OEB Study Sheds New Light on Evolution

Most evolutionary pathways closed off by natural selection, biologists find

Working with antibiotic-resistant bacteria, a new study by evolutionary biologists at Harvard suggests that a vast majority of evolutionary pathways for organisms are closed off by natural selection.

Daniel M. Weinreich, a post-doctoral fellow in the Department of Organismic and Evolutionary Biology, headed a study—published in the journal Science last week—of the development of antibiotic resistance in E. coli.

In particular, the scientists studied five point mutations that increases anti-biotic resistance in bacteria by five orders of magnitude. Since the overall mutation requires five sequential steps, there are 120 pathways from the initial, unresistant bacteria to the later drug-resistant bacteria. The study found that of 120 pathways, only ten are feasible.

Weinreich has generalized his results to argue that when bacteria independently develop resistance to the same drug, they likely follow similar, if not identical pathways.

To visualize this result, picture Harvard Yard during a rainstorm. While a student might take several paths from Thayer to Sever Hall, a majority of these would be muddier than others. Just as most people will walk on the few paved paths between buildings, organisms will only follow a limited number of paths to a particular adaptation.

Soon after their publication, Weinreich’s results were picked up by intelligent design advocate William Dembski. Dembski posted a link to Weinreich’s paper on his blog and asked his readers to ponder whether or not Weinreich’s experiments provided a means to falsify evolution.

Weinreich said that his experiment has implicitly tested the ability of natural selection to direct evolution, but he emphasized that “We have tested the evolutionary hypothesis and have found no reason to reject it.”

Weinreich said that he would like to follow up his research with a study on how the malaria parasite acquires drug resistance. Although the Science publication has no bearing on medical treatment because of the widespread resistance to its tested antibiotic, he hopes that study of the malaria parasite might provide doctors with advice on how to help prevent the possibility of the parasite evolving drug resistance.

In addition, Weinreich expressed hopes to develop newer, more effective anti-malarial drugs that disease-causing parasites will find particularly hard to defend against. He said he has applied for a grant for the project from the National Institute of Health.