Does Evolution Evolve?
If phrases like “the conservation of conservatism” or “the production of productivity” leave you scratching your head, you may wear off a few hairs thinking about a paper in PNAS1 on the “evolution of evolvability.” Entitled, “Is evolvability a selectable trait?”, this paper by two scientists at Rice University considers whether the rate of change of evolution can change. (Pause here to think about that.) In other words, can the ability of a population of critters to adapt to its environment quickly be selected by natural selection? Might some critters become sluggish in their ability to change, while others develop flexibility in adapting to changing conditions? Why is anyone even asking this question?
It’s not that no one has thought about this before, but the idea has been shrugged off by other evolutionists in the past. How could a population plan ahead to be flexible? For this reason, the authors seem a little defensive writing this paper:
Whether the propensity to evolve, or evolvability, can be an object of Darwinian natural selection is a topic of interest. Causality would suggest not because of the apparently anticipatory nature of evolvability. Many within the field of evolutionary biology are uncomfortable with the concept that evolvability is a selectable trait. A growing body of experimental data, however, would be explained if evolvability were a selectable trait.
Higher organisms cannot evolve, or adapt, by germ-line mutation to an environmental change within their own lifetime. Does this mean that lineages and individuals cannot be under selection for evolvability?….
….Although the use of the term evolvability has only recently come into vogue in the scientific community, investigations into the evolution of adaptation go back several decades. Prominent from a theoretical perspective are works in population genetics and game theory [see 02/10/2004 headline]. Despite the insights that these studies give as to the origin and maintenance of evolvability, evolution of and selection for evolvability remains a contested issue primarily because of the causality principle
So the burden of proof is on them to show that evolvability evolves. Their paper is primarily a mathematical model, similar to computer models of evolution (see 07/04/2004 headline). A model is needed, they say, because of the difficulty of measuring the effect in the wild:
Whether evolvability is selectable has been a difficult question to answer, primarily because observations in evolutionary biology tend to be correlative in nature and difficult on which to make mechanistic conclusions. Therefore, we consider here the dynamics of evolvability in a well defined theoretical model of protein evolution. Within this model of protein structure and function, we have a fixed population of proteins, which we take to be 1,000. We have a microscopic selection criterion, which we take to be the folding and binding of a protein to a substrate. And we have a means of inducing constant, random environmental change.
They claim the model shows that evolvability is a function of environmental change; the more dynamic the environment, the more evolvable the protein. This, they emphasize, is their important finding. It’s kind of like physics:
An analogy with thermodynamics illuminates the issue: How is free energy minimized in a physical system of particles despite the difficulty in defining the entropy of a given configuration of the particles? An ensemble of particle configurations allows the definition of free energy and the approach to thermodynamic equilibrium just as a population of evolving organisms allows the definition of and selection for evolvability.
They seem to be viewing individual organisms as molecules, and treating Darwinian selection as a force acting on the ensemble– a form of group selection (see 05/31/2004 headline for opposing view). Is there any evidence in nature for their position? They point to a few possibilities:
Many observations within evolutionary biology, heretofore considered evolutionary happenstance or accidents, are explained by selection for evolvability. For example, the vertebrate immune system shows that the variable environment of antigens has provided selective pressure for the use of adaptable codons and low-fidelity polymerases during somatic hypermutation. A similar driving force for biased codon usage as a result of productively high mutation rates is observed in the hemagglutinin protein of influenza A. Selection for evolvability explains the prevalence of transposons among bacteria and recombination among higher organisms.
Is this concept useful? The authors feel that “therapeutics also confer selective pressure on the evolvability of pathogens, and that this driving force for antigenic drift should be considered in drug- and vaccine-design efforts.”
The believe their model shows that “The rates at which the various events within the hierarchy of evolutionary moves occur are not random or arbitrary but are selected by Darwinian evolution. Sensibly, rapid or extreme environmental change leads to selection for greater evolvability. This selection is not forbidden by causality and is strongest on the largest-scale moves within the mutational hierarchy.”
One of their concluding statements summarizes their view into a pithy sound bite: “Not only has life evolved, but life has evolved to evolve.”
1David J. Earl and Michael W. Deem, “Evolvability is a selectable trait,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0404656101.
It must get boring at the Darwin Party storytelling banquets (see 12/22/2003 commentary), so every once in awhile someone has to come up with a new plot to argue about. To these guys, proteins in a test tube are a microcosm of caribou in the tundra or humans in Manhattan. This paper might suggest a short story or novel on whether New Yorkers are evolving evolvability in response to terrorist attacks. If so, terrorism might be a good thing; it makes the species more adaptable to sudden change.
It doesn’t matter whether the model corresponds to reality or not, or can be observed or not, as long as it makes entertaining reading, generates lively discussions and opens new markets for GameBoy.