4.1.3 Classification and Evolution

• Genetic variation


  Occurs within a species
• Alleles


  New alleles arise by mutation
• Organisms

  • Whose characteristics are best adapted to a selection pressure such as predation, competition or disease will have an increased chance of survival and reproductive success
  • Less well-adapted organisms will die or fail to reproduce
• Natural selection


  1. Successful organisms will pass on their alleles with the advantageous characteristic to their offspring

  2. This process is repeated for every generation

  3. Over time, the frequency of the allele that codes for this characteristic increases
• Over very long periods of time and many generations, often involving multiple genes, this process can lead to the evolution of a new species
• • Originally classification systems were based on observable features
  • Now scientists study the genetics and biological make-up of organisms to classify them, called: molecular systematics

  For characteristics to change, DNA must also change. By comparing similarities between DNA and the proteins of the different species, the evolutionary relationships between organisms can be discovered.
• Molecular systematics uses molecules such as DNA, RNA and proteins (amino acids) to interpret the evolutionary relationships between organisms
• • The three domains:
  • Eukarya
  • Archaea
  • Bacteria
• • It recognises the fundamental differences between the prokaryotes and the eukaryotes. 
  • Also recognises that the prokaryotes should be split as there are fundamental differences between the Archaea and Bacteria.
  • Structural differences between Bacteria and the other 2 domains:
  • Different cell membrane structure
  • Flagella with a different internal structure
  • Different enzymes for synthesising RNA
  • No proteins bound to their genetic material
  • Different mechanisms for DNA replication and for synthesising RNA
• • Although both Archaebacteria and Eubacteria are single-celled organisms, Eubacteria are classified in their own kingdom because they have a different chemical makeup.
  • Archaebacteria: ancient bacteria; can live in extreme environments (hot vents, anaerobic conditions and highly acidic environments)
  • Eubacteria: found in all environments – they types of bacteria you are more familiar with
• Alfred Wallace was working on his own theory of evolution. He sent his ideas to Darwin for peer review and as their ideas were so similar, they presented two scientific papers jointly in 1858. E.g., warning colours to deter predators.
• • Fossils are formed when animal and plant remains are preserved in rocks.
  • Over a long period of time sediment is deposited and layers of rock are formed. The different layers correspond to the different geological eras. This is known as the fossil record.
• • A homologous structure is one that superficially appears to be different in different organisms but has the same underlying structure.
  • This can be seen in the pentadactyl limbs – the functions of which have evolved over time but are from a common ancestor. We call this divergent evolution.
• Highly conserved


  Molecules that remain unchanged among species
• Highly conserved


  Molecules that remain unchanged among species
• Common molecules studied

  • cytochrome c
  • ribosomal RNA
• Common molecules studied

  • cytochrome c
  • ribosomal RNA
• Substitution rate


  The known rate at which mutations occur
• Substitution rate


  The known rate at which mutations occur
• Using substitution rate to determine common ancestor


  1. Know average mutation rate

  2. Extrapolate how long ago two organisms shared a common ancestor
• Using substitution rate to determine common ancestor


  1. Know average mutation rate

  2. Extrapolate how long ago two organisms shared a common ancestor
• Ribosomal RNA


  • Has a very slow rate of substitution
  • Commonly used together with fossil record to determine relationships between ancient species
• Ribosomal RNA


  • Has a very slow rate of substitution
  • Commonly used together with fossil record to determine relationships between ancient species
• • Why do we classify organisms?
  • To identify species and avoid confusion
  • To predict characteristics – if several members in a group have a characteristic, it is likely that another species in the group will have the same characteristic
  • To find evolutionary links – shared characteristics because they evolved from a common ancestor
• • The grouping of organisms is classification
• • The theory and practice of classification is called taxonomy.
• Taxonomy: a form of classification that focusses on similarities between different species for ease of naming and identification
• • Phylogeny: a way of classifying organisms to show the evolutionary relationships between them, so that every group shows a common ancestor
• • Artificial classification– divides organisms according to observable similarities and differences e.g. colour, size, number of legs, leaf shape etc.  
  • These characteristics may have the same function, but they evolved separately.
  • Does not reflect any evolutionary relationships.
• • Natural classification – based upon evolutionary relationships between organisms and their ancestors' -> shared features are derived from their ancestors.  
  • May change with advancing knowledge.
• Domain

  Kingdom

  Phylum

  Class

  Order

  Family

  Genus

  Species
• • When a new species is identified, it is always given a binomial name. 
  • The first part indicates the genus (starts with a capital letter) to which the organism belongs. 
  • The second part is the species (starts with a lower-case letter) to which the organism belongs. 
• Why is binomial naming still used now?

  • Linnaeus used Latin as a universal language.
  • This means that whenever a species is named, it is given a universal name.
  • Every scientist in every country will use the same name.
  • Avoids the potential confusion caused by using common names.
  • Allows you to identify the species and genus
• • The five-kingdom classification system:
  • Prokaryotae (bacteria) (prokaryotes)
  • Protoctista 
  • Fungi 
  • Plantae 
  • Animalia
• Prokaryotae

  • Unicellular
  • No nucleus or other membrane-bound organelles
  • A ring of ‘naked’ DNA
  • Small ribosomes
  • No visible feeding mechanism – nutrients absorbed through cell wall or produced by photosynthesis
• Protoctista

  • Most of them are unicellular
  • Have a nucleus and other membrane-bound organelles
  • Some have chloroplasts
  • Some are sessile, but others move by cilia, flagella or amoeboid mechanism
  • Nutrients acquired by photosynthesis (autotrophs) or ingestion of other organisms (heterotrophic); or both
  • Some are parasitic
• Fungi

  • Unicellular or multicellular
  • Have a nucleus and other membrane-bound organelles
  • Have a cell wall composed of chitin
  • No chloroplasts, no chlorophyll
  • No mechanism for locomotion
  • Most have a body or mycelium made of threads or hyphae
  • Saprophytic feeders – acquire nutrients from decaying material by absorption
  • Most store their food as glycogen (insoluble)
• Plantae

  • Multicellular
  • Have a nucleus and other membrane-bound organelles
  • Have chloroplasts and chlorophyll
  • Cells have cell walls consisting of cellulose
  • Most do not move, although the gametes of some plants can move using a flagella/cilia
  • Nutrients acquired by photosynthesis (autotrophic)
  • Store food as starch
• Animalia

  • Multicellular
  • Have a nucleus and other membrane-bound organelles
  • No cell walls
  • No chloroplasts
  • Move with the aid of cilia, flagella or contractile proteins, sometimes in the form of muscular organs
  • Nutrients acquired by ingestion (heterotrophic feeders)
  • Food is stored as glycogen
• Greater standard deviation = the data is more spread out around the mean it is less consistent and has low repeatability.