Rapid Microevolution and Fitness Cost


By Stephen Caesar

Associates for Biblical Research

“Rapid Microevolution and Fitness Cost”

Frequently in this column, the phenomenon of fitness cost has been discussed. This is when an organism, as it undergoes minor changes in reaction to changes in its environment, improves in one area of its abilities but pays a price by becoming less fit in another area. These minor changes are known as microevolution, in which the individual organism may undergo noticeable alterations and improvements, but does not become a new, superior species.

One of the best-known and most easily observable examples of this is in the never-ending battle between bacteria and their most common attackers, viruses known as bacteriophages (“bacteria eaters”). As these “phages” attack bacteria, the bacteria micro-evolve to improve their defenses, while the phages micro-evolve to outpace the improving bacteria. This “arms race” is known as coevolution, but the end result has never been an observed, brand-new, separate species in either the bacteria or the phages. In addition, these micro-evolutionary changes occur rapidly, not slowly, and they incur a fitness cost that makes the bacteria weaker in other areas.

As Pedro Gómez and Angus Buckling of the Department of Zoology at Oxford University in England pointed out: “Bacteria and their viruses (phages) undergo rapid coevolution in test tubes” (2011: 106), but they wanted to see if the same thing would happen in a more natural environment, referred to as a “soil community.” The result of their experiment was that “we showed rapid coevolution of bacteria and phages in a soil community,” but the bacteria also experienced “fitness costs constraining the evolution of high levels of resistance” to attacks by phages (Ibid.).

By observing bacteria and their enemy phages in a natural environment, Gómez and Buckling learned that, as in the case with the same organisms in test tubes, “bacteria and phage rapidly evolve” (Ibid. 107). Furthermore, they reported that “there is evidence for bacteria bearings costs of resistance to phage, and that these costs are greater in less productive environments” (Ibid. 108).

In other words, the bacteria in the experiment were not evolving into a brand-new, superior species. Rather, they remained exactly the same species (Pseudomonas fluorescens), improving in one area while growing weaker and less fit in another. Moreover, these micro-evolutionary changes occurred rapidly, not gradually over thousands of years. Both in the laboratory and in natural settings, scientists have observed that, when evolution does occur, it is only within the bounds of a single species, without any new, superior, separate species coming into existence.


Gómez, P., and A. Buckling. 2011. “Bacteria-Phage Antagonistic Coevolution in Soil.” Science 332, no. 6025.

Stephen Caesar holds his master’s degree in anthropology/archaeology from Harvard. He is a staff member at Associates for Biblical Research.

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