Since Darwin, biological taxonomy has attempted to reflect the phylogeny of organisms, i.e. the tree of life. The classifications used in taxonomy attempt to represent the precise interrelatedness of the various taxa. However, the taxonomy of the Gastropoda is constantly being revised and because of this, the versions shown in various texts can differ in major ways.
In the older classification of the gastropods, there were four subclasses:
- Opisthobranchia (gills to the right and behind the heart).
- Gymnomorpha (no shell)
- Prosobranchia (gills in front of the heart).
- Pulmonata (with a lung instead of gills)
The taxonomy of the Gastropoda is still under revision, and more and more of the old taxonomy is being abandoned, as the results of DNA studies slowly become clearer. Nevertheless a few of the older terms such as "opisthobranch" and "prosobranch" are still sometimes used in a descriptive way.
New insights based on DNA sequencing of gastropods have produced some revolutionary new taxonomic insights. In the case of the Gastropoda, the taxonomy is now gradually being rewritten to embody strictly monophyletic groups (only one lineage of gastropods in each group). Integrating new findings into a working taxonomy will continue to be a challenge in coming years. Consistent ranks within the taxonomy at the level of subclass, superorder, order and suborder have already been abandoned as unworkable. Ongoing revisions of the higher taxonomic levels are to be expected in the near future.
Convergent evolution, which appears to exist at especially high frequency in the Gastropoda class, may account for the observed differences between the older phylogenies which were based on morphological data, and more recent gene-sequencing studies.
Bouchet & Rocroi (2005) made sweeping changes in the systematics, resulting in a taxonomy that is a step closer to the evolutionary history of the phylum.
The Bouchet & Rocroi classification system is based partly on the older systems of classification, and partly on new cladistic research. In the past, the taxonomy of gastropods was largely based on phenetic morphological characters of the taxa. The recent advances are more based on molecular characters from DNA and RNA research. This has made the taxonomical ranks and their hierarchy controversial. The debate about these issues is not likely to end soon.
In the Bouchet, Rocroi et al. taxonomy, the authors have used unranked clades for taxa above the rank of superfamily (replacing the ranks suborder, order, superorder and subclass), while using the traditional Linnaean approach for all taxa below the rank of superfamily. Whenever monophyly has not been tested, or is known to be paraphyletic or polyphyletic, the term "group" or "informal group" has been used. The classification of families into subfamilies is often not well resolved, and should be regarded as the best possible hypothesis.
In 2004, Brian Simison and David R. Lindberg showed possible diphyletic origins of the Gastropoda based on mitochondrial gene order and amino acid sequence analyses of complete genes.
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