Playing with flagellum

Evolution takes place as a result of random, spontaneous changes in genetic material.

Intelligent design has beset educators, scientists, school boards and the general population. It is the notion that a divine hand rather than evolution has shaped the complex structures and processes of biology. There are two questions that should be addressed:

1. Is there a problem that has necessitated the idea of intelligent design?

2. If so, what is the solution?

Evolution takes place as a result of random, spontaneous changes in the genetic material of an organism. If those changes improve the fitness of the organism, they are maintained in the organism’s genome, and the property is selected for. If they do not, the changes may eventually disappear. That is the process of microevolution; it is not a hypothesis, but an incontrovertible fact. As biologists, we see it before us every day. However, there is another aspect of evolution that is not quite as clear-cut: The process of macroevolution, i.e., the evolution of new species ” dogs, cats, horses, frogs, humans and bacteria ” and of the complex processes necessary to evolve the new structures and mechanisms that compose that new creature. While some biologists argue that macroevolution is simply the sum total of many microevolutionary events, there are good reasons that a clear explanation for the generation of biological complexity has eluded biologists for the 150 years since

Darwin. First, there are serious gaps in the fossil record that preclude piecing together an unbroken record of the steps leading from one species to another. Second, there is the difficult matter of understanding the evolution of irreducible complexity, which refers to a biological process or structure that is complex and cannot operate unless all its component parts are present and functional. We argue that, with regard to biological complexity, the answer to the first question is “Yes,” and that it does not serve the scientific community well to deny this potential problem. With regard to the second question, we suggest that it is a fundamental lack of confidence in the process of science to conclude that the answer is divine intervention.

The poster child for irreducible complexity that has been often cited by the intelligent design advocates is the flagellar apparatus of bacteria. Flagella are complex and efficient microbial rotary nanomotors that allow bacteria to swim. More than 50 different proteins are required to build a fully functional flagellum. The fact that genetic disruption of any member of this large set of proteins abolishes or greatly impairs swimming behavior suggests to proponents of intelligent design that the flagellum is irreducibly complex and that evolutionary models cannot account for the emergence of such a structure by stepwise modification of existing structures with different functions. To the contrary, credible evolutionary models to explain the genesis of flagella, as well as other complex structures and processes, can be developed readily without invoking any supernatural design process.

Although space limitations preclude a detailed description of a plausible model for evolution of a functional flagellum in this essay, the main features are as follows: A key aspect of how new functions evolve is that existing genes serving one function can be modified to serve the new function. The process is called cooptation. The original function of the gene need not be lost because duplications of fairly large regions of the genome occur fairly often and microbes can also transmit large amounts of DNA from one cell to another. These processes give the cell extra genetic material to “play with” in evolutionary terms, while enabling the cell to maintain its original set of essential functions. One plausible scenario for the origin of the primitive flagellum postulates the cooptation of a gene that codes for an enzyme that normally functions to provide energy to the cell. This enzyme, called ATPase, physically rotates as protons rush through it as part of the process that produces ATP, the energy currency of the cell. A strong case can be made for cooptation of the ATPase for generation of a primitive machine to export primitive flagellar components from inside the cell to the surface. The rotation of the ATPase enzyme also could have served as a primitive stirring device or as a little motor. Once the coopted ATPase assumed a new function, it could be evolutionarily improved in a stepwise fashion.

What is the advantage of an evolutionary model over an intelligent design model, since they are both theoretical? The critical point is that the evolutionary model makes specific predictions that can be tested by collection and analysis of more experimental data. This serves to either validate and refine the model or cause it to be rejected in favor of better models ” and finally, our understanding of the process is advanced. In contrast, when we throw up our hands and proclaim that biological complexity cannot be explained by science, we prevent ourselves from coming to a better understanding of our universe.

For a more detailed description of the bacterial flagellum and a speculative scenario for its evolution see:

Musgrave, I. “Evolution of the Bacterial Flagellum,” in Young, M and Edis, T. (Eds) “Why Intelligent Design Fails: A Scientific Critique of Neocreationism.” Rutgers University Press, Piscataway, N.J.

Martin Dworkin and Gary Dunny are University professors of microbiology. Please send comments to [email protected]