Assessing physical features is one way of determing the relatedness of organisms.
When assessing similar physcial features a homologous structure should be looked for: this is two structures of similar arrangement of component parts and a similar developmental origin, but a different function.
E.G. the Pentadactyl limb of the vertebrate
Its basic structure is common to the four terrestrial vertebrates: amphibians, mammals, reptiles and birds, but they all have different functions (e.g. grasping, flying).
This provides an example of divergent evolution: a structure of a common ancestor has evolved.
Analogous structures are structures of similar shape and function, but a different developmental origin.
E.G. Butterfly, sparrow and a bat.
These three organisms all have wings, however, one is an insect, one is a bird and one is a mammal, and so they do not share a recent common ancestor. Instead, their ancestors must have adapted to similar environmental conditions and evolved wings.
This is an example of convergent evolution: the structure has evolved a similar shape and function, but comes from a different environmental origin.
During the course of evolution when one species evolves to form another, the new species will have slight differences in the base sequences of its DNA. Over time, this species will accumulate more differences in its DNA base sequences. Therefore, the more closely related two species are, the greater the similarity in their DNA base sequences.
DNA analysis allows for us to correct mistakes made in classification due to convergent evolution.
The relatedness of two organisms can be determined by coimparing the DNA base sequences of two species through DNA hybridisation.
The DNA from the two organisms is extractd, separated, split into fragments and then mixed. Any DNA with complimentary base pairs will hybridise to form a DNA double helix. Therefore, the greater the number of hybrid DNA helicases, the more closely related two species are.
Amino Acid sequencing
The amino acid sequence of a protein is determined by the DNA base sequences. Therefore, the amino acid sequences of a specific protein of two organisms can be compared. The greater the similarity in the sequences, the more closely related the two species are.
The antigens of one species can be mixed with specific antibodies from another species. These will react to form a precipitate. Therefore, the more closely related two species are, the more the antigens and antibodies that will react and so the volume of precipitate produced will be greater.