If there’s a change in the environment of a species then selective pressure is put on the species to change in response to this. This is known as directional selection. This type of natural selection is common because many types of environment change, for example due to a natural disaster or sea levels.

One example is the peppered moth. It used to be that being a light coloured moth was favoured against being a mutant dark one because they could camouflage themselves from predator birds against birch trees which have a pale bark. However, during the 1800s the industrial revolution caused birch trees close to industrial factories to blacken from pollution. This gave being dark coloured the selective advantage. The tables turned: dark moths then became more populous and the pale coloured moths became the rare ones.

Another example is antibiotic resistant bacteria. Antibiotics are the main defence used by humans against bacterial infections. However, mutants can appear that are resistant to the drug. If put into an environment in which antibiotics is present this bacteria has a selective advantage and, as the other bacteria are killed, it can reproduce and colonise without having to deal with any competition.

In order for a population to survive after an environmental change the allele mutation must already be present. This happens by chance; populations don’t decide to mutate or adapt.

An ‘environment’ can be biotic as well as abiotic. For example, if predators begin to run faster then there’s a selective pressure for prey to also run faster, or if one species of tree begins to grow taller then there’s a selective pressure for other species to grow taller too.