Photo: Keith Wilbourn
Genetic abnormalities occur in all species, including insects. Each generation, the genes of those individuals that do not mate are eliminated from the gene pool. Over time, a phenotype best adapted to its environment in all respects is generated. Once a well adapted phenotype is established, mutations produce a phenotype that is less well adapted. These mutations are rarely inherited, must be generated anew and are present in low numbers.
Source: Living With Insects Blog