Alternative taxonomical classifications
are those which differ from the commonly accepted classifications in fundamental ways. One could say that every single classification system since Linnaeus
has been alternative at some point, before it was either completely rejected, or accepted as the standard. The current trend seems to be to replace older systems which use paraphyletic or polyphyletic taxa with those with monophyletic groups. This is facilitated by the use of clades, which abandon the use of conventional taxa, such as kingdoms and phyla, and use instead monophyletic groups nested inside one another.
Historical alternative classifications
These are systems developed in the past that have been either ultimately rejected, or ultimately accepted by the scientific community
, who created the first classification system, divided the known universe into three kingdoms: Animalia
, and Mineralia
. This system recognized the differences between the two most visible forms of life, plants and animals, and placed everything else (rocks, clouds, water, fire, etc.) in a catch-all category. This system was the standard for many years, and provided the basis for the "kingdom" taxon in all subsequent taxonomical systems.
The five-kingdom system
As scientific methods of experimentation improved, in particular with the use of the microscope, it became increasingly obvious that the world of life could not be divided simply into plant and animal kingdoms. Many microscopic organisms, or "animalcules" as they were called, did not fit well into either category. Therefore, the kingdom Protista (or Protoctista) was proposed by Ernst Haeckel to comprise the unicellular organisms. It was not widely accepted at first, but it was the first step towards improving the Linnaean system.
The next problem with Linnaean taxonomy was the prokaryotic cell. Again, as methods of scientific research improved, it became apparent that there were two distinct types of cells - those of eukaryotes, and those of prokaryotes.