Problems in Classifying Organisms: Species are always changing. It is also important to realize that most species are physically and genetically diverse. Many are far more varied than humans. New species are defined based on minor differences between organisms. This is the splitter approach. The second tends to ignore minor differences and to emphasize major similarities. This lumper approach results in fewer species being defined. It should be noted that members of very closely related species can sometimes produce offspring together, and a small fraction of those may be fertile. This is the case with mules, which are the product of mating between female horses and male donkeys. About one out of 10,000 mules is fertile. Comparisons of DNA sequences are now becoming more commonly used as an aid in distinguishing species. If two animals share a great many DNA sequences, it is likely that they are at least closely related.
Linnaeus’s classification system: The Linnaean scheme for classification of living things groups together organisms based on presumed homologies. The assumption is that the more homologies two organisms share, the closer they must be in terms of evolutionary distance. It is a hierarchical system of classification with the highest category consisting of all living things and the lowest category consists of a single species.
Cladistics approach to classification: Modern day researchers take a cladistics approach to classification. This involves making a distinction between derived and primitive traits when evaluating the importance of homologies in determining where to place organisms within the Linnaean classification system. Derived traits are those that have changed from the ancestral form or function.