Q.Why are the classification systems changing every now and then?

Classification systems keep changing because our knowledge of organisms keeps growing and improving. Classification is based on the characters of organisms, so whenever new or better information about those characters becomes available, the way we group organisms has to be revised.

Reasons the systems change include:

• New organisms are constantly being discovered and described, and they must be fitted in.
• Better tools let us study not just external form but also internal structure, cell structure, development and ecology, which can reveal relationships we missed earlier.
• Systematics takes evolutionary relationships into account, and our understanding of how organisms are related keeps deepening.
• As understanding improves, earlier groupings are corrected so that organisms are placed more soundly.

Classification is never a finished, fixed thing. It is the process of arranging organisms into groups based on their characters, and it is only as good as the information those characters provide. Since that information is always expanding and being refined, the systems built upon it naturally shift over time.

Several forces drive this change:

• Continuing discovery. A large number of organisms are still unknown. Every newly found and described organism has to be given its correct place, and accommodating it can reshape existing groups.
• Deeper sources of evidence. Modern taxonomy no longer relies on external appearance alone. It draws on external and internal structure, cell structure, developmental processes and ecological information. As these finer characters are studied, hidden similarities and differences emerge, and earlier groupings may need adjusting.
• A focus on relationships. Interest in how organisms are related to one another gave rise to systematics, the systematic arrangement of organisms, which takes evolutionary relationships into account. As our grasp of these relationships improves, the arrangement that best reflects them also changes.

So the changes are not a sign of weakness in the science; they are a sign of progress. Each revision is an attempt to make the groupings reflect the organisms' real characters and relationships more faithfully than before.