Is macroevolution impossible to study?

Once again, the Discovery Institute is playing word games with educational systems, trying to give legal protection to religion-based incompetence. I refer, of course, to the ongoing debate about standards in Texas, and the insidious influence that the DI is wielding.

As Wesley Elsberry notes in his summary of the alleged weaknesses of evolutionary theory, an oft-repeated mantra rears its head yet again. This ID tenet holds that macroevolution is either not possible, or cannot be observed, or cannot be studied (or any combination of the these). Apparently, Board of Education member Ken Mercer is of the opinion that macroevolution has not been observed.

This is one of the “weaknesses” that really betrays the many levels of ignorance of the ID proponent. For it implies that, because it would seem to be impossible to replicate a complete macroevolutionary transition that is expected to take millions of years, it is not possible to test specific questions about the process using the time-honored process of hypothesize, test using controlled and repeatable experiment, and revise. This is patently ludicrous.

Worse still, however, this ID claim implies that macroevolution cannot be observed in controlled experimental settings. As the following discusses, this is false. (Of course, I must add a caveat here. When cornered, the ID proponent will define macroevolution as any evolution that has not been observed. Of course, this deflates their objections, or should if one cares a whit about the irrelevance of circular reasoning, but logical consistency isn’t something we expect from the Discovery Institute.)

To begin with, it helps to reflect on what might constitute a macroevolutionary transition. Ask yourself, if you might, what distinguishes, say, animals from plants. Curious and perceptive students could come up with an interesting list, and I daresay many of the distinguishing characteristics would trace themselves to a fundamental difference that is seen at the cellular level. Of course, this difference is the presence of an organelle - the chloroplast - in plant cells that is absent from animal cells. This difference distinguishes entire kingdoms, and changes that modified an organism such that it possesses a new, additional, organelle would undeniably be considered as macroevolutionary. Similarly, changes that are clearly in such a pathway would have to be considered macroevolutionary.

Enter into the picture the long-running studies of Kwang W. Jeon. More than 40 years ago, Jeon and Lorch reported the discovery of a novel strain of Amoeba proteus, so-called xD amoeba (2). This strain differed from its parent (the D strain) in that it possessed a bacteria-like endosymbiont (termed XB, and shown later to be related to Legionella species). xD amoeba are totally dependent on their endosymbiont, as removal of the XB bacteria from the xD strain is lethal to the amoeba. xD amoeba possess a number of novel proteins; one of these interferes with lysosomal recognition of the endosymbiont, one (a nuclear protein) inhibits growth of D amoeba, and one (coded for by the so-called s29x gene) is encoded by the XB genome and exported to the cytoplasm. (This is likely far from an exhaustive list of novel proteins found in xD amoeba - I am not aware of proteomic studies that have been conducted with xD amoeba.) More recent studies have shown that the XB endosymbiont directly controls the expression of at least one nuclear gene, possibly via adenine methylation. (This modification is typically found in prokaryotes, and its occurrence in the xD strain suggests that the XB Dam methylation system can access the nucleus of the xD strain, perhaps by the same mechanisms that allow the s29x protein to pass from endosymbiont to cytoplasm.)

Reflect, now, on the ramifications of this system. Organelles such as the chloroplast and mitochondria arose by endosymbiotic events. What we see with xD amoeba are the early stages of another such event. The xD strain is dependent on the endosymbiont, much as plant cells depend on chloroplasts, and eukaryotic cells on mitochondria. Moreover, as is the case with more recognizable organelles, gene expression and cellular physiology in the xD strain have become interdependent, such that endosymbiont and nucleus communicate and control expression and metabolism. This system is arguably the beginnings of the evolution of a new organelle, something that would be tantamount to the origination of a new kingdom.* By any reasonable measure, what Jeon and his coworkers have been studying is an example of macroevolution. His system stands out as a refutation (NOT the only one, but merely one interesting example of what are likely many) of this oft-repeated (and erroneous) ID claim, that macroevolution has not been, and cannot be, observed or studied.

A recent review that can be used to follow the history of this fascinating system:

1. Jeon, K. W. 2004. Genetic and physiological interactions in the amoeba-bacteria symbiosis. J Eukaryot Microbiol 51:502-8.

A few other selected references:

1. Jeon, K. W., and I. J. Lorch. 1967. Unusual intra-cellular bacterial infection in large, free-living amoebae. Exp Cell Res 48:236-40.
2. Jeon, T. J. 2008. DNA adenine methylation of sams1 gene in symbiont-bearing Amoeba proteus. J Microbiol 46:564-70.
3. Jeon, T. J., and K. W. Jeon. 2004. Gene switching in Amoeba proteus caused by endosymbiotic bacteria. J Cell Sci 117:535-43.
4. Lorch, I. J., and K. W. Jeon. 1981. Rapid induction of cellular strain specificity by newly acquired cytoplasmic components in amoebas. Science 211:949-51.
5. Pak, J. W., and K. W. Jeon. 1997. A symbiont-produced protein and bacterial symbiosis in Amoeba proteus. J Eukaryot Microbiol 44:614-9.

* - what seems to be missing from the xD amoeba are examples of bacteria genes that have picked up and moved into the nucleus. Such examples may remain to be discovered, or it may be too early in the evolution of the system to expect such events. Regardless, if and when the pertinent experiments are done, they can be expected to shed interesting insight into the movement of DNA between compartments in other systems.

Postscript - I would appreciate any pointers to studies of this system that have been authored by other groups, or papers that do not appear in Pubmed or Google Scholar. These may be left in the comments, either on The Panda’s Thumb or here.