Natural selection
Although members of a species have many features in common they are not identical to each other.

Variation occurs as a result of mutations that create new forms of genes, known as alleles, which can be inherited by the next generation.

Members of a species reproduce at a rate that creates more offspring than the environment can support, and some die because of factors such as predation, food shortage or disease.

Well-adapted individuals are those which have inherited characteristics such as the ability to avoid predators, compete strongly for food, or resist disease. They survive and reproduce, passing on the genes that make them successful. They are said to have a selective advantage.

This process is called natural selection, and it ensures that most members of a species are well adapted to their environment.

Natural selection allows species to respond to the changes in the environment.

Industrial melanism

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Industrial melanism
Light peppered moths camouflage themselves against light lichens on trees.

During the nineteenth century pollution killed off some of the lichens and soot deposits caused the bark to appear darker. Light coloured moths were no longer camouflaged and were eaten by birds. The dark moths had a better camouflage.

As a result, dark moths had a greater chance of reproducing and passing on the alleles that made them dark. This led to a gradual increase in the proportion of dark moths until light moths became very rare in industrial areas.

This change from predominantly light to dark forms in industrial areas is called industrial melanism.

Note that this change was not due to pollution making the moths darker. The dark variety had always existed but only had a selective advantage when the selection pressure changed.

The following video demonstrates the advantages of natural selection.





Speciation
A species is a group of organisms able to interbreed and produce fertile offspring.

As long as a population has the opportunity to interbreed and exchange genes, they remain one species. A population of one species can only evolve into more than one species if groups within the population become isolated from each other by barriers that prevent exchange of genes.

There are three types of isolating barrier:

  • Geographical

    Features such as rivers or mountain ranges isolate groups. Movement of land-masses by continental drift led to geographical isolation millions of years ago.

  • Ecological

    Although groups are not geographically isolated from each other they may be isolated by such things as occupying different habitats or breeding areas, pH, salinity.

  • Reproductive

    Breeding between groups within a population may not be possible because of differences in courtship behaviour, physical differences which prevent mating, or failure of gametes to fuse.

Speciation

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Speciation
The diagram illustrates what could happen to populations of animals, which become geographically isolated. Once two groups are isolated different mutations occur in each group. If the environments differ, different adaptations are favoured by natural selection. This leads to different characteristics evolving in each group as time passes. Eventually the groups become so different that if they come together again they are unable to interbreed and are now separate species.

A common pattern resulting from isolation and speciation is adaptive radiation. This is where a common ancestor evolves into a number of species. These have adaptations allowing them to occupy different niches.

A niche is the role of an organism in its community and consists of features such as its feeding methods and habitat.

Speciation in honeycreepers

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Speciation in honeycreepers
The islands of Hawaii were colonised by a single species of finch-like birds. Eventually populations were isolated from each other on separate islands. The diagram shows how they evolved into the modern honeycreeper species with beak shapes, which adapt them for different feeding methods.



Natural selection and speciation exam skills
Questions in Section B of the exam paper often describe examples of natural selection and ask for explanations of the process.

It is important to be familiar with a number of other examples in addition to industrial melanism, such as the evolution of antibiotic resistance in bacteria and heavy metal tolerance in grasses.

When explaining the process in any of these cases it's important to highlight the key features of natural selection.

Remember that variation occurs in the existing population and is not caused by external factors such as pollution or antibiotics.

The surviving individuals in the species such as dark moths or antibiotic resistant bacteria, reproduce and pass on the genes that helped them become well adapted.

Extended writing questions on speciation occur frequently in Section C of the exam paper and it is important to give a clear step-by-step account of the process. A common error in describing the effect of isolating barriers is to say that the barrier separates different species.

Remember that barriers separate groups of the same species, which then undergo mutation and natural selection until they become separate species.

Questions are often asked in which a situation such as the evolution of finches in the Galapagos Islands is described, and you are asked to explain how this illustrates the process of adaptive radiation.

Remember that the two key features of adaptive radiation are that the species described have all evolved from a common ancestor and that they are all adapted to occupy different niches.