The Evolution of New Species Is Referred to as Speciation

Speciation is the evolutionary process by which populations diverge to form distinct new species. In short, the evolution of new species is referred to as speciation. A species is generally defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions. When gene flow between populations stops—whether through geographic separation, genetic mutations, or behavioral changes—those populations can follow separate evolutionary paths and eventually become reproductively isolated.

Speciation Defined

Speciation is the formation of one or more new species from a single ancestral lineage. It occurs when populations become reproductively isolated, meaning they can no longer successfully interbreed to produce fertile offspring. According to Encyclopaedia Britannica, speciation is the result of genetic divergence accumulated over generations, often accelerated by natural selection and environmental pressures.

Speciation matters because it explains biodiversity. Every distinct species on Earth—from microscopic bacteria to whales—originated through some form of speciation event. The process usually requires thousands to millions of years, though under strong selection pressures it can happen faster. It is not a single event but a gradual continuum.

The most widely used framework for identifying species is the biological species concept, which defines a species by reproductive compatibility. Two populations belong to the same species if they interbreed in nature and produce viable, fertile offspring. When that compatibility breaks down, speciation has occurred. Other species concepts exist—such as the ecological and phylogenetic species concepts—because the biological concept does not apply to asexual organisms.

Types of Speciation

Scientists classify speciation into four main types based on how geographic separation and gene flow interact.

Allopatric speciation occurs when a physical barrier—such as a mountain range, river, or ocean—separates a population into isolated groups. Over time, the separated populations accumulate different genetic changes and diverge. A classic example is the squirrel populations on opposite sides of the Grand Canyon. The Kaibab squirrel on the north rim and the Abert’s squirrel on the south rim are considered distinct species, likely diverged after the canyon formed. Allopatric speciation is thought to be the most common form of speciation among animals.

Sympatric speciation happens without geographic separation: new species arise within the same habitat. This is less common in animals but well-documented in plants through polyploidy—when an organism inherits extra sets of chromosomes. Polyploid plants often cannot breed with their diploid ancestors, creating instant reproductive isolation. Understanding Evolution (UC Berkeley) notes that polyploidy is a major driver of plant speciation.

Parapatric speciation occurs when populations are partially separated along an environmental gradient, with some gene flow continuing across a contact zone. Divergence is driven by different selective pressures on each side of the gradient. A well-studied example is the grass Anthoxanthum odoratum growing on contaminated mine soils in the UK. Heavy-metal tolerance evolved in the mine population, and flowering times shifted so much that cross-pollination with the adjacent uncontaminated population became rare.

Peripatric speciation is a special case of allopatric speciation in which a small peripheral population becomes isolated from the main population. Because the founding group is tiny, genetic drift and the founder effect strongly influence its evolution. The classic example is the radiation of Drosophila fruit flies on the Hawaiian Islands, where founder populations repeatedly colonized new islands and diverged into new species.

Mechanisms and Barriers

Reproductive isolating mechanisms are the biological barriers that prevent gene flow between populations and allow them to diverge into separate species. NCBI organizes these barriers into two broad categories: prezygotic and postzygotic.

Prezygotic barriers prevent mating or fertilization from happening at all. They include habitat isolation (populations live in different environments and rarely meet), temporal isolation (populations breed at different times), behavioral isolation (courtship signals differ), mechanical isolation (reproductive anatomy is incompatible), and gametic isolation (sperm and egg cannot fuse even if mating occurs).

Postzygotic barriers act after fertilization, reducing the fitness of hybrid offspring. They include hybrid inviability (the hybrid dies before reaching reproductive age), hybrid sterility (the hybrid is healthy but cannot reproduce—like a mule), and hybrid breakdown (the first generation is fertile but later generations are weak or sterile). These barriers reinforce divergence over time.

Four evolutionary forces shape speciation: natural selection, genetic drift, mutation, and gene flow. When gene flow stops or becomes negligible, the other three forces push populations apart. Natural selection is usually the most powerful because it adapts each population to its local environment.

Evidence and Examples

Evidence for speciation comes from both direct observation and the fossil record.

Scientists have documented speciation as it occurs. The apple maggot fly, Rhagoletis pomonella, is a frequently cited example. Originally parasitic on hawthorn fruit, a population shifted to introduced domestic apples. The apple-infesting flies now mate on apples rather than hawthorns, and the two groups show reduced interbreeding—an early stage of sympatric speciation.

Cichlid fish in Africa’s Lake Victoria provide another striking case. Hundreds of cichlid species evolved there in a relatively short time following a population bottleneck. Each species adapted to a distinct ecological niche, from algae grazing to snail crushing to open-water predation.

The fossil record preserves transitions that indicate speciation. Trilobites, ammonites, and marine microfossils show gradual morphological changes across strata that align with lineage divergence. Paleontologists use these patterns to study both gradualism (slow, steady change) and punctuated equilibrium (long periods of stability interrupted by rapid bursts of speciation). While fossils rarely preserve the exact moment of reproductive isolation, the morphological splits they document correlate with recognized species boundaries.

Consider a beetle population split by a new river. The southern group adapts to a warmer, drier climate; the northern group to a cooler, wetter one. After thousands of generations, their mating seasons shift, their body sizes diverge, and their courtship pheromones change. If they meet again, they no longer recognize each other as mates. That is allopatric speciation in action: geographic separation, genetic divergence, and reproductive isolation reinforcing each other.

Key Takeaways

• Speciation is the evolution of new species, defined by the development of reproductive isolation between populations.
• The four main types are allopatric (geographic separation), sympatric (same area, often via polyploidy in plants), parapatric (partial separation along a gradient), and peripatric (small peripheral populations).
• Reproductive barriers are prezygotic (prevent mating or fertilization) or postzygotic (reduce hybrid fitness).
• Natural selection, genetic drift, mutation, and gene flow interact to drive or slow divergence.
• Real-world evidence includes apple maggot flies, Lake Victoria cichlids, polyploid plants, and transitional fossils in the stratigraphic record.