Mike the Mad Biologist

The Macroevolution ‘Controversy’

I’ve always thought if the evolutionary biologists who invented the term macroevolution–any evolutionary change at or above the level of species–knew the mischief that the creationists would do with it, they would have ‘uninvented’ the term right then and there.

I’ve been meaning to write about this for some time, but this post by ScienceBlogling Mike Dunford where he discusses a creationist who misuses macroevolution finally gave me the much needed kick in the assreason to do so. Creationists–some anyway–have built a cottage industry out of claiming that while they might accept ‘microevolution’ (i.e., small genetic changes in populations like coloration), ‘macroevolution’ (e.g., ‘large’ changes such as the evolution of tetrapods from fish) is impossible.

Of course, the creationists conveniently gloss over the fact that most ‘macroevolutionists’ clearly state that the mechanism of evolutionary change occurs through population genetic changes. In other words, macroevolutionary patterns and trends are derived from microevolutionary processes. So why invent the term macroevolution in the first place? To understand this, we need to examine the history of evolutionary biology.

Flashback to the 60s and 70s. At that time, population genetics, for all intents and purposes, was evolutionary biology, to the virtual exclusion of other disciplines such as paleontology. Despite George Gaylord Simpson’s claim that paleontology had ‘a seat at the high table of evolution’, the published evolutionary biology literature certainly didn’t reflect it. A common notion among evolutionary biologists was that if we understood population genetics better, we would understand the evolution of life (an aside: some population geneticists were bright enough to realize that this wasn’t the case). Paleontology simply was not incorporated into evolutionary theory at that time, and was even occasionally disparaged as “bone collecting” or “stamp collecting.” Even worse, it was regarded as only ‘systematics’ or ‘taxonomy’ (which is ironic in light of the phylogenetics revolution that would later sweep through evolutionary biology and revitalize population genetics through coalescent theory).

Population genetics is an incredibly powerful tool (and having considerable population genetics training, one I use quite often). Among other things, it can inform us about evolution within a population, potential mechanisms of selection, and even enable short term prediction of genetic changes. However, there are entire realms of questions that can not be answered by population genetics. Biogeography is one of those areas. The Wikipedia definition of biogeography makes clear the shortcomings of population genetics:

Biogeography is the science which deals with patterns of species distribution and the processes that result in such patterns. The patterns of species distribution at this level can usually be explained through a combination of historical factors such as speciation, extinction, continental drift, glaciation (and associated variations in sea level, river routes, and so on), and river capture, in combination with the area and isolation of landmasses (geographic constraints) and available energy supplies.

These phenomena are all vital to understanding evolution; however, theories about the change of gene frequencies in populations do not incorporate many of these processes and were never designed to do so. While the separation of land masses can result in evolutionary change due to different selection pressures, and, thus, a change in gene frequencies, a population genetic explanation simply can not explain the cause of these changes. There is no population genetic framework to explain a priori the effects of glaciation, sea level variation, or plate tectonics. Retrospectively, we can use population genetic methods to infer the past effects of glaciation. However, there is no prospective or predictive theory of glaciation incorporated into population genetics.

For example, on each side of the Isthmus of Panama, sister species of marine organisms are observed. There is no population genetic framework to explain this. What happened is that when the Central American land bridge formed, one continuous species was subdivided into two (shrimp can’t crawl across all of Panama). Understanding the sister species pattern requires explanations that are not solely based on population genetics.

Perhaps the first noticeable use of the term was Steven Stanley’s book Macroevolution in 1979. What Stanley and others were trying to do was (among other things):

1) Make the case that paleontology and paleontological data were more than ‘stamp collecting.’ Paleontology was essential for understanding how life evolved, not just as a record of what happened, but in terms of elucidating evolutionary mechanism.

2) Reincorporate macroevolutionary analyses into the evolutionary synthesis. Examining traits across species (not within species as population biologists were already doing) was in its infancy, and largely the purview of community ecologists.

3) Related to point 2, reincorporate community approaches into evolutionary biology. While particular species-interactions had been studied (i.e., host-pollinator interactions, mimicry), evolutionary biology was largely divorced from community ecology.

4) Incorporate phylogenetics and systematics into the modern synthesis. While this sounds like a truly odd statement to make in the era of coalescent theory (the combination of phylogenetics and molecular population genetics), a lineage-based approach to populations and species was not being used in mainstream evolutionary biology.

Overall, I would argue that the ‘macroevolutionists’ were successful, in part because as each of the subdisciplines matured, the gaps between the disciplines could begin to be closed. But it’s still annoying as hell to listen to creationists lie invent arbitrary distinctions. Most ‘macroevolutionists’ (barring a brief interlude of loopiness by Gould and a few others) argued that population processes could explain much of the diversity of life, but that other data were valuable and needed to fully understand evolution.

Not that any of this will convince creationists….

Comments

  1. #1 Jud
    July 6, 2007

    So if taxonomists re-classify two groups of organisms that were formerly considered distinct species as two variants of a single species, does that mean it’s now OK for IDers to believe one can evolve into the other?

  2. #2 mirc
    April 1, 2009

    thanks..