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1.6The Species Concept

When an organism related to our lives is recognized, we conventionally name it for the purpose of transmission of information. Species is a fundamental unit for recognizing and classifying organisms (see Column at the bottom). A species is a collection of individuals with similar characteristics; however, a uniform definition is difficult because of the presence of various organisms.

Although organisms can be classified in many ways considering species criteria, the most widely used is the biological species concept. This species concept was defined by Ernst Mayer as a group of organisms capable of interbreeding and producing fertile offspring (reproductively isolated from other groups). Here reproductive isolation has been selected as species criteria with two groups being termed different species if they do not produce fertile offspring, even if their phenotypes are similar. Conversely, two groups are considered to be of the same species if they can produce fertile offspring, even if they differ in appearance. For example, donkeys and horses can mate to produce hybrid offspring, but the hybrid offspring of a male donkey and a female horse that is called a mule is infertile. Thus, donkeys and horses have been determined to belong to different species. On the other hand, although there are many breeds of dogs varying in appearance and size, they all belong to the species of dog because they can interbreed and produce fertile offspring.

Biological species and traditionally understood species, recognized by similarities and differences in form, agree in many cases. However, although the biological species concept is an excellent species concept that emphasizes genetic relationships between organisms, it is not applicable to organisms that reproduce asexually or those known only from fossils. In addition, practical evaluation of the breeding potential between two groups is highly labor intensive. Thus, the morphological species concept, discerning by appearance and internal structures, has also been widely used as evaluation criteria for species. Other species concepts used include those that use the evolutionary path of descent and ecological characteristics as criteria.

The biological species concept is not applicable to prokaryotic bacterial species because they do not reproduce sexually. Since they generally have a simple structure, understanding the diversity of bacterial species is difficult. Thus, in case of bacteria, their structure, biochemical properties, and differences in DNA sequences are used as species criteria.

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Nomenclature of Organisms

Although organisms have Japanese names, the Latin binomial nomenclature is used in academic literature. This nomenclature was popularized by Carolus Linnaeus, the father of taxonomy, and scholars throughout the world use this naming convention to ensure uniformity. The binomial nomenclature is a combination of a noun indicating the genus name, which is the species group, and an adjective that explains the noun [the present participle of a verb or possessive case of a noun (formally the genitive case)], indicating the specific species of the organism. For example, humans are Homo sapiens. Latin names are generally italicized. Homo is a masculine noun indicating the human genus (In Italian, a descendant of Latin, it is “uomo”). Sapiens is the masculine nominative singular form of the present participle of the verb “sapere” (the same in Italian), which means “to know.” Similar to French and Italian, Latin modifiers follow nouns and therefore the notation is written in this manner. The ending of the adjective changes in case of feminine or gender-neutral nouns. There is no change in the genitive; for example, Pisum sativum (the pea plant: sativum is the gender-neutral form of the adjective that means “cultivated”). Another example is Escherichia coli (E. coli: coli means “of the colon”). Knowing these rules may make remembering these names a little easier.

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