Variation and natural selection, Darwin's theory of evolution by natural selection, the development of antibiotic-resistant bacteria, the evidence for evolution from fossils and from resistant bacteria, extinction, and the formation of new species by isolation.

What this dot point is asking

OCR wants you to explain how variation and natural selection lead to evolution, describe Darwin's theory of evolution by natural selection, use it to explain antibiotic resistance, give the evidence for evolution (fossils and resistant bacteria), and explain extinction and how new species form.

Variation, selection and evolution

The steps of natural selection, which OCR expects you to give in order, are:

1. There is variation in a population, caused by differences in alleles (the source of new alleles is mutation).
2. Organisms produce more offspring than the environment can support, so they compete to survive.
3. Individuals with characteristics best suited to the environment are more likely to survive and reproduce.
4. The survivors pass on the useful alleles to their offspring.
5. Over many generations, the useful alleles become more common in the population, and the species changes (evolves).

Darwin's theory of evolution by natural selection

Charles Darwin proposed the theory of evolution by natural selection. He observed the variation in organisms (for example finches with different beaks on the Galapagos Islands) and reasoned that those best adapted to their conditions would survive and breed, passing on their characteristics. His ideas were only gradually accepted, partly because they challenged religious beliefs, and partly because the mechanism of inheritance (genes and DNA) was not yet understood. The later discovery of genes and DNA provided the explanation for how characteristics are inherited and how mutation creates variation, which strengthened the theory.

Antibiotic-resistant bacteria

The evidence for evolution

Two main lines of evidence are examined:

• Fossils. Fossils are the preserved remains, or traces, of organisms from long ago. By comparing fossils in rock layers of different ages, scientists can see how organisms have changed gradually over millions of years and how modern species developed from earlier ones. Gaps in the fossil record exist because soft-bodied organisms rarely fossilise and many fossils have not been found or were destroyed.
• Antibiotic-resistant bacteria. Because bacteria reproduce so quickly, the rise of resistant strains can be seen within years, showing natural selection acting in real time exactly as the theory predicts.

Extinction and new species

Extinction is the permanent loss of all members of a species. It can be caused by a change the species cannot survive, such as a new disease, a new predator or competitor, the loss of habitat or food, or a sudden catastrophe (for example the asteroid linked to the dinosaurs).

A new species can form when populations of the same species become isolated (for example by a mountain range or a stretch of sea). The separated populations experience different conditions, so different alleles are selected for in each. Over many generations they become so different that, even if they meet again, they can no longer interbreed to produce fertile offspring. They are then separate species.