The Science of Evolution

Evolution of Pesticide Resistance in a Population of Insects

by Ardea Skybreak

Consider a population of insects (such as some kind of crop pest) which is being largely wiped out by the introduction into their environment of some kind of poisonous insecticide. If at least some of the individual insects happen to be genetically resistant to the effects of that insecticide and remain healthy, they will likely live long enough to reproduce and be able to contribute more descendants to the next generations than individuals who are not resistant to the pesticide. In turn, those among their descendants who inherit the particular "genetic variation" which makes the insects resistant to the poison will also, on average, contribute more descendants to future generations than any remaining non-pesticide resistant insects. So what happens is that, some generations later, the entire local insect population will be largely (or maybe even completely) made up of pesticide-resistant individuals. At that point, we would say that this local population of insects has evolved pesticide resistance. This is something most farmers are very aware of, because it causes big problems for pest control in agriculture. This kind of change is a very common form of evolution by natural selection, and is very frequently observed in all sorts of species, both in nature and in laboratory populations. All the ingredients are there for evolution by natural selection to take place automatically, without the need for any conscious force or guiding hand. Natural selection will simply tend to "sort out" the individuals at each generation: the offspring at each generation which have inherited features which confer a definite "reproductive advantage" (such as pesticide resistance in this case) will on average be much more likely to survive and produce descendants than individuals who don't have this feature. Therefore, over time, this transmittable feature (pesticide resistance) will spread throughout the population. Keep in mind that this doesn't happen because the insects have previously come across a pesticide: the emergence in the population of some individuals with pesticide resistance is simply due to the kind of genetic mutations and recombinations which are always shuffling the genes around like a deck of cards whenever new generations are produced, especially in species with sexual reproduction.*

* Evolutionary change can take place over the generations even in asexual organisms (which reproduce by simple division or cloning of a parent organism) thanks to random mutations taking place in the genetic material and even such phenomena as lateral gene transfers (occasional transfers of genes between organisms which haven't evolved actual sexual reproduction). But, in the overall history of the evolution of life on this planet,the innovation of a capacity for true sexual reproduction represents an incredibly significant milestone which made possible an unprecedented degree of evolutionary modification and diversification of life-forms. The fact that the offspring of sexually reproducing organisms inherit part of their genetic variation from one parent, and part from the other, means that there are a great many more options for genetic reshuffling and recombination at each generation, and this in turn increases the overall genetic variation available in a total population and significantly increases the odds that brand-new evolutionary "novelties" will emerge and be subjected to natural selection.

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