What are the difficulties you would face in classifying animals without taking the common characters into consideration?

Answer 1

For the purposes of this answer common is understood to mean "usual, widespread, belonging to more than an individual" Therefore the question is understood to ask about the problems in looking at the most unique characteristics of an animal for purposes of classification. The unique characteristic approach to classification can work. The question inquires to the difficulties of this approach. The answer follows.

The practioner must be aware of exactly what are the most unique characteristics of the animal being classified e.g. one can examine an animal that has a nematocyte. If the individual properly recognizes this as a nemotocyte then the classification can be made that the animal is a cnidarian. Difficulties will occur when the wrong "unique" characteristics are grouped to come to a conclusion.

Take the Portuguese-Man-O-War. It has long tentacles, is jelly-like and has nematocytes. If the classifier looked at these characteristics alone then the Man-O-War could be mislabeled as belonging to the subphylum Medusozoa as a jellyfish.

This would be incorrect however since jellyfish exist as single organisms and Portuguese-Man-O-War exist in integrated floating polyp colonies of the class Hydrazoa. The more specific feature concering the colony characteristic of this organism would allow the correct distinction.

Similarly Horshoe Crabs could be mislabeled as crustaceans since they are oceanic creatures that crawl and have shells but this would also be incorrect.

Conclusion: the difficulties in classifying animals consist of the requirement of knowledge of the most specific characteristics of an animal, grouping these characteristics correctly, not using standalone traits and recognizing the difference between convergent evolution and divergent evolution in producing these characteristics.