The first scenario that was explored was bacteria evolving in a dynamic environment, using an adaptive landscape in which a second peak that sequentially increases in height was introduced alongside a static initial peak.
There is not much data available on how quickly, and by what mechanisms, free-living non-pathogenic bacteria evolve in their natural environments, but a recent study by Denef and Banfield [ 28] showed that a series of important divergence events occurred over a time scale of years to decades in Leptospirillum biofilms in a mine drainage system.
This is a possible scenario for bacteria evolving in several species hundreds or thousands of years ago, with occasional CST between species.
A family tree showing how different strains of Bifidobacteriaceae bacteria evolved in humans (blue) and our hominid relatives, the gorilla (green), chimp (yellow) and bonobo (red).
To explore this hypothesis, we characterized host sex-specific effects of the parasite Pasteuria ramosa, a bacterium evolving in naturally, strongly, female-biased populations of its host Daphnia magna.
A third scenario where the bacteria evolve first in one species, and then CST occurs, is intermediate between the two extreme scenarios presented.
In contrast, when bacteria evolve resistance by acquiring a plasmid they are obtaining a resistance determinant that has already experienced selection to minimize its cost in previous hosts.
To illustrate the negative frequency-dependent selection pressure imposed on bacteria by density-dependent phage predation, simulations were performed in which bacteria evolved on a smooth single-peak adaptive landscape.
It is hardly imaginable that these bacteria evolved a distinct immune system in the short time elapsed since the loss of CASS.
First, bacteria evolve to use the basic elemental nutrients in that environment that are common to and essential for all growth: C, N, P, S, O, H, etc.
