In bacteriology, a taxon in disguise is a species, genus or higher unit of biological classification whose evolutionary history reveals has evolved from another unit of a similar or lower rank, making the parent unit paraphyletic. 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 group, which is left as an evolutionary grade.
While the term is from bacteriology, parallel examples are found throughout the tree of life. For example, four-footed animals have evolved from piscine ancestors but since they are not generally considered 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 pathogens, it is generally avoided for some groups.
Examples.
"Shigella".
The bacterial genus "Shigella" is the cause of bacillary dysentery, a potentially-severe infection that kills over a million people every year. The genus ("S. dysenteriae", "S. flexneri", "S. boydii", "S. sonnei") have evolved from the common intestinal bacterium "Escherichia coli", which renders that species paraphyletic. "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 ("B. anthracis", "B. cereus", "B . thuringiensis", "B. mycoides", "B. pseudomycoides", "B. weihenstephanensis" and "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.
References.
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