Natural Selection

DDT Resistance in Anopheline Mosquitoes

The malarial parasite is spread by anopheline mosquitoes

The spread of malaria can be controlled by controlling mosquito numbers

One way of controlling mosquito numbers is to use an insecticide like DDT

DDT

Not every mosquito will be killed each time we spray

Some will survive to repopulate the area, so… …we must spray frequently.

Random mutation may produce mosquitoes which are resistant to the effects of DDT…

…these are more likely to survive and pass on their genes to the next generation

NOTE

A resistant mosquito does not need to be totally immune to the effects of DDT…

… it may just be able to survive higher dose of DDT than ‘normal’ mosquitoes.

The next generation contains more resistant mosquitoes

Again, they are more likely to survive to reproduce, so…

…the proportion of the population which is resistant to DDT increases

With each successive generation…

…the proportion of the mosquito population which is resistant to DDT increases.

Eventually,…

…the whole population may consist of resistant mosquitoes

Spraying with DDT produces the selective pressure which favours the resistant mosquitoes.

Because they can resist the effects of DDT, the resistant mosquitoes are said to have a selective advantage

It may not be able to increase the dose of DDT used:

- higher doses may be dangerous to humans

- higher doses may be too damaging to other wildlife

Using higher doses of DDT will also produce the selective pressure which will favour mosquitoes with even higher levels of resistance