By Angelique Jammal, Marina Mechail and Rachelle Tran The mosquito population decreased when DDT was introduced to their environment because it causes the mosquitoes nerve cells to keep firing and the insect goes into a spasm before it dies. Mosquitoes eventually evolved over time so they became resistant to the pesticide DDT and the insecticide could not bind to the nerve endings to kill them. When DDT is sprayed or dusted on people, it can leach off the soil in to nearby rivers. There it contributes to the growth of algae. When algae die, they sink to the bottom of the river bed where decomposing bacteria use the oxygen dissolved in water to decompose the algae. This causes the aquatic life to suffocate and die and also contaminates the water making it not suitable for drinking. DDT becomes more toxic as it goes along the food chain and it becomes most toxic at the highest trophic level. This is because the organism in each trophic level will feed on more numbers of organisms in the previous trophic level. The pesticide DDT in the environment is an example of chemical change Selective pressure is any environmental change that alters the behaviour or fitness of living organisms, and this is the driving force of natural selection. The insecticide DDT was a way to decrease the mosquito population in some areas to reduce cases of malaria, which is spread by mosquitoes. By spraying DDT or dusting people with it, it causes mosquitoes nerve cells to keep firing and the insect goes into a spasm before it dies. This causes selective pressure on the mosquito population which favours the resistant mosquitoes. As previously mentioned, DDT in the environment is an example of a chemical change which placed selective pressure on the mosquito. Random mutation and variation of genes gave mosquitoes favourable adaptations so they were resistant to DDT and the insecticide could not bind to their nerve endings to send them into spasm. These mosquitoes were more likely to survive, thrive and pass on their advantageous genes in the new environment. The next generation contained more mosquitoes which were resistant to DDT, and over time, the proportion of mosquitoes able to resist DDT were be greater than those which couldn’t. Eventually, the majority of the population was filled with DDT resistant mosquitoes and now the insecticide rarely has an effect on them. Macroevolution refers to major evolutionary changes over time, the origin of new types of organisms from previously existing but different ancestral types. On the other hand, microevolution refers to a small evolutionary change and to varieties within a given type. The change in mosquitoes is an example of micro-evolution. This is because the mosquitoes still have the same ancestor specie which hadn’t changed even though variation and random mutation of genetic material. The DDT resistant mosquitoes can also still reproduce with the non DDT resistant mosquitoes, which is impossible if it was macroevolution.