11. Classification

• Classification is the way in which living organisms are divided into groups.
• Taxonomy is the scientific practice of grouping organisms based on shared characteristics.
• Hierarchal means that it consists of smaller groups within larger groups, with no overlap between groups.
• Phylogenetic means it is based on the evolutionary history of organisms, i.e their common ancestry.
• Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
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• Binomial name is the genus and the species.
• The phylogenetic system reflects the evolutionary relationship between organisms, looking at similarities and differences in structure and function and usually represented by a phylogenetic tree. The oldest species are at the base of the tree and the most recent ones at the ends of the tree. Each branch represents where divergence occurred (where a common ancestor evolves to become new species). The closer the branches, the closer the evolutionary relationship, therefore the more recently they shared a common ancestor.
• A species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring.
• Two organisms belong to the same species if they have the potential to breed with each other and their offspring are fertile.
• If two different species breed and there is an odd number of chromosomes, the offspring will be infertile as meiosis cannot be completed and gametes cannot form.
• Problems classifying organisms as different species:

  The extinct species greatly outnumber the living.

  Most species did not leave fossils/fossils are incomplete.

  If organisms only known from fossils, we can’t test if they can interbreed and produce fertile offspring.

  There is considerable variation within any one species.

  Groups of organisms that are isolated form each other by oceans etc may be classified as different species but can interbreed.

  Ability to interbreed cannot be tested on organisms which use asexual reproduction.
• Courtship behaviour is genetically determined. All members of same species programmed to show identical courtship behaviour. This means it can be used to identify individuals of the same species.
• Species recognition allows one species to recognise other members of the same species. Chain if actions which occur are the same for all members of a species but differs for members of other species.
• Synchronising reproductive behaviour is when behaviour of one individual stimulates a response from the other, such as both individuals being in the correct stage of their reproductive cycle, and if they are not they don't respond.
• Identifying a mate that is capable of breeding ensures both partners and sexually mature and receptive to breeding.
• Formation of a pair bond keeps the male and female together, increasing the survival chances of the offspring.
• Becoming able to breed involves bringing a member of the opposite sex inti a physiological state that allows breeding to occur such as by stimulating the production of gametes.
• Courtship behaviour can take many forms, such as displaying breeding colours, a dance or a song. This depends on the habitat the species live in, such as woodlands or nocturnal animals rely more heavily on sking.
• Comparing the frequency of measurable or observable characteristics is how genetic diversity was tested before modern gene technology. Differences were measured through careful detailed observations of the anatomy and physiology of different individuals. However, observable characteristics are usually coded for by more than one gene and the environment can also influence some characteristics.
• Comparing the base sequence of DNA can be used to investigate diversity as members of the same species will have very similar DNA base sequences. As populations evolve, random mutations will accumulate and cause genetic variation. Species with closer evolutionary relationships and share a more recent common ancestor will have more similar DNA sequences.
• Comparing the base sequence of mRNA is sometimes more useful as it looks at what parts of their genome are expressed. The base sequence of mRNA can be compared in different organisms and can be used to determine evolutionary relationships and genetic diversity.
• Comparing the amino acid sequence of the proteins encoded by DNA and mRNA can be used to investigate diversity as organisms that share a recent common ancestor and are more closely related will have similar amino acid sequences when looking at a particular protein.
• Immunological techniques can be used to investigate diversity because if antibodies that are specific to antigens from a particular species are complementary to antigens from a different species then this suggests they are closely related and share a recent common ancestor.
• Large samples are more representative of the population than small samples and minimise the influence of chance on the result. However sample size is often influenced by the time available to collect sample.