Taxa in disguise
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In
bacteriology Bacteriology is the branch and specialty of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
, a taxon in disguise is a
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
,
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
or higher unit of
biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...
whose evolutionary history reveals has evolved from another unit of a similar or lower rank, making the parent unit
paraphyletic In taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only ...
. That happens when rapid evolution makes a new species appear so radically different from the ancestral group that it is not (initially) recognised as belonging to the parent
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

phylogenetic
group, which is left as an
evolutionary grade In , a grade is a taxon united by a level of morphological or physiological Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of s ...
. While the term is from bacteriology, parallel examples are found throughout the
tree of life #REDIRECT Tree of life#REDIRECT Tree of life The tree of life is a fundamental widespread mytheme or archetype in many of the world's mythology, mythologies, religion, religious and philosophy, philosophical traditions. It is closely related ...
. For example,
four-footed animals
four-footed animals
have evolved from piscine ancestors but since they are not generally considered
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
, they can be said to be "fish in disguise". In many cases, the paraphyly can be resolved by reclassifying the taxon in question under the parent group. However, in bacteriology, since renaming groups may have serious consequences since by causing confusion over the identity of
pathogen In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
s, it is generally avoided for some groups.


Examples


''Shigella''

The bacterial genus ''
Shigella ''Shigella'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circum ...
'' is the cause of shigellosis, bacillary dysentery, a potentially-severe infection that kills over a million people every year. The genus (''Shigella dysenteriae, S. dysenteriae'', ''Shigella flexneri, S. flexneri'', ''Shigella boydii, S. boydii'', ''Shigella sonnei, S. sonnei'') have evolved from the common intestinal bacterium ''Escherichia coli'', which renders that species parephyletic. ''E. coli'' itself can also cause serious dysentery, but differences in genetic makeup between ''E. coli'' and ''Shigella'' cause different medical conditions and symptoms. ''Escherichia coli'' is a badly-classified species since some strains share only 20% of their genome. It is so diverse that it should be given a higher taxonomic ranking. However, medical conditions associated with ''E. coli'' itself and ''Shigella'' make the current classification not to be changed to avoid confusion in medical context. ''Shigella'' will thus remain "''E. coli'' in disguise".


''B. cereus''-group

Similarly, the ''Bacillus'' species of the ''B. cereus''-group (''Bacillus anthracis, B. anthracis'', ''Bacillus cereus, B. cereus'', ''Bacillus thuringiensis, B . thuringiensis'', ''Bacillus mycoides, B. mycoides'', ''Bacillus pseudomycoides, B. pseudomycoides'', ''Bacillus weihenstephanensis, B. weihenstephanensis'' and ''Bacillus medusa, B. medusa'') have 99-100% similar 16S rRNA sequence (97% is a commonly-cited adequate species limit) and should be considered a single species. Some members of the group appear to have arisen from other ''Bacillus'' strains by acquiring a protein coding plasmid and so the group may thus be polyphyletic. For medical reasons, such as anthrax, the current arrangement of separate species has remained intact.


Large genera of microbes

* The bacterial genus ''Pseudomonas'' has enlarged through several generations of taxonomic methods, bringing the species count to alarming proportions, with around 800 species recognized by the mid-20th century. The nitrogen-fixing bacteria of the genus ''Azotobacter'' and the species ''Azomonas macrocytogenes'' have evolved from a species in the genus ''Pseudomonas''. Its nitrogen-fixing capabilities and deviant features have caused ''Azotobacter'' to be described as "''Pseudomonas'' in disguise". * The genus ''Bacillus'' was described early in the history of microbiology and so is a large genus that is very genetically diverse, 266 species. The genera ''Paenibacillus'' and ''Brevibacillus'' are clades that are nested within ''Bacillus''. Since ''Bacillus'' is highly medically relevant and ''Paenibacillus'' is a model organism that is used in research, renaming them to reflect phylogeny would result in confusion.


See also

* Monophyly * Paraphyly * Polyphyly * Species problem * Evolutionary grade * Cryptic species complex * Synonym (taxonomy) * Taxonomy (biology), Taxonomy * LPSN, a list of accepted bacterial and archaeal names * Bacterial phyla, a complex classification scheme * Cyanobacteria, a phylum of common bacteria that remain poorly classified


References

{{reflist, 2 Biological classification Bacteriology Evolutionary biology