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Ecosystem > Variation
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Normal distribution about the mean

  • Mode (most frequent) = median (mid) = mean (average value)
  • Bell-shaped/ even distributions of values above and below mean
  • Standard error (SE)
    • True mean of SE is ±1.96
    • In a number of samples each sample will have its own mean
    • Standard error measures how much the value of a sample mean is likely to vary
    • The greater the standard error, the greater the variation of the mean
  • Standard deviation (σ)
    • Measure of the spread of results about the mean of a normal distribution curve
    • Thinner bell-shape / smaller standard deviation / less variation
    • Same pattern with bigger bell-shape

Causes of variation

Independent assortment of bivalents at the equator during anaphase I

  • Chromosomes of bivalents pulled to opposite poles at random
  • 2n different combinations of chromosomes in four haploid cells produced where n is the haploid number of chromosomes

Crossing over between non-sister chromatids during prophase I

  • At synapsis, non-sister chromatids of homologous pairs cross over at chiasmata
  • Homologous chromatids (corresponding pieces of genetic material) break and exchange equivalent segments between maternal and paternal chromatids
  • Results in new combination of genes from the two parents

Fertilization / random fusion of gametes

  • Genetic difference amongst the zygote
  • New combinations of alleles

Gene mutation / increased by environmental factors (eg radiation)

  • Addition / at least one base is added during DNA replication
  • Deletion / at least one base is not copied (frameshift)
  • Substitution / at least one base is copied wrongly
  • Interferences with normal base pairing (A-T;C-G)
  • Degenerate code / different triplets can code for same amino acids

Discontinuous variation

  • IMG 5-14-2
  • Limited number of distinct phenotypes / categories (e.g. blood group)
  • Strong genetic factor controlled by alleles on one gene
  • Frequency histogram has separate bars
  • Unaffected by the environment

Continuous variation

  • Continuous range of values / class intervals (e.g. human height)
  • Alleles on many genes located on different chromosomes / polygenic inheritance
  • Frequency histogram is a smooth (normal distribution) curve
  • Phenotype is affected by environmental factors
    • Lower skin temperature activates a gene for pigment production
    • Diet affects individual's size and health. Malnourishment results in shorter height
    • Therefore, genes + environment → phenotype (continuous variation)

Advantageous of variation to species

  • Allows different adaptations / some better adapted
  • Some survive / reproduce / pass on gene/allele
  • Allows for changing environment / different environment