Species

A species is a taxonomic unit grouping organisms by shared characteristics and evolutionary relationships. Its boundaries depend on the evidence and species concept used.

Population: A group of organisms of the same species living in a defined area and interacting. Populations are the units whose variation and connections help determine species boundaries.
Reproductive isolation: Barriers that reduce or prevent gene flow between populations. It can keep diverging populations distinct even when they live near one another.
Evolutionary species concept: A species concept defining a species as a lineage with its own evolutionary tendencies and historical fate. It emphasizes lineage continuity rather than interbreeding alone.
Eastern and western meadowlarks: Two North American bird species with similar appearance but distinct songs and limited hybridization. Their narrow hybrid zone tests how much interbreeding a species boundary can tolerate.
Biological species concept: A species concept defining species as groups that interbreed and are reproductively isolated from other such groups. It makes reproductive compatibility the central test for grouping organisms.
Gene flow: The transfer of genetic variants between populations through migration and reproduction. Its reduction allows populations to accumulate differences that can mark species boundaries.
Ecological species concept: A species concept that distinguishes species by their occupation of distinct ecological niches. It can separate populations by ecological role even when reproductive evidence is unclear.
Ensatina eschscholtzii: A salamander complex whose populations form a ring around California's Central Valley. Its connected populations illustrate gradual divergence and terminal reproductive isolation.
Phylogenetic species concept: A species concept that identifies the smallest diagnosable lineage sharing a common ancestor. It bases species recognition on ancestry and distinguishing traits.
Genetic drift: Random changes in allele frequencies, especially influential in small populations. It can drive divergence between isolated populations without adaptive differences.

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