Compare and contrast the classification system used by Linnaeus to system used now?

Answer 1

Over time, our understanding of the relationships between living things has changed. Linnaeus could only base his scheme on the structural similarities of the different organisms. The greatest change was the widespread acceptance of evolution as the mechanism of biological diversity and species formation. It then became generally understood that classifications ought to reflect the phylogeny of organisms, by grouping each taxon so as to include the common ancestor of the group's members (and thus to avoid polyphyly). Such taxa may be either monophyletic (including all descendants) such as genus Homo, or paraphyletic (excluding some descendants), such as genus Australopithecus. Originally, Linnaeus established three kingdoms in his scheme, namely Plantae, Animalia and an additional group for minerals, which has long since been abandoned. Since then, various life forms have been moved into three new kingdoms: Monera, for prokaryotes (i.e., bacteria); Protista, for protozoans and most algae; and Fungi. This five kingdom scheme is still far from the phylogenetic ideal and has largely been supplanted in modern taxonomic work by a division into three domains: Bacteria and Archaea, which contain the prokaryotes, and Eukaryota, comprising the remaining forms. This change was precipitated by the discovery of the Archaea. These arrangements should not be seen as definitive. They are based on the genomes of the organisms; as knowledge on this increases, so will the categories change. Reflecting truly evolutionary relationships, especially given the wide acceptance of cladistic methodology and numerous molecular phylogenies that have challenged long-accepted classifications, has proved problematic within the framework of Linnaean taxonomy. Therefore, some systematists have proposed a Phylocode to replace it. Source: http://en.wikipedia.org/wiki/Linnaean_taxonomy