Adaptive Mutation

Do organisms guide their own evolution?

In 1988, a paper by Cairns et al. bravely examined the core assumptions of neoDarwinian ideology. In the paper titled, “The origin of mutants”, Cairns and colleagues explored the question of whether organisms inherently have the capacity to “choose” their own mutations.¹ Such a question was (and still is) taboo from the perspective of the modern synthesis of Darwin’s conception of evolution by natural selection. This modern synthesis posits that evolution proceeds strictly by random mutations. If a mutation in the hereditary material produced offspring that were more suited to their environment, they would reproduce more than those that didn’t — the unstoppable march of evolution. To the neoDarwinist, the mechanism was purely and unquestionably random. But Cairns’ paper clearly demonstrated something different.

One particular case in the paper documented an elegant and ambitious experiment on E. coli. The group cultured two groups of E. coli lac^— that lacked the gene required to produce the enzyme β-galactosidase. One was cultured on a carbon source of lactose, which would require the enzyme else they would starve, and one on yeast extract that did not require the enzyme to eat. From a neoDarwinian perspective, one would expect to see the β-galactosidase mutation appear at the same rate in both groups as the process is supposedly governed by randomness. However, they found that the adaptive mutation occurred at a much higher rate in the E. coli cultured on lactose — a carbon source they would otherwise not be able to use. Was this evidence that organisms can seek specific outcomes by purposively guiding genetic mutation?

Lamarckian Evolution. Jean-Baptiste Lamarck was a French naturalist who’s theories of evolution overlapped in critical areas with those of Charles Darwin. Lamarck and Darwin corresponded with each other during their lifetimes, both in agreement that “use and disuse” influenced the yet-to-be-discovered hereditary material. Lamarck’s most famous example is that giraffes who extended their necks to reach higher foliage would influence their hereditary material such that their offspring would have longer necks, and thus, be more fit in the context of their environment. Darwin too imagined these instances throughout the lifetime of the organism to influence the traits of the offspring via “gemmules” — material that could pass from the soma to the germ cells. “Use and disuse” influencing the process of heredity has become a topic of serious debate since the dawn of epigenetics and the increasing porosity of Weismann’s “barrier”. Image from Britannica.

The authors waste little time in the discussion to address the dogma they were clearly questioning:

“The main purpose of this paper is to show how insecure is our belief in the spontaneity (randomness) of most mutations. It seems to be a doctrine that has never been properly put to the test. We describe here a few experiments and some circumstantial evidence suggesting that bacteria can choose which mutations they should produce.” (emphasis added)

— Cairns et al. 1988