Misjudgment of the scale of local adaptation midge also lead to problems. For example, if parasites adapt to individual hosts rather then to host populations, detection of local adaptation across host populations might be difficult. To our knowledge, no evidence has been presented against local adaptation. There are likely to have been numerous failed introductions, which have passed unnoticed. In summary, genetic diversity of host populations appears to be crucial in hindering the parasite to evolve an optimal level of virulence.
Sexual recombination benefits an outcrossing host through the production of variable offspring.
Asexual offspring, in contrast, cannot escape the antagonistic advances made during the previous generations by their parent's parasites. Genetic variability among hosts forces a parasite to adapt anew whenever it encounters a new host genotype. The more different genotypes a host population consists of, the lower the frequency of each and the smaller the chance that a parasite will encounter the same genotype in successive hosts. So far, direct evidence for the benefits of genetic diversity for host populations is weak, although numerous studies suggest such benefits. The vulnerability of mono-cultures to pathogen attack is notorious.