Variation and Evolution

• No two living things are absolutely identical. In every population of every living thing, there is variation of a huge number of features.
• Heritable variation


  Caused by differences in genes and can be passed on from parent to child (e.g. hair colour, height, shape of nose)
• Environmental variation

  Caused by the environment in which the organism has developed (e.g. scars, piercings, muscularity), cannot be passed on to the next generation
• Continuous variation


  A continuous range with no "categories" (e.g. height in humans)
• Discontinuous variation


  Distinct groups (e.g. fingerprints can be arch, whorl or loop)
• Origins of variation


  1. Sexual reproduction mixes genes from mother and father

  2. Asexual reproduction produces genetically identical clones
• The genes of a species do not remain the same for all time. Novel variations are constantly appearing.
• Mutation


  A random change in the structure of a gene
• Mutations are very common. Exposure to radiation can increase the rate of mutation.
• Most mutations cause small changes that have no effect. When a change does show, it may be harmful, or occasionally beneficial.
• Mutations in the sex cells (gametes) will be passed on to the next generation. Mutation in body cells will not.
• Inherited disease


  A disease caused by a harmful allele that can be inherited
• Cystic fibrosis allele


  Recessive, so the disease only appears when an individual has the cystic fibrosis allele on both chromosomes
• If two people carrying the cystic fibrosis allele have children, there is a 1 in 4 chance that any child they have will suffer from the disease.
• Examination of a family tree can indicate how a disease like cystic fibrosis is inherited.
• Gene therapy


  Techniques that can be used to remove the effects of a harmful allele
• Gene therapy methods


  • Introducing a 'healthy' allele into the person's DNA
  • 'Switching off' the harmful allele
• There are ethical issues around gene therapy, including religious objections and concerns about cost and prioritisation of healthcare resources.
• Evolution is the process by which living species have gradually changed and developed from earlier forms over a long period of time.
• Evolution


  Results in organisms becoming better adapted (suited) to their environment, due to variation allowing more suitable variants to succeed
• Theory of evolution


  Living things have evolved (changed) over a period of time
• Theory of natural selection


  Explains how evolution has occurred, by the survival and reproduction of organisms with beneficial variations
• Charles Darwin proposed the theory of evolution by natural selection in 1858. It has been refined over time, but is still accepted as the mechanism of evolution by the vast majority of scientists.
• The theory of evolution by natural selection


  1. All populations vary, due to past mutations

  2. Most organisms over-produce offspring, leading to competition for survival

  3. Organisms with variations that allow them to survive better in their environment are more likely to survive

  4. The organisms that survive breed and pass on their genes to the next generation

  5. The next generation will have more of the beneficial variations, and the process repeats, generation after generation

  6. The beneficial characteristics become more common, eventually spreading to the whole population, which will have changed, or evolved, as a result
• Millions of species that existed in the past are no longer found on Earth - they have become extinct. This could happen for a variety of reasons
• Reasons for extinction


  • The organism has failed to adapt quickly enough to an environment
  • The organism has adapted to its environment to some extent, but another similar organism has adapted better
  • The organism has adapted to its environment well, but the environment suddenly changes and the organism cannot survive the new conditions
• Evolution is a slow process and it is very difficult for populations to adapt to sudden drastic changes. Such changes rarely occur naturally, and nearly always result from human interference
• Natural selection

  A constant and ongoing process that usually takes a very long time, but in certain circumstances can happen quite quickly
• Superbugs

  • Bacteria that have become resistant to the antibiotics normally used to treat infections