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By Angelique Jammal, Marina Mechail and Rachelle Tran

The

mosquito

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The mosquito population

decreased when DDT wasintroduced to theirenvironment because itcauses the mosquitoesnerve cells to keep ringand the insect goes into aspasm be!ore it dies"

Mosquitoes eventuallyevolved over time so theybecame resistant to thepesticide DDT and the

insecticide could not bind tothe nerve endings to killthem"

The change inspecies

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#hen DDT is sprayed or dustedon people, it can leach o$ thesoil in to nearby rivers" There itcontributes to the growth o!

algae" #hen algae die, theysink to the bottom o! the riverbed where decomposingbacteria use the o%ygendissolved in water todecompose the algae" This

causes the aquatic li!e tosu$ocate and die and alsocontaminates the water makingit not suitable !or drinking" DDTbecomes more to%ic as it goesalong the !ood chain and itbecomes most to%ic at thehighest trophic level" This isbecause the organism in eachtrophic level will !eed on morenumbers o! organisms in theprevious trophic level"

The change inenvironment

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The pesticide DDT in theenvironment is an e%ampleo! chemicalchange

Type o!change

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&elective pressure is anyenvironmental change thatalters the behaviour or tness

o! living organisms, and thisis the driving !orce o! naturalselection" The insecticideDDT was a way to decreasethe mosquito population insome areas to reduce cases

o! malaria, which is spread bymosquitoes" By spraying DDTor dusting people with it, itcauses mosquitoes nervecells to keep ring and theinsect goes into a spasm

be!ore it dies" This causesselective pressure on themosquito population which!avours the resistantmosquitoes"

&elective

pressureacting on the

organism

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As previously mentioned, DDT inthe environment is an e%ample o!a chemical change which placedselective pressure on the mosquito"

Random mutation and variation o!genes gave mosquitoes !avourableadaptations so they were resistantto DDT and the insecticide couldnot bind to their nerve endings tosend them into spasm" Thesemosquitoes were more likely to

survive, thrive and pass on theiradvantageous genes in the newenvironment" The ne%t generationcontained more mosquitoes whichwere resistant to DDT, and overtime, the proportion o! mosquitoesable to resist DDT were be greater

than those which couldn't"(ventually, the ma)ority o! thepopulation was lled with DDTresistant mosquitoes and now theinsecticide rarely has an e$ect onthem"

*ow theselective

pressure ledto a change in

species

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Macroevolution re!ers to ma)orevolutionary changes overtime, the origin o! new types o!organisms !rom previously

e%isting but di$erent ancestraltypes" +n the other hand,microevolution re!ers to a smallevolutionary change and tovarieties within a given type"The change in mosquitoes is an

e%ample o! microevolution"This is because the mosquitoesstill have the same ancestorspecie which hadn't changedeven though variation andrandom mutation o! genetic

material" The DDT resistantmosquitoes can also stillreproduce with the non DDTresistant mosquitoes, which isimpossible i! it wasmacroevolution"

Macro or

microevolution