Abstract
The link between fitness and reproduction rate is a central tenet in evolutionary biology: mutants reproducing faster than the dominant wild-type are favoured by selection, otherwise the mutation is lost. This link is given by Fisher’s theorem under the assumption that fitness can only change through mutations. Here I show that fitness, as formalised by Fisher, changes through time without invoking new mutations—allowing the co-maintenance of fast-and slow-growing genotypes. The theorem does not account for changes in population growth that naturally occur due to resource depletion, but it is key to this unforeseen co-maintenance of growth strategies. To demonstrate that fitness is not constant, as Fisher’s theorem predicates, I co-maintained a construct of Escherichia coli harbouring pGW155B, a non-transmissible plasmid that protects against tetracycline, in competition assays without using antibiotics. Despite growing 40% slower than its drug-sensitive counterpart, the construct with pGW155B persisted throughout the competition without tetracycline—maintaining the plasmid. Thus, predicting the selection of microbial genotypes may be more difficult than previously thought.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Sections re-written for clarity
