the study of evolutionary relationships between species

Phylogenetics is the study of evolutionary relationships and the history between species. Phylogenetic trees can be used to illustrate phylogenetics. It is important in showing the gradual changes that have occurred between organisms which has caused one species to develop into the next. Without phylogenetic trees we would not be able to trace back to common ancestors and discover from whereabouts certain characters have emerged.

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Biological species is the most widely accepted definition of a species; individuals that can interbreed with each other to produce fertile offspring. It is directed more towards the idea of interbreeding. The morphological species concept, on the other hand, is based on the appearance of the organism rather than their ability to interbreed. This concept should only be used when the biological species concept cannot be used, for example when fossils are the only obtainable evidence as the organism no longer exists.
A character is a feature that separates one species from another using the morphological species concept or behavioural or molecular attributes. A morphological example would be the presence or absence of a tail. To determine whether a character is good or less-good, the organism and its habitat must be taken into account. A good character will increase the chance of a species survival. The presence of wings in birds is clearly advantageous as all birds have wings. A less-good character to use would be one that is not so commonly found throughout the animal kingdom, indicating that it has no real advantage, for example hair found only on mammals.
Shared ancestral characters and shared derived characters have an effect on phylogenetic trees. A shared ancestral character is where the common ancestor of a taxon has that particular character and has passed it on to its descendants. A shared derived character is where a character evolves and the common ancestor did not have that character. The shared derived character is what is needed to separate species when constructing a tree because it determines the evolutionary relationship. If systematists were to define humans using the character ‘hair’ it would be fairly useless as all non-human mammals also have hair, therefore making the character ‘hair’ in this case a shared ancestral character for humans.
Systematists use parsimony to construct phylogenetic trees. Concerning phylogenetics, parsimony is the least complex and most likely evolutionary pathway. For example, it is more likely that a backbone only evolved once, producing vertebrates, rather than several times for many different species of vertebrate.
To construct a phylogenetic tree the biological species concept can be used as well as the morphological one. Systematists use characters that separate different taxa in order to construct such trees. Examples of characters that could be utilised include the absence or presence of hair, teeth, eyes, a hinged jaw, vertebral column etc. A computer programme is used to construct the tree accurately.
Having carried out a practical experiment I can now use evidence to support my ideas. During the practical I constructed two trees. The initial tree was inaccurate as very few of the organisms were placed next to their closest relative. The absence or presence of eyes was a less-good character to use for my classification because it didn’t separate the different species correctly. It is not specific enough and it is a shared ancestral character, therefore not reflecting an evolutionary relationship. The character unicellular or multicellular was, on the other hand, a good character to use because it separated the Amoeba from the other organisms.
After having reconstructed the tree by changing some of the characters, improvements were made , but it was still not entirely accurate. The oystercatcher (Haematopus ostralegus) and Brontosaurus are grouped together which correctly suggests they are related, as dinosaurs are an ancestor of birds. However, the whale (Cetacea sp), human (Homo sapien) and badger (Meles meles) should be grouped together as they are all mammals.
The mistake I made was choosing some characters that were analogous as opposed to homologous. Analogous characters are when species may look alike but are genetically completely dissimilar. They have been subjected to selection pressures which has caused them to develop similar characters needed to survive in their environment. A good example of this is the bat wing and the bird wing. They have not evolved from a common ancestor, but instead have converged from different ancestors. Homologous characters are when species may look completely different but they are very similar genetically because they have both descended from a common ancestor. These characters are useful for the classification of species and should therefore be used for constructing phylogenetic trees.
To conclude, the most useful characters I used for this practical were unicellular or multicellular as this separated the Amoeba from the others, and the presence of a backbone as this grouped most of the vertebrates together. A homologous character is the most use when creating a phylogenetic tree and should be used in preference to analogous characters.  
Bibliography

Lecture notes
Campbell and Reece
Robyn Beck