Plants are indeed classified as species, meaning they represent one of the basic units of biological classification and biodiversity. The science of botany organizes the plant kingdom into a hierarchical structure, with the species level representing a group of organisms sharing the closest similarities. However, the process of determining exactly where one plant species ends and another begins is one of the most complicated tasks in biological science. This complexity arises because the standard rules used for classifying many other life forms often break down when applied to the unique reproductive biology of plants.
The Concept of a Biological Species
The most widely taught definition of a species, known as the Biological Species Concept (BSC), was developed primarily with animals in mind. This concept defines a species as a group of natural populations that can interbreed with one another to produce viable, fertile offspring, but which are reproductively isolated from other such groups. Reproductive isolation acts as a boundary, preventing gene flow between two distinct groups.
This definition relies on reproductive barriers, which can be prezygotic (preventing mating or fertilization) or postzygotic (leading to sterile offspring). For example, the mule, the sterile hybrid of a horse and a donkey, is the classic example of postzygotic isolation maintaining two separate species.
Why Defining Plant Species Is Complex
The strict application of the Biological Species Concept frequently fails in the plant kingdom, largely due to plants’ high tolerance for genetic mixing. Plants exhibit much higher rates of successful interspecific hybridization—the crossing of individuals from two different species. Unlike in animals, where such crosses often fail or produce sterile offspring, many plant hybrids are successful and can even be fertile.
This genetic permeability blurs the lines of reproductive isolation that the BSC relies upon to define species boundaries. Furthermore, polyploidy, the doubling or multiplication of the entire set of chromosomes, is widespread in plants. Polyploidy can instantly restore fertility to a hybrid that would otherwise be sterile, effectively creating a new, reproductively distinct species in a single generation.
The prevalence of asexual reproduction, such as through runners or root sprouts, also complicates the issue. Many plants can reproduce without sexual processes, a biological pathway that bypasses the need for reproductive isolation entirely. Since the BSC is rooted in the ability to sexually interbreed, it cannot be applied to plants that form clonal lineages through non-sexual means.
Tools for Plant Species Classification
Because the reproductive isolation test is unreliable for many plants, botanists rely on a combination of alternative frameworks and practical tools to classify species.
Morphological Species Concept
The oldest method is the Morphological Species Concept, which groups organisms based on shared physical characteristics, such as flower shape, leaf structure, and size. This approach is practical for classifying the millions of preserved specimens in herbaria and remains a foundational tool in taxonomy.
Phylogenetic Species Concept
Modern classification uses the Phylogenetic Species Concept, which defines a species as the smallest group of organisms sharing a common ancestor and distinguished by a unique set of genetic or physical traits. This method uses DNA sequencing to construct evolutionary family trees, identifying distinct genetic lineages. This genetic-based approach is useful for separating “cryptic species” that look identical but do not interbreed.
Ecological Species Concept
Botanists also employ the Ecological Species Concept, which defines a species as a population adapted to a specific ecological niche or environment. For instance, two morphologically similar plant populations may be classified as separate species if one is adapted to a high-altitude, rocky soil environment while the other is restricted to a lowland, moist habitat. By integrating morphological analysis, genetic data, and ecological context, botanists establish the practical boundaries of plant species.
Binomial Nomenclature and Species Naming
The result of this classification process is the standardized scientific name given to every recognized plant species, known as binomial nomenclature. This system, formally established by Carl Linnaeus, provides a two-part Latinized name that is universally recognized by scientists. The structure consists of the generic name, which is capitalized, followed by the specific epithet, which is lowercase.
The two names are always written in italics, such as Quercus alba for the White Oak, where Quercus is the genus and alba is the species. The system’s purpose is to provide a unique identifier that avoids the confusion caused by common names, which often vary by region and language. This standardized scientific name reflects the plant’s place in the taxonomic hierarchy.