PLASMID CARRIAGE AND THE UNORTHODOX USE OF FISHER’S THEOREM IN EVOLUTIONARY BIOLOGY

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.