A species is, roughly, a group of organisms that can interbreed and produce fertile offspring. This is the biological species concept, proposed by Ernst Mayr in 1942, and it works well for many sexually reproducing animals. It fails for asexual organisms (most prokaryotes), for organisms that hybridize freely (many plants, some animals), and for extinct organisms known only from fossils.
Because no single definition works across all cases, biologists use several species concepts depending on context. The morphological species concept groups organisms by physical similarity. The phylogenetic species concept defines species as the smallest monophyletic group — the smallest set of organisms sharing a common ancestor to the exclusion of others. The ecological species concept defines species by the niche they occupy. Each captures something real; none is universal.
Speciation — the formation of new species — occurs when populations become reproductively isolated and diverge genetically over time. Isolation can be geographic (allopatric speciation), ecological (different niches within the same area), or behavioral. The process is usually gradual, meaning that the boundary between “one species diverging” and “two species existing” is often blurry in real time, even if the endpoint is clear in retrospect.
Species are the basic units of biological classification (taxonomy). Linnaeus’s hierarchical system groups species into genera, genera into families, families into orders, and so on up to kingdoms and domains. This system reflects evolutionary relationships: organisms grouped together share a more recent common ancestor than organisms in different groups. Modern taxonomy is increasingly based on molecular phylogenetics — comparing DNA sequences to reconstruct evolutionary trees.
Related terms
- Organism — the individual members of a species
- Natural selection — the primary mechanism driving species divergence
- Gene — the molecular basis of the heritable variation on which speciation depends
- Symbiosis — cross-species relationships that can shape speciation