This is the grouping of plants and animals based on a number of factors. Once a simple task the process has become more and more complex as new discoveries have been made.

Classification System

Specification Reference

"Know that the classification system consists of a hierarchy of domain, kingdom, phylum, class, order, family, genus and species."
This is the systematic grouping of organisms based upon appearance, heiarchy or phylogenetic relationships.
  • Taxonomy - the science of describing and classifying living things based upon physical similarities.
  • Phylogeny - classification based upon evolutionary relationships determined by DNA or RNA.
  • Analogous features - look similar and have the same function but do not have the same origin.
  • Homologous features - show genuine common ancestry (pentodactyl limb).
Animals are named using a binomial system. This means that and organism is named using its genus and species. This overcomes regional variations in naming the same organism.

Classification Heirarchy
Domain Do
Kingdom Keep
Phylum Pots
Class Clean
Order Or
Family Family
Genus Gets
Species Sick
This allows species to be easily identified - Homo Sapiens.

Defection of a Species

Specification Reference

"Understand the limitations of the definition of a species as a group of organisms with similar characteristics that interbreed to produce fertile offspring."
Understand why it is often difficult to assign organisms to any one species or to identify new species.
What a species is can be very hard to define as nearly every model has its limitations. A few examples are shown below with their limitations:

This looks at appearance and physical characteristics. Those animals that look similar will be put into the same category and be defined as a species.
  • Does not account for sexual dimorphism
  • Organisms change over time making some hard to trace through the fossil record
This defines a species as a group of organisms with similar characteristics that can reproduces to form fertile offspring.
  • Does not account for asexual reproduction
  • Some animals have never been seen mating so we are only presuming that they do
An organisms DNA is analysed and compared with another sample. The degree of similarity determines if they are from the same species.
  • How "similar" do they need to be for them to be classed as the same species
  • DNA can degrade over time
  • The genome of animals changes significantly over long periods of time
Overall Limitations
  • Finding evidence
  • When does a hybrid become a new species
  • Some do not account for asexual reproduction
  • Fossils need to be accounted for
  • Sexual dimorphism
  • Some species mimic each other

Evolutionary Relationships

Specification Reference

"Understand how gel electrophoresis can be used to distinguish between species and determine evolutionary relationships."
DNA analysis allows us to asses the similarities between two organisms. This can be achieved using two different techniques.

DNA Hybridisation

  1. Samples of DNA from each organism are taken and heated in separate tubes
  2. The hydrogen bonds break releasing the double helix
  3. The two samples are mixed
  4. The less hydrogen bonds that reform when these two samples are mixed the less similar the organisms are
  5. The number of hydrogen bonds present can be determined by the temperature at which the two strands of DNA separate

DNA Sequencing

This is determining the sequence of bases within a DNA molecule and is achieved through a process known as gel electrophoresis.
  1. DNA is isolated and hydrolysed into single strands by restriction endonucleases
  2. These fragments are mixed with DNA polymerase and nucleotides
  3. Some nucleotides have a fluorescent marker, these stop further nucleotides being added
  4. The solution is then placed in gel and a current is passed through the solution drawing the negatively charged DNA towards the positive electrode
  5. Smaller fragments move more quickly and are detected first allowing the base sequence to be deduced


Specification Reference

"Know that DNA sequencing and bioinformatics can be used to distinguish between species and determine evolutionary relationships."
This is the use of technology and software to analyse large amounts of biological data, spotting patterns and trends.
  • This enables comparisons of entire genomes to be made, looking at mutations and how many there are to create a phylogenetic tree.
  • These mutations may have caused a divergence in the phylogenetic tree allowing relationships between organisms to be mapped out.

The Peer review Process

Specification Reference

"Understand the role of scientific journals, the peer review process and scientific conferences in validating new evidence supporting the accepted scientific theory of evolution."
When new research is completed, it must first be validated by the scientific community. The process is as follows:
  1. Published in a journal
  2. The sent to experts for review
  3. This checks for validity
  4. Presented at a conference
  5. Questions are asked
This is known as the scientific method of enquiry.

Models of Classification

Specification Reference

"Understand the evidence for the three-domain model of classification as an alternative to the five-kingdom model and the role of the scientific community in validating this evidence."
There are currently two major models of classification structure. These are outlined below:

Five-Kingdom Model

  • Prokaryotic
  • Autotrophs and hetrotrophs
  • Eukaryotic
  • Unicells, colonies and filaments
  • Autotrophs (algae) and hetrotophs (protozoa)
  • Eukaryotic
  • Cell walls of chitin
  • All are hetrotrophs
  • Saprobionts or parasites
  • Eukaryotic
  • Cellulose cell walls
  • All autotrophs
  • Eukaryotic
  • No cell wall
  • Multicellular
  • Hetrotrophs
  • Develop from a blastocyst
This model relies heavily on physical observations. In light of new DNA sequencing and molecular phylogency th model was revised. The new model better shows evolutionary relationships.

Three-Kingdom Model

This takes into account research of the ribosomal unit known as 16s RNA.

  • Prokaryotic
  • Circular DNA
  • No introns
  • Cell wall present
  • L-glycerol (ability to live in extreme habitats.
  • Prokaryotic
  • Circular DNA
  • No introns
  • Peptidoglycan cell wall
  • Unbranched fatty acids
  • D-glycerol
  • Eukaryotic
  • Linear DNA with histones
  • Has introns
  • Cell walls in some (cellulose or chitin)
  • Unbranched fatty acids
  • D-glycerol
An Autotroph is an organism that synthesises its own food using an external energy source. A hetrotroph is an organism that feeds on other organisms.


The endosymbiotic theory provides compelling evidence that all life on Earth has a common ancestor. The theory is as follows:
  1. A large host cell ingested a bacteria
  2. This bacteria was not consumed
  3. The ingested cell became useful to the host
  4. This created a symbiotic relationship
This is not just a wild theory, there is actually a substantial amount of evidence that backs it all up. Take a chloroplast and you will see that:
  • They have their own DNA
  • Are double membrane bound
  • Reproduce like bacteria
  • Contain 70s ribosomes (like bacteria)
All the evidence suggest that in some point in history, organelles such as chloroplasts where actually independent bacterial cells capable of living outside of a eukaryotic cell.