Speciation Explained: How New Species Form

What Is a Species

Before understanding how new species form, it is important to define what a species is. The biological species concept, proposed by Ernst Mayr, defines a species as a group of organisms that can interbreed and produce fertile offspring under natural conditions. This definition works well for most sexually reproducing animals and plants, but it has limitations. It cannot be applied to asexual organisms, fossils, or populations that never encounter each other in nature.

Other species concepts address these limitations. The morphological species concept defines species by physical characteristics. The phylogenetic species concept defines species as the smallest group of organisms that share a common ancestor and can be distinguished from other such groups. In practice, biologists often use multiple criteria to identify and define species, recognizing that no single definition works perfectly in all cases.

Allopatric Speciation

Allopatric speciation is the most common and best-understood mode of species formation. It occurs when a physical barrier divides a population into two or more geographically isolated groups. Once separated, the populations evolve independently through natural selection, genetic drift, and mutation. If they accumulate enough genetic and phenotypic differences, they become unable to interbreed successfully even if the barrier is later removed.

Geographic barriers that drive allopatric speciation include mountain ranges, rivers, oceans, glaciers, and deserts. The formation of the Isthmus of Panama approximately three million years ago separated marine populations in the Atlantic and Pacific oceans, leading to the formation of numerous paired species on either side. The uplift of mountain ranges in East Africa created isolated habitats that contributed to the diversification of many plant and animal species.

A special case of allopatric speciation is peripatric speciation, which occurs when a small group of individuals colonizes a new habitat at the edge of a species range. Because the founding population is small, genetic drift plays a larger role than in typical allopatric speciation, potentially leading to rapid genetic divergence. The diverse species of honeycreepers in Hawaii likely arose through peripatric speciation as small groups of ancestral birds colonized different islands.

Sympatric Speciation

Sympatric speciation occurs without geographic isolation, when new species arise from populations living in the same area. This mode of speciation was long considered rare or even impossible, but growing evidence suggests it may be more common than previously thought.

In plants, polyploidy is a well-documented mechanism of sympatric speciation. Polyploidy occurs when an error in cell division produces an organism with extra sets of chromosomes. A polyploid individual is often reproductively isolated from the parent species because crosses produce offspring with uneven chromosome numbers that are usually sterile. Many important crop plants, including wheat, cotton, and strawberries, are polyploids that arose through this mechanism.

Ecological speciation can also occur in sympatry. When subpopulations within the same area specialize on different resources or habitats, they may gradually become reproductively isolated. The apple maggot fly in North America provides a compelling example. Originally a parasite of hawthorn berries, a subpopulation shifted to feeding on domesticated apples after European colonization. The apple-feeding and hawthorn-feeding populations now mate at different times of year and show measurable genetic divergence, suggesting that speciation is in progress.

Reproductive Isolation

Reproductive isolation is the defining criterion for speciation under the biological species concept. Reproductive barriers are classified as either prezygotic (preventing mating or fertilization) or postzygotic (reducing the fitness of hybrid offspring).

Prezygotic barriers include habitat isolation (species occupy different habitats within the same area), temporal isolation (species breed at different times), behavioral isolation (species have different courtship behaviors or mating signals), mechanical isolation (reproductive structures are physically incompatible), and gametic isolation (eggs and sperm are chemically incompatible). These barriers prevent the formation of hybrid zygotes in the first place.

Postzygotic barriers include hybrid inviability (hybrid embryos fail to develop properly), hybrid sterility (hybrids are viable but cannot reproduce, as in mules), and hybrid breakdown (first-generation hybrids are fertile but their offspring have reduced fitness). These barriers reduce the effectiveness of interbreeding even when it occurs, maintaining species boundaries over evolutionary time.

Ring Species and Speciation Continua

Ring species provide a fascinating window into the speciation process. In a ring species, a series of neighboring populations can each interbreed with adjacent populations, but the populations at the two ends of the ring cannot interbreed with each other. The Ensatina salamanders of California form a ring around the Central Valley, with neighboring populations interbreeding freely but the terminal populations in southern California behaving as distinct species.

Ring species illustrate that speciation is not an instantaneous event but a gradual process. At any given time, populations exist at various stages along a continuum from full interbreeding to complete reproductive isolation. This continuum challenges the idea that species are discrete, clearly bounded entities and highlights the dynamic nature of the speciation process.

Rates of Speciation

The time required for speciation varies enormously depending on the organisms involved, the strength of selection, the degree of geographic isolation, and other factors. In some cases, such as polyploid speciation in plants, new species can arise in a single generation. In other cases, populations may remain partially isolated for millions of years before completing the speciation process.

Studies of known speciation events suggest that most animal species take between 100,000 and several million years to form through allopatric speciation. However, rapid ecological speciation has been documented in some groups, including cichlid fish in African lakes, where hundreds of species have evolved within the last few hundred thousand years. The rate of speciation depends on the rate of environmental change, the strength of reproductive barriers, and the population sizes involved.

Adaptive radiation is a pattern in which a single ancestral species rapidly diversifies into many new species, each adapted to a different ecological niche. Classic examples include Darwin finches on the Galapagos Islands, Hawaiian honeycreepers, and the cichlid fish of the African Great Lakes. Adaptive radiations typically occur when a lineage colonizes a new environment with many available ecological opportunities, such as an island archipelago or a newly formed lake.