Urban adaptation
The changes in the distribution of different colour forms of the peppered moth in industrialised parts of Britain is one of the best-known examples of adaptation to living in cities.
Natural selection and evolution
The first few simple living creatures appeared on earth about 4000 million years ago. Today there are an enormous number and variety of living organisms ranging from single celled species to highly complex groups such as the mammals. How and why these changes occurred had been a puzzle for a long time.
Charles Darwin was a great 19th century naturalist. During his famous 5 year trip around the world in HMS Beagle he made detailed field observations that convinced him that species gradually changed as one generation succeeded another - they evolved. This was not a new suggestion but before Darwin there had been little evidence to support it. Back home studies of fossils showed Darwin that relationships could also be found between animals living in the past and those living today. Where there were enough fossils it was even possible to show how one type of organism had gradually evolved into others which could be related to organisms living today. But what made evolution happen? Linking his ideas with those of others Darwin proposed that evolution took place by 'natural selection'.
Natural selection works as follows:
- Within a species there are always slight differences between individuals.
- Some individuals may have characteristics which make them better able to survive than others (they are better adapted).
- These individuals are likely to live longer, breed and produce more offspring
- If the characteristic which helped the parent survive is passed on to the offspring there will be more individuals with this character.
- After several generations individuals with the favourable character will be the most common.
- Over a long period of time new varieties and species can evolve
- If environmental conditions change different characteristics may be favoured and selected.
Darwin 's book The origin of species caused great controversy when it was published in 1859. Many people at this time thought that all living organisms had been created and specially designed by God whilst the thought that man might be descended from apes horrified others.
Today more and more evidence is being found particularly in the study of genetics to support Darwin's theory of evolution by natural selection.
Most scientists today believe that the great variety of living organisms found on earth today is the product of millions of years of natural selection and evolution.
Case study in the urban environment: the peppered moth
The typical form of the peppered moth (Biston betularia) has a white background speckled with black. During the day time it rests on the underside of lichen covered branches and twigs in the tree canopy. Here it is well camouflaged and protected from bird predation.
In 1848 a black form of this moth was recorded in Manchester . This rapidly became much more abundant than the typical form in industrial areas. Industrial pollution, particularly soot, and the higher levels of sulphur dioxide (which killed tree lichens), darkened branches and twigs. The black form now had better protective camouflage than the typical form. More survived and were able to breed. It seemed that the black variety was being naturally selected
More evidence for natural selection came after the passing of antipollution and smoke control legislation in the 1950's when the numbers of moths of the black variety declined.
Further studies on the peppered moth have shown that this explanation is oversimplified and that predation by birds is only one of the factors influencing the natural selection of the different coloured varieties.
