The Importance Of Genetic Variability

How important is genetic diversity itself to the persistence of insect populations? This question is difficult to answer. Clearly, the presence of resistant alleles in pest populations has led to the development of resistance to numerous insecticides or to the development of virulence on resistant plant varieties. For example, it is not uncommon to find alleles for resistance in mass screening that takes place through field applications of insecticides or in the field use of transgenic plants expressing insecticidal proteins. By contrast, it is more difficult to detect such alleles in laboratory studies in which only a relatively small handful of individuals are sampled. Yet, for many species, genetic diversity seems to matter little. Many parasitoids, for example, are notoriously lacking in genetic polymorphisms, perhaps as a result of years of brother—sister mating. Invasive species such as the Mediterranean fruit fly, Ceratitis capitata, have reduced genetic variability, likely as a result of successive population bottlenecks, yet survive well while expanding their range. Indeed, the lack of variability may explain the success of some invasive species. Invasive populations of the Argentine ant, Linepithema humile, for example, have lost the genetic ability to distinguish one colony from another, thus escaping from population control imposed by intercolony conflict.

In these examples, high levels of genetic variability do not seem to be important for survival. However, it should be noted that the pests and invasive species that are the largest economic problems are only a very small sample of the diversity of insect species and that these species have experienced selection over many generations to be successful as pests or invasives. During this period such species may have overcome potential genetic obstacles associated with low levels of genetic variability. In natural populations of insects, however, the importance of genetic diversity still remains poorly understood.

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