January 23, 2022
Species - in biology, the basic taxonomic unit, one of the systematic categories, and the unit of biological diversity. Establishing an exhaustive definition of species has proved difficult. Such attempts are still being made as it is a key concept for the research practice of scientists and the activities of the conservation movement. Carl Linnaeus (and many others after him) assumed that species are something constant, but it is now known that evolutionary processes cause their transformation and gradual transition from one species to another. Often a species is defined as the largest group of organisms out of which two individuals are able to reproduce (usually sexually), producing fertile offspring. This definition is generally accurate, but wrong in many specific cases. For example, it is not applicable to hybrids, in species complexes composed of hundreds of similar micrographs and ring species. In their case, the line between closely related species becomes blurred. It is also not applicable to asexual reproduction, or to fossil organisms that cannot be tested for reproduction. Thus, in palaeontology, the concept of chronogenesis is used. Other ways of defining a species may be through karyotyping, DNA sequence, morphology, behavior, or ecological niche studies. Each species is given a two-part (binomial) name. The first part identifies the genus it belongs to, and the second is known as a species epithet or species name. In English, there is a distinction between a specific name and a specific epithet (usually used in botanical and sometimes zoological nomenclature). For example, Boa constrictor is one of the species of the genus Boa. From the time of Aristotle to the eighteenth century, the species was seen as something permanent, with a designated place in the hierarchical great chain of being. In the 19th century, biologists realized that species could evolve over time. Charles Darwin's The Origin of Species (1859) explains how new species arise through natural selection. Understanding of this process was deepened in the twentieth thanks to genetics and population ecology. Genetic variation occurs as a result of mutation and recombination, while the organisms themselves move, leading to geographic isolation and genetic drift with varying selection pressures. Genes are sometimes exchanged between species through horizontal gene transfer. New species may emerge through hybridization and polyploidy. Species are also dying out for a variety of reasons. Viruses are a special case, as they are shaped by a mutation-selection equilibrium that makes them quasi-species. In practice, various species definitions are used to measure biodiversity, but the accuracy of these measurements is debatable.