Evidence For Evolution (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Fossil Evidence

• Evidence for early life forms on Earth can be found in the fossil record

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

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

• Fossils may be formed in several ways:

  

Evolutionary trees

• Evolutionary trees are diagrams that show the relationship between species over evolutionary time

• A new branch in the tree shows where speciation has occurred (when a new species has evolved)

• In the evolutionary tree below, for example:

  • Chimpanzees and bonobos share a recent common ancestor. Chimpanzees are therefore most similar to bonobos (more similar than they are to any other primate species)

  • Humans share a more recent common ancestor with gorillas than they do with orangutans – this means we are more closely related to gorillas than we are to orangutans

  • All five primate species shown here share a common ancestor (from the distant past)

An evolutionary tree for five species of primate

Evolution in Bacteria

• The theory of evolution by natural selection is now widely accepted and many sources of data are now available to support the theory of evolution

• One very clear piece of evidence for evolution is antibiotic resistance in bacteria

  • An antibiotic is a chemical that can kill or inhibit the growth and reproduction of bacteria

  • Antibiotics are extremely useful to humans as some bacteria are pathogenic and can cause life-threatening disease

• Bacteria reproduce, on average, every 20 minutes and therefore evolution occurs in a much shorter time span

• Antibiotic resistant strains of bacteria develop by the following mechanism:

  1. Like all other organisms, within a population, there will be variation caused by mutations

  2. A chance mutation might cause some bacteria to become resistant to an antibiotic (eg penicillin)

  3. When the population is treated with this antibiotic, the resistant bacteria do not die

  4. This means they can continue to reproduce with less competition from non-resistant bacteria, which are now dead

  5. Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation

  6. Over time the whole population of bacteria becomes antibiotic-resistant because the bacteria are best suited to their environment