Evolution is a change in the inherited characteristics of a population over time through the process of natural selection, which may result in the formation of new species.
Natural selection is a process where organisms that are better adapted to an environment will survive and reproduce. This means that the advantageous alleles of this variant organism are passed on to offspring. Over many generations, the process of natural selection leads to evolution occurring.
Charles Darwin was a famous English naturalist. During his life he came up with the theory of natural selection and how this drives evolution of new species.
Darwin is associated with the term 'survival of the fittest', which describes how natural selection works. Individual organisms in an environment are ‘selected for’. This means that only the organisms that have the best characteristics for that particular environment will survive. If they survive then they are the ‘fittest’ for this environment, they reproduce and pass on the advantageous characteristics to their offspring.
Before the industrial revolution in Britain, in the early 1800s, most peppered moths were of the pale variety. This meant that they were camouflaged against the pale birch trees that they rest on. Moths with a mutant black colouring were easily spotted and eaten by birds. This gave the white variety an advantage, and they were more likely to survive to reproduce.
During the last half of the 1800s, airborne pollution in industrial areas blackened the birch tree bark with soot. This meant that the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators. This gave the black variety an advantage, and they were more likely to survive and reproduce. The dark moths passed on the alleles for black wing colour leading to offspring with the black wing colour phenotype. Over time, the black peppered moths became far more common in urban areas than the pale variety.
Note that this change in phenotype was not due to pollution making the moths darker. The dark variety had always existed, but was the best suited variant when the environment changed. It took many generations before the population of moths was mainly black in colour.