One hundred and fifty years ago, British naturalist Alfred Russel Wallace wrote an essay describing some of his ideas on the origin of new species and survival of the fittest species in an environment. Knowing that Charles Darwin had been kicking around some similar ideas, Wallace sent him a copy so the two might compare notes. Darwin, who indeed had formed some seemingly identical conclusions to Wallace's, asked close friend and prominent geologist of the time, Charles Lyell, for advice on how to deal with the similar theories. Lyell suggested that Darwin and Wallace's findings be presented together at the annual meeting of the Linnean Society, held that July in London.
Since that famed, side-by-side presentation of the two naturalists' theories and Darwin's subsequent publishing of On the Origin of Species, Wallace's views have, for the most part, been deemed synonymous with Darwin's. And attribution to the founding theories of evolution has, on the whole, gone to Darwin. A small phrase in Wallace's original essay, however, was his alone: He suggested that certain organisms, or systems that make up organisms, have evolved a way to direct the course of their own evolution, rather than be purely subject to natural selection. As a result, this mechanism could affect whether traits ever get expressed, and therefore subjected to the forces of natural selection. Now, a century-and-a-half later, a group of Princeton University researchers say they're the first to provide evidence to support Wallace's claim.
In 2000, two biochemistry researchers, George McLendon and Stacey Springs, were investigating the proteins that constitute the electron transport chain (ETC). An ancient chain of protein interactions in the mitochondria, the ETC is responsible for passing electrons derived from metabolic sources (food) from one protein to another in a series of redox reactions. As a result of this electron hot-potato, certain proteins release protons into the mitochondrial membrane, creating an unbalanced gradient of protons between the membrane and inner mitochondrial matrix. When enough protons build up, a channel opens on the membrane, and protons stream into the matrix, simultaneously providing the energy to form ATP.
The Princeton group was looking at the molecular mechanism of each protein in the ETC chain. To do so, the researchers mutated each protein's structure one by one, and watched how well the protein was able to transfer electrons, measured as the redox potential in millivolts (mV).
When McLendon mapped the redox potential measurements after each mutation, he saw a striking pattern. Typical redox potentials for each wild-type protein range from -8mV to about 130mV. But regardless of each protein's redox potential in its wild-type version, all mutated versions shot up to around +150mV. This was not a random response, the authors reasoned.
Not sure what to make of the results, McLendon brought a figure illustrating the reactions to Herschel Rabitz, professor of chemistry at Princeton. "McLendon showed me that figure and I said, 'Oh my god, is this true?'" says Rabitz.
Recruiting chemistry research scholar Raj Chakrabarti, the two went about mathematically proving that the ETC proteins were operating under some kind of internal control, suggesting an evolutionary mechanism (Phys Rev Lett, 100:258103, 2008). "The ETC is evolutionarily conserved among all organisms," says Chakrabarti. "There are various variations in terms of exact structure of proteins, but it's one of the most conserved biochemical networks, the base of metabolism." This study was conducted in vitro, so it's not clear whether the mutations disable the ETC, and what influence that disabling might have on redox potentials. But Chakrabarti and his colleagues suggest the ETC has evolved an intolerance to certain mutations that acts as an internal mechanism to control its evolution—hello, Wallace.
"Our work is a piece of evidence that says Wallace's theory was not exactly identical to Darwin's—he had this additional component," says Chakrabarti. The researchers assume that a redox potential of +150mV represents some kind of maximum level. But even if such values derail the ability of the ETC to produce ATP, the theory is still valid, says Chakrabarti: Whatever results the team sees in vivo, an evolutionary mechanism that pushes the redox values to an extreme with every mutation has never been seen before. "Now the next question becomes: How did nature actually do this?" says Chakrabarti.
Update: This article was changed on 12/03/08 to reflect that Alfred Russel Wallace was never knighted, as the original image caption indicated, and that Wallace wrote his essay on evolution before it was presented in July, 1858.