variation and evolution

• Variation is the difference in features between individuals

  There are three causes of variation :

  • Genetic - inheriting different genes
  • Environment - living in different environmental conditions
  • A combination of both

  some variations are not visible as they occur inside of the cells
• Discontinuous variation

  The variants fit into one group or another. There are no intermediates e.g. human blood groups
• Continuous variation

  There are many intermediates. An individual can fit at any point along the range e.g. height
• Reproduction

  • is the production of new individuals of the same species
• Asexual reproduction

  • does not involve the production of gametes
  • only one parent needed
  • no mixing of genetic information
  • the offspring are genetically identical
  • offspring are called clones
• Sexual reproduction

  • involves the production of sex cells or gametes (sperm and eggs)
  • during fertilisation the two gametes are brought together and fuse to form the offspring
  • we inherit half our genes in the sperm
  • half from our mother in the egg
  • we are genetically different from our parents
  • so sexual reproduction leads to variation
• Mutations

  • a mutation is a change in the gene
  • some are helpful e.g longer necks in giraffes
  • some are neutral e.g blue eye colour
  • some are harmful e.g genetic disorders
• Evolution

  • the process of gradual change over time in the characteritics of an organisms or population
• Natural selection

  • individual organisms within a particular species may show a wide range of variation because of the differences in their genes
  • predation, disease, environmental changes and competition cause large numbers of individuals to die
  • individuals with characteristics most suited to the environment are most likely to survive and breed successfully
  • the genes which have enabled these individuals to survive are then passed on to the next generation
• Extinction

  • those individuals whose characteristics are not so favourable may decline in number
  • these organisms cannot evolve fast enough to survive in the new environmental conditions