The Pleistocene Theater
The temperate deciduous forests along Turkey's Black Sea coast are a living laboratory for evolution. Here, four distinct species of wild hazelnut grow in close proximity: the common hazel (Corylus avellana), the Turkish hazel (Corylus colurna), the filbert (Corylus maxima), and the Pontic hazel (Corylus pontica). Genetic analyses show these species began to diverge during the Pleistocene epoch, which lasted from about 2.6 million to 11,700 years ago.
This era of recurring ice ages dramatically reshaped global ecosystems. As vast ice sheets expanded across northern Europe, many plant and animal species were forced into southern sanctuaries known as "glacial refugia". Anatolia, particularly the mountainous coastal regions along the Black Sea, was a critical refuge for temperate flora. Shielded from the harshest glacial conditions, isolated populations of ancestral hazelnuts survived. This geographic separation, lasting for thousands of years, prevented gene flow and enabled allopatric speciation, where isolated groups evolve independently. Phylogenomic data confirm that recent speciation in many regional plants was driven by this type of geographical separation during the Pleistocene.
A Genetic Crossroads
When the last glacial period ended and the climate warmed, these separated hazel populations expanded their ranges. Their descendants, now genetically distinct species, eventually met again in the Turkish forests. The result is a complex mosaic of pure species and naturally occurring hybrid zones.
One of the most studied interactions is between the common hazel, a multi-stemmed shrub, and the Turkish hazel, a large, single-trunk tree that can grow up to 25 meters tall. Where their ranges overlap, they can still interbreed, a phenomenon called incomplete reproductive isolation. The resulting hybrids, sometimes referred to as Corylus × colurnoides, often display intermediate characteristics, such as a tree-like form with multiple trunks.
Scientists use molecular markers, including simple sequence repeats (SSRs), to map the genetic structure of these populations. These studies reveal the extent of hybridization and gene flow. While natural hybridization between C. avellana and C. colurna is possible, it is not always common. Some studies have detected clear differentiation between the two species with no evidence of recent hybridization in the analyzed populations. This dynamic environment, where distinct species maintain their integrity while occasionally forming hybrids, shows scientists a direct view of the complex processes that create new species over evolutionary time. The genetic diversity harbored in these Black Sea forests is among the highest for hazelnut populations worldwide.