A species is most commonly defined as a group of organisms that can interbreed with each other and produce fertile offspring, but cannot successfully do so with members of other groups. This definition, known as the biological species concept, has been the standard in biology for decades. The reality, though, is messier than that single sentence suggests. Scientists use at least four major frameworks to decide where one species ends and another begins, and no single definition works perfectly for every organism on Earth.
The Biological Species Concept
The most widely taught definition centers on reproduction. If two populations can mate and produce offspring that are themselves fertile, those populations belong to the same species. If they can’t, or if the offspring are infertile, they’re separate species. The mule is the classic example: horses and donkeys can mate, but the resulting mule is sterile, which keeps horses and donkeys classified as distinct species.
This framework is intuitive and powerful, but it has obvious limits. It can’t apply to organisms that reproduce asexually, like bacteria. It’s useless for fossils, since you can’t test whether two extinct animals could have interbred. And it struggles with populations that are geographically separated and never encounter each other in the wild. For those cases, biologists turn to other definitions.
Other Ways to Define a Species
The morphological species concept is the oldest approach: a species is the smallest group of organisms that can be reliably distinguished by their physical features. This works for fossils, asexual organisms, and any situation where all you have is a body to examine. The drawback is subjectivity. Two scientists can disagree on whether physical differences are “enough” to justify a separate species name.
The ecological species concept focuses on the role an organism plays in its environment. Under this view, a species is a group that occupies its own distinct ecological niche, competing more with its own kind than with members of other groups. Two populations of fish in the same lake that feed at different depths and eat different prey could qualify as separate species even if they look almost identical.
The phylogenetic species concept defines a species as the smallest group of organisms that all share a common ancestor and can be distinguished by unique inherited traits. This approach leans heavily on evolutionary family trees and is especially popular among zoologists. It applies to both sexually and asexually reproducing organisms and doesn’t require testing whether two populations can interbreed.
No single concept has won out. Which one a scientist uses often depends on the organisms they’re studying and the tools available. In practice, researchers frequently combine two or more frameworks to reach a classification.
What Keeps Species Apart
For sexually reproducing organisms, species stay separate because of reproductive barriers. These fall into two broad categories: those that prevent mating from happening in the first place, and those that make the results of mating unsuccessful.
Barriers that prevent mating include:
- Timing: Two closely related frog species might live in the same pond but breed in different seasons, so they never get the chance to mate.
- Behavior: Bird species often have unique songs or courtship dances. A female simply won’t respond to the wrong song, no matter how closely related the singer is.
- Physical incompatibility: Some insect species have differently shaped reproductive organs, making mating physically impossible.
Even when mating does occur, the offspring may not survive. Crosses between certain frog species produce embryos that fail to develop or larvae that die before reaching adulthood. And when hybrids do survive, they may be sterile, like the mule. Both outcomes keep the parent species genetically separate over time.
How New Species Form
New species arise when populations of the same species stop interbreeding long enough to become permanently different. The most common path is geographic separation. A mountain range rises, a river changes course, or a population colonizes an island. Cut off from each other, the two groups accumulate different mutations, adapt to different environments, and eventually can no longer interbreed even if they come back into contact. This is called allopatric speciation.
Sympatric speciation is rarer and more controversial. Here, a new species emerges within the same geographic area, with no physical barrier separating the populations. This requires strong natural selection pushing different groups toward different lifestyles or food sources. Over time, those groups may begin to prefer mating with their own type, and the split deepens into full reproductive isolation. Allopatric and sympatric speciation aren’t really separate categories so much as two ends of a spectrum, ranging from zero gene flow between populations to substantial gene flow that must be overcome by strong selective pressure.
DNA as a Species Identifier
Modern genetics has transformed how scientists identify species. A technique called DNA barcoding uses a short, standardized stretch of genetic code to tell species apart. In animals, this typically involves a gene found in mitochondria (the energy-producing structures inside cells). By comparing the sequence of this gene across specimens, researchers can detect species that look nearly identical to the naked eye.
In a study of fly species relevant to forensic science, genetic differences between species ranged from 4.7% to 19.8% in this barcoding gene, making identification straightforward even when physical features were ambiguous. This kind of molecular tool has been especially valuable for discovering “cryptic species,” organisms that were lumped together as one species for decades until DNA analysis revealed they were actually two or more distinct groups.
For bacteria, which don’t reproduce sexually, species boundaries are drawn using overall genetic similarity. A common benchmark compares the average similarity across entire genomes. Two bacterial strains that share roughly 95 to 96% of their genome-wide sequence identity are generally considered the same species, though exceptions exist when ecological differences are dramatic. One pair of bacterial species shares 98.7% genetic similarity but is classified separately because one underwent massive genome reduction after shifting to an exclusively disease-causing lifestyle.
Where the Boundaries Blur
Nature doesn’t always respect clean categories. Hybridization, where members of different species successfully interbreed, is far more common than most people realize. At least 25% of vascular plant species and about 10% of animal species are known to hybridize in nature, and the true numbers are likely higher because many hybrid offspring look so similar to one parent species that they go undetected.
Darwin’s finches on the Galápagos illustrate how environmental disruption can redraw species lines. Before a major El Niño event, hybrids between three finch species were rare and didn’t reproduce successfully. After the climate shifted, certain hybrid and backcross finches actually survived, bred, and recruited into the population at higher rates than the parent species. The species boundary, in other words, behaved more like a continuum than a wall.
Ring species pose another challenge. In this pattern, a species expands its range around a geographic barrier like a mountain range or valley. Neighboring populations along the ring can interbreed with each other, but when the two ends of the ring eventually meet on the other side of the barrier, those endpoint populations can no longer interbreed. They behave like separate species even though they’re connected by a chain of interbreeding populations. This makes it impossible to draw a clean line between “same species” and “different species.”
How Many Species Exist
The most widely cited estimate puts the total number of living species on Earth at roughly 8.75 million. That includes about 7.8 million animal species, 298,000 plants, 611,000 fungi, and 64,000 single-celled organisms with complex cells. Of those 8.75 million, only about 1.2 million have been formally described and named, including roughly 950,000 animal species. That means approximately 80% of Earth’s species are still hypothetical, predicted by statistical models but not yet identified by scientists. Every year, thousands of new species are cataloged, many of them discovered not by exploring remote jungles but by reexamining specimens in museum collections with DNA tools that reveal hidden diversity.