In Which Larry Moran Learns the Difference Between Comparative Biology and Evolution - Evolution News & Views


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In Which Larry Moran Learns the Difference Between Comparative Biology and Evolution

I recently pointed out that biochemistry research does not depend on evolutionary theory. Biochemistry depends of course on knowledge of similarities between species, but the similarities are established in the usual way of comparing biochemical, genetic, anatomical and physiological properties of organisms. We can use laboratory rats to study some aspects of human biochemistry, not because evolutionary theory tells us that rats are closely related to humans, but because rats and humans have many actual similarities that make the research plausible.

The inference that rats and humans are close to each other on the evolutionary tree is drawn (partly) from the study of the biochemistry of the two species. Our study of the biochemistry of the two species cannot then be said to draw an inference from evolutionary theory, which itself is a set of inferences drawn from biochemistry. That would be to reason in a circle. 

Dr. Larry Moran begs to differ. He writes:

At least Dr. Egnor has done us a favor. Since he is a physician, he must have taken a biochemistry course at some point in his life. It is plainly obvious that the course did not include much about evolution. Thus, Dr. Egnor is inadvertently demonstrating that biochemistry should be taught as a concept-driven course and that evolution should be one of the fundamental concepts. Otherwise, we end up with doctors that are as ignorant of evolution as Michael Egnor.

I took a few biochemistry courses in college, being a biochem major and all. And Larry's right: I never encountered evolution in my biochem courses, even at the graduate level. The reason seems obvious: the study of biochemistry does not depend on evolution. 

Moran wasn't specific about precisely what he thinks I've gotten wrong, but one of his commenters singles out my error, with which I'll assume Moran agrees:

Many new insights into human biochemistry and gene function are gleaned by studies in mice and other animals. This is only possible due to shared biochemistry as a result of shared ancestry. The ever-growing library of sequenced genomes is only underscoring this fact. It's so unfortunate to have doctors like Egnor who are so hopelessly ignorant.

The study of biochemistry does depend on comparative biology. It does not depend on evolutionary biology. Larry and his Darwin Youth seem to confuse the two. I'll help them understand the difference.

Comparative biology is the oldest discipline in biology. It is the systematic study of the diversity and similarities of living things.

Evolutionary biology is a subdiscipline of comparative biology -- it is the study of how populations change with time. 

Comparative biology was founded by Aristotle. He (not Darwin) is the author of the scala naturae -- the Ladder of Life. He classified organisms according to their general and specific similarities and differences (genus, species, and differentia). The relatedness and differences between species has been a prime focus of biological research for 2200 years -- 2050 years before Darwin. 

From Aristotle's day to the 20th century, nearly all human biology was actually comparative biology. A few biologists studied the human body first-hand -- Vesalius for example -- but most biological research for two millennia was done on animals and lower organisms, and the knowledge obtained was cautiously applied to man. Galen, the father of anatomy and the doyen of anatomy for 1500 years, almost certainly never dissected a human corpse. He dissected Barbary apes and other animals, and extrapolated to humans (which was the source of some of his errors).

Carl Linnaeus developed our modern system of biological classification, working out a system of biological relatedness that is still largely unchanged two centuries later. Linnaeus worked a century before Darwin, and died thirty years before Darwin was born. 

Darwin's theory of evolution had nothing to do with comparative biology until the late 19th century, and it is not the basis our modern system of classifying organisms and ascribing relatedness. Darwin's theory is inferred (in large part) from comparative biology. 

An example of the absurdity of the assertion that evolution is a foundational concept in biochemistry is the Ames test. The Ames test was developed in the 1970s by Bruce Ames at Berkeley to test the carcinogenicity of chemicals by testing their mutagenicity. It has become a very widely used measure of carcinogenicity, and is part of a standard battery of tests required by U.S. law to evaluate the potential that a given agent will cause cancer. 

The Ames test uses salmonella bacteria that carry mutations in genes for histidine synthesis. The bacteria need exogenous histidine for growth. Mutagenic chemicals induce mutations in the bacteria that allow them to revert to making their own histidine. This new mutation is easy to detect in assays, and is the basis for the usefulness of the test. 

Why do biologists use bacteria as a model for human carcinogenicity, rather than organisms evolutionarily much closer to humans, such as great apes? After all, bacteria are, on an evolutionary scale, as far away as it gets from human beings. If evolution were the "indispensable foundation of biochemical research," why would our most important test for carcinogenesis employ organisms that are as unrelated to humans as it's possible to get, rather than any of our closer relatives (mice, dogs, apes, etc.).

The reasons are straightforward and well known. Bacteria are small, cheap, easy to work with, have simple biochemistry and genetics that facilitates investigation, and have fast generation times to allow simulation of longer periods in humans. Their biochemistry is close enough to that of humans -- even though they are very far from humans on an evolutionary scale -- to be valuable in testing. 

Note that one factor is utterly irrelevant to the usefulness of the Ames test-- the evolutionary relationship between humans and the organisms being tested. It's irrelevant where salmonella and humans fall on the evolutionary tree. The same is true of a broad spectrum of biological research, for which evolution is no more than a narrative gloss. 

