Species

A species is the fundamental category used to classify living things and measure biodiversity. It is most commonly defined as the largest group of organisms where any two individuals of the right sexes or mating types can produce fertile offspring, usually through sexual reproduction. Species can also be identified by other traits, such as their chromosome structure, DNA sequence, physical form, behavior, or the ecological role they play. For paleontologists, who cannot observe reproduction in fossils, the idea of a chronospecies—a lineage that changes enough over time to be considered a new species—is used instead.

The most recent careful estimate suggests there are between 8 and 8.7 million species of eukaryotes, and by 2011, only about 14% of these had been described. Every species gets a two-part binomial name: the first part is the genus it belongs to, and the second part is its specific name or epithet. For instance, Boa constrictor is a species in the genus Boa, with constrictor as the specific name.

These definitions, while seemingly straightforward, become problematic on closer inspection. Boundaries between closely related species can blur due to hybridization, in species complexes with hundreds of similar microspecies, or in ring species. For organisms that reproduce only asexually, the reproductive definition breaks down, and each separate clonal line can be considered a microspecies.

None of these concepts are perfect, and the species idea is not a flawless model of life, but it remains a useful tool for scientists and conservationists. If species were fixed and clearly distinct, there would be no difficulty, but evolution constantly changes them. This forces taxonomists to decide, for example, when a fossil lineage has changed enough to be split into multiple chronospecies, or when populations have diverged enough in key traits to be described as cladistic species.

From Aristotle until the 18th century, species and higher groups were seen as fixed categories in a hierarchy called the great chain of being. In the 19th century, biologists realized that species could evolve over time. Charles Darwin’s 1859 book On the Origin of Species explained how natural selection could produce new species. This understanding deepened in the 20th century with genetics and population ecology.