Biological classification is in a continual state of flux. As our knowledge of organisms expands, we gain a better understanding of the similarities and differences among various groups of organisms. In turn, those similarities and differences shape how we assign animals to the various groups (taxa).

taxon (pl. taxa) - taxonomic unit, a group of organisms that has been named

Factors That Shaped High-Order Taxonomy

The invention of the microscope in the mid-sixteenth century, revealed a minute world filled with countless new organisms that had previously escaped classification because they were too tiny to see with the naked eye.

Throughout the past century, rapid advances in evolution and genetics (as well as a host of related fields such as cell biology, molecular biology, molecular genetics, and biochemistry, to name just a few) constantly reshape our understanding of how organisms related to one another and shed new light on previous classifications. Science is constantly reorganizing the branches and leaves of the tree of life.

The vast changes to classification that have occurred throughout the history of taxonomy can best be understood by examining how the highest level taxa (domain, kingdom, phylum) have changed throughout history.