Variation and selection

• Variation in Biology


  Differences between individuals of the same species
• Phenotypic variation


  Difference in features between individuals of the same species
• Genetic variation


  Differences in genes between individuals of the same species
• Continuous variation


  • Very many small degrees of difference for a particular characteristic between individuals, arranged in order and can usually be measured on a scale
• Continuous variation


  • Height, mass, finger length
• Discontinuous variation


  • Distinct differences for a characteristic, no 'inbetweens'
• Discontinuous variation


  • Blood group, gender, ability to roll tongue, free or fixed earlobes
• Continuous variation gives smooth bell curves, discontinuous variation gives a 'step-like' shape
• Continuous variation


  • Height
• Discontinuous variation


  • Blood group
• Genetic variation


  Variation controlled entirely by genes
• Genetic variation in humans

  • Blood group
  • Eye colour
  • Gender
  • Ability to roll tongue
  • Whether ear lobes are free or fixed
• Environmental variation


  Variation caused entirely by the environment in which the organism lives
• Environmental variation


  • Scarring from accidents
  • Weight gain from overeating and lack of exercise
  • Speaking a certain language with a certain accent from being raised in a certain country
  • A plant growing taller to reach more light in the shade
• Discontinuous variation is usually caused by genetic variation alone
• Continuous features often vary because of a combination of genetic and environmental causes
• Mutation


  Genetic changes
• Most mutations have no effect on the phenotype as the protein produced may work just as well as the non-mutated protein
• Rarely, mutations lead to new alleles and new phenotypes, and if they do, most have a small effect
• Occasionally, a new allele gives the individual a survival advantage
• Mutation giving survival advantage


  • A bird developing a mutation leading to different feather colours, making it more attractive to mates
• Mutations can also lead to harmful changes, like sickle cell anaemia
• Factors that can increase mutation frequency

  • Ionising radiation (gamma rays, X-rays)
  • Some non-ionising radiation (ultraviolet)
  • Certain chemicals (e.g. tar in tobacco)
• Increased mutation rates can cause cells to become cancerous
• Sources of genetic variation in populations

  • Mutations
  • Meiosis (new allele combinations)
  • Random mating
  • Random fertilisation
• Adaptive features


  Inherited functional features of an organism that increase its fitness
• Fitness


  The probability of an organism surviving and reproducing in the environment in which it is found
• You should be able to interpret images or other information about a species in order to describe its adaptive features
• Plants that live in different types of habitat

  • Have leaves that show adaptations for survival
• Features of leaves of three plant species from different habitats

  • Orientation of the leaves
  • Individual leaf area
  • Mean stomatal density on upper and lower epidermis
• A typical question here might be to explain how the leaf area and distribution and density of stomata help different species of plant survive in their different habitats
• Hydrophytes


  Plants adapted to live in extremely wet conditions
• Hydrophyte adaptations


  • Large air spaces in leaves for flotation and gas exchange
  • Small roots to extract nutrients from water
  • Stomata open all the time and mainly on upper epidermis
• Xerophytes


  Plants adapted to live in extremely dry conditions
• Xerophyte adaptations


  • Thick waxy cuticle to reduce water loss
  • Sunken stomata to reduce evaporation
  • Leaf rolled with hairs to trap moist air
  • Small needle-shaped leaves to reduce surface area
  • Extensive shallow roots for quick water absorption
  • Thickened stems/leaves to store water
• Natural selection


  Individuals with the best adaptive features are most likely to survive and reproduce
• Natural selection


  1. Variation in individuals

  2. More offspring than environment can support

  3. Competition for resources

  4. Individuals with better adapted characteristics survive and reproduce more

  5. Alleles for better adapted characteristics passed on at higher rate
• This theory of natural selection was put forward by Charles Darwin and became known as 'survival of the fittest'
• Natural selection illustrated by snail shell colour

  • Population has variation in shell colour
  • Mutation produces white shell phenotype
  • White shell provides survival advantage
  • White shell phenotype becomes more common
• Allele for white shells passed onto offspring more frequently than alleles for black or grey shells


  1. Over generations, this is repeated

  2. Majority of snails in the population have white shells