the sunset, current debates within evolutionary theory about the genetic and developmental bases of major transitions would have been settled long ago. But those debates continue.

Evolutionary developmental biologist Wallace Arthur, of the National University of Ireland, expresses the key issue as follows:

Here is a question of utmost importance for our understanding of what has been called the 'big picture' of evolution (Simpson 1944, 1953): are the divergences that lead ultimately to higher-level groups, such as those that would typically be labeled as orders, classes, and phyla, qualitatively or quantitatively different from those that lead to low-level sister groups, such as races, species and genera? (2008, 30)

It would be odd even to ask this question, much less to describe it as "of utmost importance," if the NCSE were right that macroevolution--the origin of higher-level differences--were generally known to be simply microevolution, plus time. The sun sets in the west, each evening: everyone knows that already, right?

But evolutionary and developmental biologists, geneticists, and paleontologists differ strongly over the sufficiency of microevolutionary processes to explain macroevolutionary differences. Caltech developmental biologist Eric Davidson, for instance, has long urged that what Arthur calls "mega-evolution"--the origin of novel body plans--is qualitatively a fundamentally different process than microevolution. Davidson (2006, 192) argues that "traditional microevolutionary theory is not useable for treatment of the molecular mechanisms by which evolution of the animal body plan has occurred." As he explains,

contrary to classic evolution theory, the processes that drive the small changes observed as species diverge cannot be taken as models for evolution of the body plans of animals. These are as apples and oranges, so to speak, and that is why it is necessary to apply new principles that derive from the structure/function relations of gene regulatory networks to approach the mechanisms of body plan evolution. (2006, 195)

This debate within evolutionary theory has, in fact, been so extensive and long-lasting that many biologists open their papers on the topic with a stock description of the controversy, along the lines of "To begin with what we all know..." For instance:

A persistent debate in evolutionary biology is one over the continuity of microevolution and macroevolution--whether macroevolutionary trends are governed by the principles of microevolution....The continuity of selective processes over microevolutionary and macroevolutionary time continues to be a source of disagreement in evolutionary biology (Solé et al. 1999; Erwin, 2000; Carroll, 2001; Plotnick & Sepkoski, 2001), one that Maynard Smith (1989) described as 'unsatisfactory.' In dispute is whether the effects of selection operating over microevolutionary time, or at the population level, account for observed trends over macroevolutionary time.

These are the opening sentences of the abstract and introduction, respectively, of University of Minnesota evolutionary biologist Andrew Simon's 2002 paper "The continuity of microevolution and macroevolution." Simons notes that "resolution of the continuity issue is critical" (2002, 688) and his paper proposes a possible solution to the problem.

Probably the most forceful statement of the micro-macro controversy within the past two decades, however, can be found in the work of NCSE President Kevin Padian. Reviewing a major evolutionary biology text by Jeffrey Levinton, Padian passionately lays out the need for new approaches to the micro-macro problem. His arguments are worth quoting at length, to end this response:

Macroevolution is evolution "above the species level," that is, evolution that supersedes that of interbreeding populations, the domain of microevolution. Just the fact of a break in the interbreeding continuity of genetic lineages at speciation demonstrates a level of phenomena of evolutionary interplay that is not microevolutionary. And this level clearly has its own patterns and processes. If we proceed from the data base, rather than from an assumption of extrapolationism, we might get a crack at understanding what these processes are.
Even on his own terms, Levinton has missed the process of macroevolution as he defines it. In supporting the conventional extrapolationist view of evolution, he excludes a lot of problems that simply have no counterpart in the orthodox literature of the Modern Synthesis. How do major evolutionary changes get started? Does anyone still believe that populations sit around for tens of thousands of years, waiting for favorable mutations to occur (and just how does that happen, by the way?), then anxiously guard them until enough accumulate for selection to push the population toward new and useful change? There you have the mathematical arguments of neodarwinism that Waddington and others rightly characterized as "vacuous." The attention is on the gauges, not on the machinery. For example, the soles of the human feet and the ventral callosities of the ostrich begin to thicken before birth; how did the response first engendered by post-natal wear become anticipatory, captured by the genome? What is the interplay between morphogenesis and genetics, and how does an organisms phenotypic behavior translate through developmental change into the hereditary program? If feathers did not evolve "for" flight, of what use is a neodarwinian explanation that demonstrates that they could evolve gradually from scales? What does that explain?
I would like to see a new evolutionary synthesis that approaches questions of how morphogenesis constructs new features, and how it does it so well, so often, and so quickly. (1989, 77)

If Padian stands by these arguments today, the NCSE (whose board of directors Padian chairs) should revisit its claim that macroevolution is only microevolution plus time.

References Cited
Arthur, Wallace. 2008. "Conflicting hypotheses on the nature of mega-evolution." In A. Minelli and G. Fusco, eds., Evolving Pathways (Cambridge: Camb. Univ. Press), pp. 50-61.

Bowler, Peter. 2003. Evolution: The History of an Idea (Berkeley: Univ. of Cal. Press).

Davidson, Eric. 2006. The Regulatory Genome (New York: Academic Press).

Mayr, Ernst. 1982. "Epilogue to special issue on Darwin." Biological Journal of the Linnean Society 17:115-25.

National Center for Science Education. 2008. Sections on "Evolution" and "Limits on Evolution" in the NCSE critique of Explore Evolution. Available at http://ncseweb.org/creationism/analysis/evolution and http://ncseweb.org/creationism/analysis/extrapolations as of January 16, 2009.

Padian, Kevin. 1989. "The Whole Real Guts of Evolution?" Paleobiology 15:73-78.

Simons, Andrew. 2002. "The continuity of microevolution and macroevolution." Journal of Evolutionary Biology 15:688-701.


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This page contains a single entry from the blog posted on February 24, 2009 1:11 PM .

The previous post in this blog was Monophyly vs. Polyphyly and Christian Schwabe .

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