For all the sexually reproducing eukarya, the fundamental taxonomic unit is the species (s. and pl.). Biological species are populations of organisms between which reproduction is prevented biologically, by reproductive isolating barriers.
The birth of new species is speciation, during which one species splits into two reproductively distinct new species. The death of species is extinction. Speciation and extinction have produced, over the past two billion years, earth's diverse assemblage of extant species of eukarya, numbering ~107. The species of today are perhaps at most 1% of all species that have ever lived. Speciation is an inevitable by-product of genetic evolution. The exact genetic events that cause speciation are under intensive investigation.
Phenotypic or Typological Species Concept
In the phenotypic concept, members of one species may vary quantitatively in form but are clearly distinct from members of another species. Individuals within species are considered variants of the same type. The phenotype is usually morphological, but it can be molecular. The Linnaean system of classification and binomial nomenclature for species (e.g., Homo sapiens) is purely phenotypic.
The main difficulty of this concept is that there is no evidence for the reality of a “true type,” from which individuals deviate, even though it is commonplace for species to appear distinct.
Biological Species Concept
According to this concept, biological species are reproductively isolated from each other. This concept applies only to sexually reproducing eukarya.