If not, they are called races, which means that a formal rank should not be given to the group, or taxonomists are unsure whether or not a formal rank should be given.
The key lime is a shrub that grows to a size of about 5 metres in height. It has many thorns. It produces a fruit that is yellow when it is ripe. This fruit is preferred by bartenders to mix cocktails. They prefer this lime, rather than the Persian lime.
The lime plant originally came from southeast Asia, where it is native. It was taken to the Middle East, and Crusaders took it to Europe and North Africa. Spanish explorers took it to the West Indies and the Florida Keys. In 1926, a hurricane destroyed most of the commercially-grown limes in the region. The Persian lime was reintroduced then.
Some of the original shrubs grew wild in the Florida Keys. It became clear that the originally introduced shrubs (now known as Mexican limes) had modified their fruits. These were darker green than the original Persian limes, they also had a thicker skin.
In biological taxonomy, race is an informal rank below the level of subspecies. It may be used as a higher rank than "strain". There are various definitions. Races may be genetically distinct populations in the same species, or they may be defined in other ways, e.g. geographically, or physiologically. Genetic isolation between races is not complete (some interbreeding takes place between the groups). However, the genetic differences are not (yet) enough to put the groups into separate species. The term race is recognized by some, but is not governed by any of the formal codes of biological classification.
In former times, human beings were often divided into races. For example, people with a dark skin were called "Negroid" or "black race". But, human gene sequences are very similar compared to many other animals. This is one reason why modern biology says that there is only one human race.:360
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- Ernst Mayr (1970). Populations, species, and evolution : an abridgment of Animal species and evolution. Cambridge, Mass: Belknap Press. ISBN 0-674-69013-3.
- Mayr, Ernst 2002. The biology of race and the concept of equality. Daedalus, Winter 2002, 89-94. 
- Morton J. 1987. Mexican lime, p.168–172. In: Fruits of warm climates. Julia F. Morton, Miami, FL. Citrus aurantiifolia Swingle
- Gotoh, T.; Bruin, J.; Sabelis, M. W.; Menken, S. B. J. (1993). "Host race formation in Tetranychus urticae: Genetic differentiation, host plant preference, and mate choice in a tomato and a cucumber strain". Entomologia Experimentalis et Applicata (Submitted manuscript). 68 (2): 171–178. doi:10.1111/j.1570-7458.1993.tb01700.x.
- Ritchie, D. F.; Dittapongpitch, V. (1991), "Copper- and Streptomycin-resistant Strains and Host Differentiated Races of Xanthomonas campestris pv. vesicatoria in North Carolina" (PDF), Plant Disease, 75 (7): 733–736, doi:10.1094/pd-75-0733
- Morris, Christopher, ed. (1992). "Race". Academic Press Dictionary of Science and Technology. San Diego / London: Academic Press (Harcourt Brace Jovanovich). "Biology" entry, p. 1777. ISBN 978-0-12-200400-1.
an interbreeding subgroup of a species whose individuals are geographically, physiologically, or chromosomally distinct from other members of the species
- Jaenike, J. (1981), "Criteria for ascertaining the existence of host races", The American Naturalist, 117 (5): 830–834, doi:10.1086/283771, JSTOR 2460772
- "The use of racial, ethnic, and ancestral categories in human genetics research". Am. J. Hum. Genet. 77 (4): 519–32. Oct 2005. doi:10.1086/491747. PMC 1275602. PMID 16175499.
- Bamshad M, Wooding S, Salisbury BA, Stephens JC (August 2004). "Deconstructing the relationship between genetics and race". Nat. Rev. Genet. 5 (8): 598–609. doi:10.1038/nrg1401. PMID 15266342.CS1 maint: multiple names: authors list (link)
- Tishkoff SA, Kidd KK (November 2004). "Implications of biogeography of human populations for 'race' and medicine". Nat. Genet. 36 (11 Suppl): S21–7. doi:10.1038/ng1438. PMID 15507999.
- Jorde LB, Wooding SP (Nov 2004). "Genetic variation, classification and 'race'". Nat. Genet. 36 (11 Suppl): S28–33. doi:10.1038/ng1435. PMID 15508000.
- American Association of Physical Anthropologists (27 March 2019). "AAPA Statement on Race and Racism". American Association of Physical Anthropologists. Retrieved 19 June 2020.
- Templeton, A. (2016). EVOLUTION AND NOTIONS OF HUMAN RACE. In Losos J. & Lenski R. (Eds.), How Evolution Shapes Our Lives: Essays on Biology and Society (pp. 346-361). Princeton; Oxford: Princeton University Press. doi:10.2307/j.ctv7h0s6j.26. That this view reflects the consenus among American anthropologists is stated in: Wagner, Jennifer K.; Yu, Joon-Ho; Ifekwunigwe, Jayne O.; Harrell, Tanya M.; Bamshad, Michael J.; Royal, Charmaine D. (February 2017). "Anthropologists' views on race, ancestry, and genetics". American Journal of Physical Anthropology. 162 (2): 318–327. doi:10.1002/ajpa.23120.