Evolution and natural selection

Living things today are not the same as those that lived millions of years ago. Over many generations, species change: this is evolution. Charles Darwin and Alfred Russel Wallace explained how it happens. Individuals vary, and those best suited to their environment survive and pass on their genes. This is natural selection.

Key facts

  • Evolution is a change in the inherited features of a population over time. All species evolved from simple life forms that first appeared more than three billion years ago.
  • Natural selection: individuals in a population vary; more are born than can survive, so they compete for food, mates and space; those best adapted are more likely to survive and reproduce; they pass on their genes; over many generations the useful feature becomes more common.
  • Darwin published his theory in “On the Origin of Species” in 1859. Wallace came to the same idea at the same time.
  • Evidence for evolution: fossils (the remains or traces of organisms preserved in rock), and evolution we can watch happening, such as bacteria becoming resistant to antibiotics.
  • Antibiotic resistance: a mutation makes a bacterium resistant; the antibiotic kills the others; the resistant ones survive and multiply. To slow it down, only use antibiotics when needed and finish the course.
  • A species becomes extinct when no individuals are left: because of new predators, new diseases, more successful competitors, changes to the environment or a disaster.
  • When two populations of one species are cut off from each other, they can evolve differently until they can no longer breed together: they have become two species (speciation).

Worked examples

Example 1

Peppered moths can be pale or dark. When soot from factories turned tree bark black, dark moths became more common. Explain why, using natural selection.

  1. Variation: some moths are pale and some are dark, because of their genes.
  2. On black bark, birds easily see and eat the pale moths; the dark moths are camouflaged.
  3. So more dark moths survive and breed, passing on the gene for dark wings.
  4. Over many generations, most of the moths in sooty areas become dark.

Answer: dark moths were better camouflaged, so fewer were eaten; more survived to breed and passed on the gene for dark wings

Example 2

Explain how a population of bacteria becomes resistant to an antibiotic.

  1. Bacteria reproduce very fast, and a random mutation can make one resistant to an antibiotic.
  2. When the antibiotic is used, it kills the bacteria that are not resistant.
  3. The resistant bacteria survive and divide, so soon most of the population is resistant (for example MRSA).

Answer: a mutation makes some bacteria resistant; the antibiotic kills the rest; the resistant ones survive and multiply

Example 3

How do fossils give evidence for evolution?

  1. Fossils form when hard parts are replaced by minerals, when organisms are trapped in amber or ice, or as traces such as footprints.
  2. Deeper rock layers are older. Comparing fossils from different layers shows how organisms changed over time.
  3. Some groups, such as the dinosaurs, appear in old rocks but not in newer ones: they became extinct.

Answer: older rock layers hold simpler organisms; fossils show how species changed and which have become extinct

Example 4

Finches on two islands far apart start from the same species. Thousands of generations later, they can no longer breed together. Explain what has happened.

  1. The sea keeps the two populations apart, so they cannot interbreed.
  2. Each island has different food and conditions, so different features are selected: for example different beak shapes.
  3. Over many generations their genes become so different that they can no longer produce fertile offspring together: two new species.

Answer: the two populations were isolated and evolved differently by natural selection until they became separate species