Parapatric speciation is one of the main geographical models for the origin of new species. Speciation is the process where an ancestral population evolves into distinct descendant populations that can no longer successfully interbreed. In parapatric speciation, this divergence happens without a complete physical barrier, making it a unique path to reproductive isolation.
The Mechanism of Parapatric Speciation
Parapatric speciation involves a population inhabiting a continuous geographic area where no absolute barrier prevents movement or mating. Within this continuous range, an environmental or selective gradient exists, meaning conditions gradually change across the habitat. This gradient might involve a shift in soil type, altitude, or the presence of a localized pollutant like heavy metals.
This environmental shift creates different selective pressures, driving a process called disruptive selection. Individuals at opposite ends of the gradient adapt to different conditions, leading to local adaptation. For instance, plants on contaminated soil may evolve metal tolerance, while neighbors on clean soil do not. This divergent selection pushes the two populations apart genetically.
A distinguishing feature of the parapatric model is the presence of partial gene flow across the continuous habitat. The diverging populations meet and interbreed in a middle zone, known as a hybrid zone, where the environmental gradient is steepest. However, the hybrid offspring produced often have reduced fitness because they are poorly adapted to either parent’s specialized environment. For example, a hybrid plant might have moderate metal tolerance but lack the competitive fitness of plants fully adapted to the clean soil.
The reduced fitness of these hybrids creates a selective disadvantage favoring traits that promote assortative mating. Assortative mating means individuals choose mates similar to themselves. Mechanisms that reduce the chance of producing unfit hybrids, such as changes in mating behavior or flowering time, are selected for in the hybrid zone. This reinforcement of reproductive isolation eventually reduces gene flow, allowing the two populations to become distinct species without physical separation.
Distinguishing Parapatric Speciation from Other Modes
The modes of speciation are categorized primarily by geographic context and the degree of gene flow. Parapatric speciation is intermediate between the other two main models. Allopatric speciation, derived from Greek words meaning “other homeland,” involves the complete geographic separation of a population by a physical barrier, such as a mountain range or a river. This total isolation prevents gene flow entirely, allowing separated populations to diverge genetically.
In contrast, sympatric speciation, meaning “same homeland,” occurs when a new species arises within the same geographic range as its ancestral species. Speciation here must be driven by non-geographic isolating mechanisms, such as a sudden change in chromosome number or a shift in host preference. These mechanisms halt gene flow even though the populations share the same space.
Parapatric speciation, or “adjacent homeland,” involves populations with adjacent ranges that maintain a continuous distribution. The separation is incomplete, allowing limited, continuous gene flow across the border between the regions. This partial gene exchange is the defining feature, differentiating it from the zero gene flow of allopatry and the unhindered gene flow of sympatry. The geographical signature of this process is the formation of a narrow hybrid zone where the diverging populations meet.
Observed Instances in Natural Populations
A frequently cited example illustrating parapatric speciation is the evolution of heavy-metal tolerance in certain plant species found near old mine workings. The sweet vernal grass, Anthoxanthum odoratum, has been studied in areas contaminated with high levels of metals. Plants growing directly on toxic mine tailings rapidly evolved specialized tolerance mechanisms, while the adjacent population on normal, uncontaminated pasture soil did not.
The selective gradient is extremely sharp, changing dramatically over just a few meters. This creates a strong barrier to gene flow despite the lack of a physical obstruction. Although pollen and seeds are exchanged between the tolerant and non-tolerant populations, the resulting hybrids have low survival rates due to intermediate adaptation. Furthermore, the metal-tolerant grass populations evolved a shift in flowering time, maturing earlier than the non-tolerant grasses. This temporal difference acts as a prezygotic isolating mechanism, reducing cross-pollination and reinforcing the speciation process.
Another classic illustration of divergence along a continuous gradient is the phenomenon of ring species. This is famously represented by the Ensatina eschscholtzii salamander complex in California. These salamanders spread southward from an ancestral population, moving around the arid Central Valley along two routes: the coastal mountain ranges and the Sierra Nevada mountains. As they moved, populations gradually diverged genetically in response to local conditions.
Where the two arms of the population meet again in Southern California, they are distinct enough that they no longer interbreed, functioning as two reproductively isolated species. However, the populations along the ring are still connected by a chain of interbreeding populations. This means gene flow occurs continuously around the central barrier. The Ensatina complex demonstrates how a continuous population, gradually adapting to local environments, can lead to the formation of distinct species where the ends of the gradient meet.