Theory of evolution

Charles Darwin, as a result of observations on a round the world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection.
- Individual organisms within a particular species show a wide range of variation for a characteristic.
- Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.
- The characteristics that have enabled these individuals to survive are then passed on to the next generation.

Darwin published his ideas in On the Origin of Species (1859). There was much controversy surrounding these revolutionary new ideas.

The theory of evolution by natural selection was only gradually accepted because:
- the theory challenged the idea that God made all the animals and plants that live on Earth
- there was insufficient evidence at the time the theory was published to convince many scientists
- the mechanism of inheritance and variation was not known until 50 years after the theory was published.

Other theories, including that of Jean-Baptiste Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. We now know that in the vast majority of cases this type of inheritance cannot occur.

A study of creationism is not required.

Speciation

Students should be able to:
- describe the work of Darwin and Wallace in the development of the theory of evolution by natural selection
- explain the impact of these ideas on biology.

Alfred Russel Wallace independently proposed the theory of evolution by natural selection. He published joint writings with Darwin in 1858 which prompted Darwin to publish On the Origin of Species (1859) the following year.

Wallace worked worldwide gathering evidence for evolutionary theory. He is best known for his work on warning colouration in animals and his theory of speciation.

Alfred Wallace did much pioneering work on speciation but more evidence over time has led to our current understanding of the theory of speciation.

Students should be able to describe the steps which give rise to new species.

The understanding of genetics

Students should be able to:
- describe the development of our understanding of genetics including the work of Mendel
- understand why the importance of Mendel’s discovery was not recognised until after his death.

In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged.

In the late 19th century behaviour of chromosomes during cell division was observed.

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In the early 20th century it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that the ‘units’, now called genes, were located on chromosomes.

In the mid-20th century the structure of DNA was determined and the mechanism of gene function worked out.

This scientific work by many scientists led to the gene theory being developed.

Evidence for evolution

Students should be able to describe the evidence for evolution including fossils and antibiotic resistance in bacteria.

The theory of evolution by natural selection is now widely accepted.

Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes. There is further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria.

Fossils

Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks.

Fossils may be formed:
- from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
- when parts of the organism are replaced by minerals as they decay
- as preserved traces of organisms, such as footprints, burrows and rootlet traces.

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Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.

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We can learn from fossils how much or how little different organisms have changed as life developed on Earth.

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Students should be able to extract and interpret information from charts, graphs and tables such as evolutionary trees.

Extinction

Extinctions occur when there are no remaining individuals of a species still alive.

Students should be able to describe factors which may contribute to the extinction of a species.

Resistant bacteria

Bacteria can evolve rapidly because they reproduce at a fast rate.

Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed. They survive and reproduce, so the population of the resistant strain rises. The resistant strain will then spread because people are not immune to it and there is no effective treatment.

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MRSA is resistant to antibiotics.

To reduce the rate of development of antibiotic resistant strains:
- doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections
- patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains
- the agricultural use of antibiotics should be restricted.

The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.