Are evolutionary changes selection-driven or are they random?

In 1943, Salvador Luria and Max Delbrück designed a simple, easy to perform experiment to answer a question that had caused many debates within the scientific community: do evolutionary mutations arise due to the stimulus that causes the selective pressure? Small E. coli cultures were grown until saturation and, then, were plated in selective media (with bacteriophage T1, which infects and kills E. coli). A small number of colonies showed up in each plate, coming from a mutant resistant to T1 infection. They predicted that, if the mutants were caused by the presence of the virus, there would be, roughly, the same amount of colonies in each plate (see figure). But that is not what they saw. They got a wide disparity in the number of colonies per plate, which led them to the conclusion that the mutations were developed before contacting the virus!
p.s.1: The Math/stats part of the paper is brutal, you should get into it at your own risk
p.s.2: A recent Microbiology discovery may make us expect the opposite results.

Image credit

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.