This is a multi-part post in response to Merlin’s paper, “Evolutionary Chance Mutation: A Defense of the Modern Synthesis’ Consensus View”. See introduction and table of contents.
In the last installment, we talked about how Merlin tried to paint a whole range of semi-directed mutations as “evolutionarily random”, and how this falls short when compared to the motivation behind Darwinism in the first place – to remove any hints of teleology from biological description.
However, Merlin also describes what she would consider as evidence of directed mutation – a bias in mutations that produce exclusively adaptive mutations. In the previous installment I showed why this was an overly stringent requirement. However, even as a requirement, there are experiments showing that at least some directed mutations occur.
The one which has been best characterized is the Glp+ (glycerol utilization) mutation of E. coli having a crp deletion. In the journal Molecular Microbiolog, Zhang and Saier described in their paper, A Mechanism of Transposon-mediated Directed Mutation how an insertion sequence, IS5, gets inserted into the promoter region of the glpFK operon. glpFK contains two genes – one for diffusing glycerol within the cell, and one for preparing it into entry into the glycolysis pathway, where it can be used to make energy for the cell.
Zhang and Saier note the following:
- The mutation happens only in the presence of glycerol
- The only detectable mutation is the insertion of IS5
- IS5 is not inserted anywhere else
- Inserting another arbitrary insertion sequence does not do the job – IS5 is required as the sequence
Zhang and Saier carefully elucidate the mechanism by which this occurs:
- Normally, a glycerol repressor binds to a regulatory region which inhibits glycerol production
- This repressor also covers an IS5 insertion target, effectively hiding the target sequence from the IS5 element
- In the presence of glycerol, the glycerol repressor is removed from the regulatory region of glpFK.
- When the glycerol repressor is removed, the target site for IS5 opens up, and IS5 inserts itself into the operon, and activates the gene
Anyway, the paper is fantastic, and I encourage anyone interested in the directed mutation controversy to give it a read. What is even more amazing, is that this appears to be a backup system. A complex of Crp and cAMP is what normally switches on glycerol production. However, this mutational process is for E. coli with a Crp deletion. Therefore, in the absence of Crp/cAMP, the genome is marked in such a way that a backup mutation can provide the necessary promoter.
This process seems to meet all of the requirements outlined by Merlin for a directed mutation, and therefore, even by her own excessively-strict requirements, shows that beneficial mutations can arise instructively (i.e. in a neo-Lamarckian manner), rather than selectively (i.e. in a neo-Darwinian manner).