Variation

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