WJEC AS Biology Unit 2 Topic 1 - Basic Revision Notes

  • Created by: Tab74
  • Created on: 01-08-17 16:31


  • Classification places organisms into discrete, hierarchical groups.  

    • It is hierarchical because it is a system of smaller groups that belong to larger groups.

    • Smaller the group, more they have in common.

    • Discrete as organisms cannot be in more than one group at the same level.

    • Another name for group is taxon.

  • The five kingdom system classifies based on physical appearance, into 5 kingdoms.

  • The 5 Kingdom system works as follows

    • Kingdom, Phylum, Class, Order, Family, Genus, Species

  • The 5 Kingdoms are

  • The classification is tentative.

    • Depends on our current knowledge

    • May be altered as our knowledge changes

    • Velvet worms have soft body and jointed limbs, characteristics of two phyla so a new phylum was created for them.

  • Phylogenetic classification groups organisms based on evolutionary relationships.

    • Further up, more recent, top row alive now.

    • Branches represent common ancestors.

Why do we need a classification system?

    • Allows you to infer evolutionary relationships and assess how closely related organisms are.

    • If new animal discovered with some characteristics of a taxon, we can predict its other characteristics based on what we know

    • Easier for communication

    • Easier to count families that species when assessing the health of an ecosystem and/or extinction.

  • Species - group of organisms that have similar features and can breed to produce fertile offspring.  

    • May show sexual dimorphism - physical difference between males and females e.g. lions.

  • Binomial Name - ‘Genus species

  • Recent biochemical evidence has shown that the kingdom prokaryotae should be split into 2 groups based on fundamental biological differences.  This has led to Three Domain System

    • States that all organisms have evolved down three distinct lines

    • Based on organisms in each domain sharing a distinctive unique pattern of Rrna

    • Eubacteria are true bacteria.

    • Archaea are extremophiles prokaryotes existing in extremes of pH, temperature, pressure and salinity and have unusual metabolisms e.g. some produce methane.

    • Eukarya/Eukaryota are organisms with eukaryotic cells.

Assessing Relatedness

  • Physical Features

    • More similar features suggests more closely related organisms.

    • Homologous Structures - evolved from same original structure for different functions through divergent evolution.  E.G. Pentadactyl limb.

    • Analogous Features - have evolved from different structures to perform the same function by convergent evolution.  E.G. wings of birds and insects.

  • DNA Sequencing

    • More similarities in dna base sequence, more closely related.  

    • Has confirmed evolutionary relationships and corrected mistakes made from physical feature classification (due to convergent evolution)

  • DNA Hybridisation

    • Compares dna base sequence of two species

    • DNA from both is extracted, separated and cut into fragments.

    • Fragments are mixed


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