Variation and natural selection

• Variation in Biology


  Differences between individuals of the same species
• Phenotypic variation


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


  • Can be caused by differences in genes (genetic variation)
  • Can be caused by the environment (environmental variation)
• 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
• Examples of continuous variation


  • Height
  • Mass
  • Finger length
• Discontinuous variation


  Distinct differences for a characteristic
• Examples of discontinuous variation

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


  Variation controlled entirely by genes
• Examples of 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
• Examples of environmental variation

  • Scarring from an accident
  • 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 of a big tree
• 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 on the organism
• Occasionally, a new allele gives the individual a survival advantage over other members of the species
• Example of beneficial mutation


  • A bird develops a mutation leading to a change in feather colours, making it more attractive to the opposite sex, causing it to breed more frequently and pass on the mutated phenotype
• Mutations can also lead to harmful changes that have dramatic effects on the organism, such as sickle cell anaemia in humans
• Factors that can increase mutation rate

  • Ionising radiation (e.g. gamma rays, X-rays)
  • Some non-ionising radiation (e.g. 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
  • 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 its environment
• Hydrophytes


  • Plants adapted to live in extremely wet conditions
  • Have large air spaces in leaves for flotation and to keep leaves close to water surface
  • Have small roots as they can extract nutrients from surrounding water
  • Have stomata open all the time, mainly on upper leaf surface for gas exchange
• Xerophytes


  • Plants adapted to live in extremely dry conditions
• Hydrophytes


  • Plants adapted to live in extremely wet conditions
  • Large air spaces in their leaves for flotation, to keep the leaves close to the surface of the water where there is more light for photosynthesis
  • Small roots as they can also extract nutrients from the surrounding water through their tissues
  • Stomata usually open all the time and mainly found on the upper epidermis of the leaf where they can exchange gases much more easily with the air
• Xerophytes


  • Plants adapted to live in extremely dry conditions
  • Thick waxy cuticle - the cuticle cuts down water loss in two ways: it acts as a barrier to evaporation and also the shiny surface reflects heat and so lowers the temperature
  • Sunken stomata: stomata may be sunk in pits in the epidermis; moist air trapped here lengthens the diffusion pathway and reduces the evaporation rate
  • Leaf rolled with stomata inside and an inner surface covered in hairs - traps moist air and prevents air movement across stomata which reduces transpiration
  • Small leaves: many xerophytic plants have small, needle-shaped leaves which reduce the surface area and therefore the evaporating surface
  • Extensive shallow roots allow for the quick absorption of large quantities of water when it rains
  • Thickened leaves or stems which contain cells that store water
• Natural Selection


  1. Individuals in a species show a range of variation caused by differences in genes

  2. When organisms reproduce, they produce more offspring than the environment is able to support

  3. This leads to competition for food and other resources which results in a 'struggle for survival'

  4. Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce

  5. Therefore the alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival

  6. This means that in the next generation, there will be a greater number of individuals with the better adapted variations in characteristics
• Survival of the fittest

  Theory of natural selection put forward by Charles Darwin
• Natural selection illustrated by snail shell colour

  • Normal varieties of shell colours in this snail species is black or grey
  • Chance mutations lead to a small number of snails / one snail having a white shell
  • The white shelled snail(s) survive longer because they are better camouflaged and less likely to be seen by predators and eaten
  • As they survive longer they get more opportunities to reproduce
  • The allele for white shells is passed onto offspring more frequently than the alleles for black or grey shells
  • Over generations, this is repeated until the majority of snails in the population have white shells
• If the environment does not change, selection does not change
• If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele
• Evolution

  The change in adaptive features of a population over time as a result of natural selection
• Natural selection results in a process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common
• Selective Breeding


  Selecting individuals with desirable characteristics and breeding them together
• Selective Breeding


  1. Select individuals with desirable characteristics and breed them together

  2. Likely that not all offspring will show the characteristics you want so offspring that do show the desired characteristics are selected and bred together

  3. This process has to be repeated for many successive generations before you can definitely say you have a 'new breed' which will reliably show those selected characteristics in all offspring
• Natural vs Artificial Selection