Evolution Icon Evolution:

“Macroevolution” and Its Discontents


desert_cottontail_rabbit_in_petrified_forest_national_park

The term “macroevolution” has problems. Why? Among other things, because it’s a term that means different things to different people. Case in point: an email correspondent points out a random usage on a BioLogos Forum thread, “Is evolution continuing? Is God still creating?” It’s a comment from a Forum participant, Socratic.Fanatic, about rabbits:

Northern and southern populations of a common species of North American rabbit is quickly becoming TWO distinct species and populations which can no longer cross-reproduce. It is a great example of macroevolution directly observable right in front of our eyes.

Without wishing to jump down Fanatic’s throat, we took a moment to Google that one and could not find anything that resembles it — nothing about speciation in a North American rabbits. It’s possible we didn’t look hard enough. In any case, the phrase “quickly becoming” suggests the populations have achieved reproductive isolation, which implies they aren’t truly distinct species.

We asked for a reference, and no doubt Fanatic will supply one in good time. It’s possible that he’s thinking of two squirrel populations on opposite sides of the Grand Canyon, which are almost identical.

In any event, let’s review two different classes of definitions of “macroevolution” found in a couple of college biology textbooks that we just grabbed off the shelf. They give two different definitions of the word.

(1) One, represented by Douglas Futuyma textbook Evolutionary Biology (1998),defines “macroevolution” in terms of the taxonomic hierarchy: “the origin and diversification of higher taxa.”

And what are “higher taxa”? Higher taxa are generally considered any taxa above “species.” That would include genera and above. That point will become important in just a moment.

(2) Others, represented by Campbell’s Biology (1999), define “macroevolution” in terms of the origin of biological novelty: “Evolutionary change on a grand scale, encompassing the origin of novel designs, evolutionary trends, adaptive radiation, and mass extinction.”

Regarding Definition (1), Michael Behe takes a similar approach in The Edge of Evolution where he says evolution at the species level is feasible.

He allows that evolution at the genera, family, or order level could be possible. But as he argues in the book, evolution at the class level or above is “beyond the edge of evolution.” So if macroevolution includes evolution at the genera, family, or order level, Behe concludes that what some consider “macroevolution” might be possible.

As for Definition (2), the theory of intelligent design has no problems with macroevolution when defined as “mass extinction.” However, many ID proponents are skeptical that material mechanisms can produce novel traits. So here it might be fair to say ID poses its biggest challenge to “macroevolution.”

But the formation of a new species isn’t necessarily a big deal for ID. In fact, pending clarification on the rabbit issue, we’re skeptical that this rabbit example entails the evolution of any kind of biological novelty. At best it’s probably just two species that are partially reproductively separated.

In short, what’s at stake are somewhat semantic questions about how we define “macroevolution.” The key point is that we know there are limits to what Darwinian evolution can accomplish. We see these limits in experiments on features that require multiple mutations before giving any advantage. Darwinian evolution gets stuck.

For example, Douglas Axe has found that in prokaryotes the limit is more than six mutations to get an advantage. Behe suggests that in multicellular organisms, any feature that requires more than two mutations to give an advantage is beyond the limit of what unguided evolution can do.

Anything below these limits is microevolution. Anything above it cannot happen by Darwinian mechanisms, mathematically speaking. Perhaps one could say that is “macroevolution.”

Photo: Desert cottontail rabbit, by Park Ranger [CC BY 2.0], via Wikimedia Commons.