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.