A subspecies is a distinct group within a species that has consistent physical or genetic differences from other populations, but can still interbreed with them and produce fertile offspring.
A variety is a more minor, naturally occurring variant within a species or subspecies that shows consistent but usually smaller differences, such as color, leaf shape and fruit size, and often occurs within or overlapping the main range.
Subspecies-level divergence usually reflects relatively recent or incomplete steps along the speciation continuum: populations are distinct enough to be recognized but still capable of interbreeding where they meet. The longer isolation and divergent selection persist without being erased by gene flow, the more likely subspecies will become strongly differentiated and eventually cross the threshold into full species.
When a subspecies is formally recognized, its scientific name usually has three parts instead of the usual two: i.e., genus, species and subspecies. Not every species is divided into subspecies, but if subspecies are recognized there must be at least two defined within that species.
Subspecies are not distributed evenly among different groups of organisms: many species have no recognized subspecies, while others have several, or even many. Some widespread vertebrates and plants have on the order of a dozen or more named subspecies (for example, certain birds, large mammals and cultivated plants), whereas many microorganisms and narrowly distributed native species have few or none, either because they are poorly studied or show little geographic variation.
Subspecies often arise first through natural causes such as partial geographic isolation, where mountains, rivers, islands, or shifting climate patterns limit movement and mating between populations of the same species. Over many generations, local environmental pressures such as temperature, food availability, predators or parasites favor different traits in each area, while random genetic drift in small or scattered groups further increases their genetic and physical distinctiveness.
Human activity can promote the formation of subspecies by fragmenting habitats with roads, farms, cities and dams. These changes create separated subpopulations that experience reduced gene flow and distinct local conditions. Conversely, human actions such as habitat destruction or long-distance transport can reconnect previously isolated populations, increasing hybridization and gene flow that erases subspecies boundaries.
As a whole, human activity tends to result in a net decrease in subspecies and in overall biodiversity. While humans can cause range expansions in some widespread species and promote divergence in a few isolated populations, the dominant effect is the reduction and fragmentation of habitats, which disproportionately harms narrow-ranged and specialized subspecies. This leads to decreased genetic diversity, local extinctions and biotic homogenization (i.e., a few common species spread and take over, replacing the many unique and vulnerable species that formerly lived there).
Subspecies designations play an important role in conservation efforts because they help identify and protect distinct populations that contribute to a species' genetic and ecological diversity. Recognizing subspecies allows conservationists to prioritize management actions for unique local lineages that may be at greater risk from habitat loss, climate change or genetic mixing with other populations. Protecting these distinct forms preserves the evolutionary potential of species by maintaining variations that could facilitate adaptation to future climatic or other environmental changes.