DDT and insects

Question

DDT was once considered a ‘silver bullet’ that would permanently eradicate insect pests. Today, instead, DDT is largely useless against many insects. Describe in specific scientific terms, applying the principles of natural selection and evolution, why many insects are DDT resistant.

Answer 1

To understand this question fully, let's recall some of Darwin's theory of natural selection. He stated that members of a population are different from one another, that there is variation between organisms. What are the causes of these differences between organisms? Differences in their DNA (since DNA codes for traits). What is the primary cause of the differences in DNA?... Mutations. Mutations are the ultimate source of genetic variation. Mutations are really the key to understanding resistance of both insects to DDT and bacteria to antibiotics. There are other ways to get genetic variation, though, such as gene flow and reproduction. Gene flow is migration - when an individual from a different population brings with them new genes. This would be like if a DDT-resistant mosquito from Alabama flew all the way to a population of non-DDT resistant mosquitoes in Florida. The Alabama mosquito is now introducing a new gene into the population. If this mosquito reproduces, then its offspring will have the DDT-resistant gene.

Let's say that in a population of 50 insects (a group of insects of the same species), 10 of them *randomly* have 6 mutations in a certain area of their DNA (and the other 40 do not have this mutation). These mutations just so happen to make these insects resistant to DDT. These 10 insects got these mutations randomly over time. When DDT is applied, these 10 insects will survive the DDT spray and let's say 28 of the non-DDT resistant insects will die. Remember that only individuals that survive can reproduce and thus pass on their DNA to their offspring. All of the insects that died during the DDT spraying cannot pass on their DNA. The 10 insects that just happened to be resistant to the DDT because of the mutation will now reproduce. All of their offspring will now be DDT resistant. Now the new population has 30 individuals that are DDT resistant and 40 that are non-resistant. When another spraying occurs, let's say all 30 of the DDT resistant insects survive and 35 of the non-resistant die. Now there are 70 DDT resistant insects and only 10 non-resistant. Because it is the environment that is determining/"choosing" who lives and who dies, this is considered to be natural selection.

**Remember that natural selection (the process of the environment determining who lives and who dies) can change depending on the environmental conditions. One minute, the non-DDT resistant insects were just fine! It was only when the environmental conditions changed (the use of the DDT) did now the mutated-DDT-resistant insects live. If someone were to stop spraying DDT, the insects that are not DDT resistant may be able to survive and reproduce more frequently and their numbers may increase again. **

Another important definition to add in would be an adaptation. Recall that an adaptation is a characteristic that is useful for the organism's survival. The resistance to DDT, in this case, is the adaptation. It is useful to the organism's survival.

Notice how there was a change in the population over time - more and more of the population were DDT resistant because those were the ones that are surviving and reproducing. This is evolution: a change in the population over time. **Please note: an individual does not evolve. Only populations can**

The cross-application to this is with bacteria and antibiotic resistance. The bacteria can randomly acquire several mutations or perhaps may exchange DNA with a bacteria that is already resistant a certain drug. When the bacteria that have the mutations (or gene of resistance from a bacteria that it exchanged DNA with) that happens to make them resistant, then they survive and reproduce while those that are not resistant die off. The gene that becomes seen more in the population of bacteria is the one that helps with resistance. Let's say a person is sick with a type of a bacteria. If the bacteria are resistant to the antibiotic methicillin, if the person is given methicillin, the bacteria that are resistant live and the rest die off. Now, within the body, the methicillin-resistant (we can use MR for short) bacteria are able to reproduce. Let's say that the bacteria were able to go from 20 MR to 50 MR. If the person takes methicillin again, it will have no effect on the 50 MR bacteria. Now the antibiotic will not work; the person will still feel sick. Doctors must now try a different antibiotic in order to kill the bacteria.This is why the over-use of antibiotics is a serious concern in the medical field.