Coevolution - Are parasites and their hosts coevolving?
Meanwhile, the rabbits were also evolving resistance.
This could be shown by challenging wild rabbits from a series of times with standard strains of the virus; now the virus is held constant and any decline in kill rate must be due to changes in the rabbits. The table shows the results of a series of such experiments through the 1960s and 1970s, in which resistance did indeed manifestly increase. Therefore, both parasitic virulence and host resistance can evolve.
Natural selection will clearly always favor increased resistance in hosts: but how will it operate on virulence in parasites?
Table: rabbits evolved resistance over time after the introduction of myxoma virus. These results are for wild rabbits (Oryctolagus cuniculus) caught at different times in two regions (called Mallee and Gippsland) of Victoria, Australia; the results were then challenged with a highly virulent laboratory strain (SLS) of the myxoma virus, the strain caused 100% mortality in unselected rabbits. From Fenner & Myers (1978): data of Douglas et al.