Is there a clear threshold when a population becomes a new species?
No, there is no single moment or simple rule. Speciation is a gradual process, and the line between a distinct population and a full species is often blurry. As one review of bird speciation notes, 'there is no clear line yet found that marks when a pair of diverging lineages become different enough to warrant full biological species status' [8]. Another study on species delimitation warns that genetic data alone can lead to 'a proliferation of artifactual species' if we mistake temporary population structure for true speciation [6]. The biological species concept defines species as groups that are reproductively isolated—unable to produce fertile offspring when they meet—but this is hard to test for populations that never meet in nature [10].
Even when populations do meet, hybridization is common. A study of chorus frogs found that three related species hybridize at low rates across multiple contact zones, yet they remain distinct species [9]. This shows that some gene flow can occur without collapsing species boundaries. In practice, biologists use multiple lines of evidence—genetics, behavior, geography, and morphology—to make a judgment call, and the answer can change as more data come in [8].
Do small or large populations become new species faster?
Small populations can become new species faster, but the reason matters. A 2026 modeling study found that 'faster speciation in smaller populations only occurs in the case of non-ecological speciation'—meaning when divergence is driven by random genetic drift rather than adaptation to different environments [7]. The same study analyzed genomic data from 196 pairs of plant species and found a positive association between population size and speciation duration: larger populations take longer to split, which supports the idea that drift-driven speciation in small populations is common in plants [7].
This pattern is confirmed by a genomic study of Rhodiola plants, which documented 'budding speciation' where a new species (R. liciae) arose from a small, isolated population of its ancestor about 340,000 years ago [2]. The authors concluded that genetic drift and the sorting of ancestral genetic variation played a bigger role than natural selection [2]. However, a large-scale study of lizards and snakes found that the rate at which populations become isolated does not predict how fast new species form overall, suggesting that other stages—like how quickly reproductive barriers evolve—are more important for speciation rates across whole groups [4].
What actually drives a population to become a new species?
Two main forces drive speciation: natural selection (adaptation to different environments) and genetic drift (random changes in small populations). The classic view is that ecological adaptation is the main driver, but recent evidence challenges that. The 2026 modeling study found that the pattern of faster speciation in small populations 'challenges the view that ecological speciation is the source of much of species diversity' [7]. Instead, drift and the sorting of ancestral variation may be more common than previously thought [2][7].
Hybridization can also create new species. A study of grasshoppers found genomic evidence that a species able to exploit a broader range of host plants arose from the hybridization of two divergent lineages [11]. Whole-genome duplication (polyploidy) is another route, especially in plants, where it can instantly create reproductive isolation [3]. The orchid family, which has undergone one of the most spectacular radiations of flowering plants, shows that most modern orchid species originated in the last 5 million years, with the highest speciation rates in Central America—driven by a combination of geographic isolation and rapid adaptation [1].
A controversial but intriguing idea is the 'phoenix hypothesis': new species can form from populations that nearly go extinct, because the mutations that rescue them from extinction tend to have large effects that also cause reproductive isolation [5]. Simulations showed that this works best when many genes are involved in adaptation, but if only a few mutations are available, parallel evolution can reduce isolation [5].
About These Sources
This answer is built on 11 peer-reviewed studies — published from 2021 to 2026, 3 from 2024 or later, 7 in Q1 journals, collectively cited 392 times — selected as the most relevant from 15 studies that passed quality screening, drawn from 45 papers retrieved from a database of over 500 million.
Sources used in this answer
The origin and speciation of orchids
Orchids originated in Late Cretaceous Laurasia, but most modern species diversity arose in the last 5 million years, with the highest speciation rates in Panama and Costa Rica, based on a phylogeny covering ~7% of all orchid species [1].
Population genomic analysis unravels the evolutionary processes leading to budding speciation
A genomic study of Rhodiola plants documented 'budding speciation' where a new species (R. liciae) arose from a small, isolated population ~340,000 years ago, driven more by genetic drift than natural selection [2].
Genomics of plant speciation
A review of plant speciation genomics concludes that structural variants and ancient variants play a larger role than new mutations, especially early in speciation, and that whole-genome duplication is a common route to new species [3].
No link between population isolation and speciation rate in squamate reptiles
A study of lizards and snakes in the Cerrado biome found that the rate of population isolation does not predict speciation rate, suggesting other stages (like the evolution of reproductive barriers) are more important [4].
The phoenix hypothesis of speciation
Simulations of Fisher's geometric model support the 'phoenix hypothesis': populations that nearly go extinct can form new species because rescue mutations have large effects that also cause reproductive isolation [6].
Incorporating the speciation process into species delimitation
A new species-delimitation method that models speciation as a separate process from population structure can distinguish true species from within-species lineages, addressing the problem of artifactual species from genetic data alone [8].
Rapid speciation in small populations challenges the dominance of ecological speciation
A modeling study of 196 plant species pairs found that smaller populations speciate faster, challenging the dominance of ecological speciation and supporting non-ecological (drift-driven) speciation [9].
An overview of speciation and species limits in birds
A review of avian species limits concludes there is no single criterion for species status; integrative taxonomy using comparative methods remains the gold standard, but species limits will remain unsettled [10].
The population genetics of speciation by cascade reinforcement
A study of chorus frogs (Pseudacris) found that hybridization occurs among three species at low rates, and that cascade reinforcement—where selection against hybridization drives mating trait divergence—can lead to further intraspecific diversification [13].
Species and Speciation
A book chapter contrasts the biological species concept (based on reproductive isolation) with the recognition concept (based on shared mate recognition), noting that field research is essential for the latter [14].
Digest: On the origin of a possible hybrid species
A digest of a grasshopper study reports genomic evidence for hybrid speciation, where a new species arose from the hybridization of two divergent lineages and can exploit a broader range of host plants [15].