Variation & Selection

• Variation


  Differences between individuals of the same species
• Phenotypic variation


  Difference in features between individuals of the same species
• Causes of phenotypic variation

  • Genetic
  • Environmental
• Genetic variation in humans

  • Blood group
  • Eye colour
  • Ability to roll tongue
  • Whether ear lobes are free or fixed
• Whether earlobes are attached (lobeless) or free (lobed) is an example of genetic variation
• Factors that can affect characteristics of species

  • Climate
  • Diet
  • Accidents
  • Culture
  • Lifestyle
• Environmental


  Outside of the organism
• Examples of environmental effects


  • Accident leading to scarring
  • Weight gain from overeating and lack of activity
  • Speaking a certain language with a certain accent from being raised in a country
  • A plant growing taller to reach more light in the shade
• 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
• Continuous variation


  • Often varies because of a combination of genetic and environmental causes
  • Gives smooth bell curves when graphed
• Discontinuous variation


  Distinct differences for a characteristic, with no intermediary values
• Examples of discontinuous variation


  • Blood group
  • Sex
  • Ability to roll tongue
  • Seed shape and colour in peas
• Discontinuous variation


  • Usually caused by genetic variation alone
  • Gives step-shaped graphs when plotted
• DNA


  The molecule that contains the instructions for growth and development of all organisms
• DNA


  • Consists of two strands wound around each other in a double helix
  • The individual units are called nucleotides
• Nucleotide


  Contains a phosphate, deoxyribose sugar, and one of four bases: adenine, cytosine, thymine, guanine
• DNA base pairing

  • A always pairs with T
  • C always pairs with G
• The phosphate and sugar section of the nucleotides form the 'backbone' of the DNA strand, and the base pairs form the 'rungs'
• The sequence of bases in DNA holds the code for the formation of proteins
• You do not need to learn the names of the DNA bases, just their letters
• Gene


  A short length of DNA that codes for a particular sequence of amino acids that make up a protein
• Examples of proteins


  • Structural proteins like collagen
  • Enzymes
  • Hormones
• From gene to protein

  Sequence of nucleotide bases in DNA codes for sequence of amino acids in protein
• Inheritance


  The transmission of genetic information from generation to generation
• Alleles


  Variations of the same gene
• We have two copies of each gene and therefore two alleles for each gene, one inherited from each parent
• Phenotype


  The observable characteristics of an organism
• Genotype


  The combination of alleles that control each characteristic
• Dominant allele


  Only needs to be inherited from one parent for the characteristic to show up in the phenotype
• Recessive allele


  Needs to be inherited from both parents for the characteristic to show up in the phenotype
• Homozygous


  Having two copies of the same allele
• Heterozygous


  Having two different alleles
• We cannot always tell the genotype of an individual just by looking at the phenotype
• If two individuals who are both identically homozygous for a particular characteristic are bred together, they will produce offspring with exactly the same genotype and phenotype as the parents</b>
• Inheritance key terms

  • Gamete
  • Chromosome
  • Gene
  • Allele
  • Dominant
  • Recessive
  • Homozygous
  • Heterozygous
  • Genotype
  • Phenotype
• Monohybrid inheritance


  Inheritance of characteristics controlled by a single gene
• Using a Punnett square

  1. Shows the possible combinations of alleles in the offspring

  2. Allows prediction of probability of different outcomes
• The dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case
• The predicted genotypes from Punnett squares are based on chance, and the observed results may differ, especially with small numbers of offspring