Yale study revises evolutionary theory

Complexity, so the saying goes, doesn’t necessarily equal perfection: There is always room to improve. Now Yale scientists are learning this adage holds true in evolutionary biology.

According to recent research by Yale scientists, organisms do not become less evolvable — that is, less responsive to natural selection — as they become more complex, a finding that refutes prevalent evolutionary theory.

The common working hypothesis within the scientific community holds that complex organisms, like highly efficient cars or machines, have little room to improve, lead author Gunter Wagner, professor and chair of the Ecology & Evolutionary Biology Department at Yale, said. Instead, he explained, these organisms face a “cost of complexity,” wherein any random mutations they acquire are much more likely to be disadvantageous than advantageous.

“There’s an intuitive feeling [that] if you randomly change a moderately complex system, the probability of hitting a combination of settings that would improve the performance of this system is fairly low,” he said. “But it is not correct that this dissuades complex organisms from evolving.”

Wagner’s findings challenge this assumption by refuting two theoretical claims, he said.

The first commonly held assumption, he said, is that mutations in a single gene tend to affect a variety of functionally unrelated characteristics — a phenomenon called pleiotropy. Instead, the study found that most mutations affect fairly few characteristics and are extremely focused, Wagner said.

Observing 70 skeletal characteristics in the mouse, the researchers identified a total of 102 genomic regions that affect the skeleton. Mutations in each genome segment were found to affect less than 7 to 10 percent of characteristics.

“You wouldn’t expect to make a lot of random adjustments — at the same time — to tune up a car,” Wagner said. “Similarly, it appears that tuning up a complex trait in a living organism is well coordinated and the effects of pleiotropy are more focused than we thought.”

Since the chance a mutation will be advantageous declines as the number of characteristics it affects increases, more complex organisms — which theoretically have more characteristics — retain their ability to evolve, he added.

The study also challenges the claim that the number of characteristics a single mutation affects is inversely related to the size of the mutational effects it confers on each characteristic. In other words, accepted theory states that mutations affecting more characteristics confer smaller effects on each of them, Wagner said.

But Wagner’s study found that this relationship is additive: The effect on each characteristic increases with the total number of traits a mutation affects, said Mihaela Pavlicev of Washington University in St. Louis, co-author of the study.

“In actuality, there is a positive relationship: The sum of the effects increases as more characteristics are affected,” she said. “This would not be the case if there were a cost for complexity.”

These two findings together provide evidence that complex organisms have found a way to organize their genomes to ensure continued evolvability, Pavlicev said, refuting the idea that complex organisms enter “a dead-end street” in which they no longer adapt to change.

But the need for research does not stop here, Wagner said.

One source of controversy that the study side-steps is whether natural selection, in theory, can improve a trait like evolvability, he said.

In other words, as he put it, “Can organisms acquire mutations that make them more responsive to change in their environment? And is evolvability an inherent trait that differs from organism to organism? Even that is a question we’re currently trying to solve.”

But a recent study by Jeremy Draghi EEB ’08 “refutes the idea that [determining the role of natural selection] is an unsolvable problem,” Wagner said.

The study, Draghi explained, shows that organisms can be evolved to show differences in the types of mutations they produce in response to the same environmental changes. This idea “shows there is a way around the idea of the ‘cost of complexity,’ ” he said, because it allows for the concept that natural selection can favor an organism’s ability to increase the number of ways it can evolve.

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