Homology means the similarity due to the common ancestor. Homoplasy, on the other hand, means similarity due to convergent evolution, but independent origins. For instance, take the fin and the caudal fin of tuna and of dolphin; they are similar but have independent histories, and their similarity comes from adaptation to similar environments and functions. This is homoplasy. However, the fin of tuna and bonito are similar because of the common ancestor, and that's homology.
To test phylogenetic hypotheses, scientists must be able to find out which similarities indicate a close relationship between species and which do not. The key to this process is determining the evolutionary origins of the similar features. Only similarities inherited from the species' common ancestor can provide evidence of phylogenetic relationship, because they are evidence of a genetic continuity from the common ancestor. Such a similarity, inherited in common form from a single common ancestor, is called homology.
To test phylogenetic hypotheses, scientists must be able to find out which similarities indicate a close relationship between species and which do not. The key to this process is determining the evolutionary origins of the similar features. Only similarities inherited from the species' common ancestor can provide evidence of phylogenetic relationship, because they are evidence of a genetic continuity from the common ancestor. Such a similarity, inherited in common form from a single common ancestor, is called homology.
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