The concept of a species, a fundamental unit in biology, seems straightforward in everyday language, where a dog is distinct from a cat. Within the scientific community, however, the seemingly simple question of “What is a species?” has led to decades of debate and more than 20 different proposed definitions. The challenge arises because life on Earth is a continuous, branching process of evolution, while human classification attempts to impose fixed, discrete boundaries. Understanding this problem requires looking beyond surface-level differences to the underlying mechanisms scientists use to categorize the vast diversity of life.
The Biological Species Concept
The most widely taught and traditional framework for classifying sexually reproducing organisms is the Biological Species Concept (BSC), articulated by Ernst Mayr in 1942. This concept defines a species as a population, or a collection of populations, whose members can interbreed in nature and produce viable, fertile offspring. The production of fertile offspring confirms that the groups share a common gene pool and are not reproductively isolated.
The core of the BSC rests on the idea of reproductive isolation, which prevents gene flow between different groups. This isolation occurs through various pre-zygotic barriers, such as differences in mating rituals or habitats, or post-zygotic barriers, where a hybrid embryo fails to develop or is infertile. The classic example is the mule, the sterile hybrid offspring of a horse and a donkey, which confirms the parent species are genetically distinct.
When the Traditional Definition Fails
The reliance on interbreeding and reproductive isolation creates significant limitations, rendering the Biological Species Concept inapplicable to a vast number of organisms. The most obvious failure is with organisms that reproduce asexually, such as bacteria, archaea, and many species of plants and fungi. Because these organisms replicate through processes like binary fission or budding, the criterion of interbreeding cannot be met or tested.
Another area where the BSC breaks down is in paleontology, which studies life forms known only from fossils. Researchers cannot observe the reproductive behavior of extinct organisms, nor can they determine if two fossilized populations separated by millions of years were capable of producing fertile offspring. In such cases, a definition based on interbreeding is impractical, forcing scientists to rely on other methods to delineate species boundaries.
Furthermore, the concept struggles with instances of viable hybridization in nature, where species boundaries are fuzzy. Plant species frequently interbreed to produce fertile hybrids, sometimes leading to new species through a process called polyploidy. A more complex example is found in ring species, such as the Ensatina salamanders, where adjacent populations can interbreed, but the populations at the end of the ring are reproductively isolated from each other.
Alternative Ways to Define a Species
Because no single definition works for all life forms, scientists employ a variety of alternative frameworks tailored to specific types of organisms or data sets.
Morphological Species Concept (MSC)
The Morphological Species Concept (MSC) is one of the oldest methods, defining a species based on shared physical characteristics, or morphology. This concept is particularly useful in paleontology, where a scientist compares the physical traits of a fossil to a known specimen.
The MSC is also practical for field guides and initial classification, as it relies on easily observable traits like size, shape, and color. However, it can be misled by polymorphism, where individuals of the same species exhibit vastly different appearances, or by cryptic species, which look identical but are genetically distinct. This underscores the need for more nuanced, data-driven concepts.
Phylogenetic Species Concept (PSC)
The Phylogenetic Species Concept (PSC) addresses the limitations of the BSC and MSC by defining a species as the smallest group of organisms that shares a common ancestor and is distinguishable by a unique set of traits. By focusing on shared ancestry and evolutionary history, the PSC is effective for classifying asexual organisms and utilizing modern DNA sequencing data. It determines species status by identifying the smallest diagnosable cluster on the evolutionary tree.
Ecological Species Concept (ESC)
The Ecological Species Concept (ESC) defines a species as a group of organisms occupying a specific ecological niche. This concept emphasizes the role of natural selection in shaping a species’ adaptations to particular environmental conditions. The ESC provides a useful way to distinguish species that might look similar but have evolved different roles, such as two species of fish that inhabit the same lake but feed on different food sources.
Why Species Definitions Matter
The academic debate over species definitions has profound consequences, directly influencing conservation, law, and public health policy. Accurate species delineation is fundamental to conservation biology because legal protections are often granted at the species level. If a distinct population is not recognized as a separate species, it may not qualify for protection under national laws, leaving it vulnerable to extinction.
Classifying species also affects the distribution of limited conservation resources. Furthermore, it is relevant to agriculture and biomedicine, particularly in tracking the transmission of zoonotic diseases. Defining the boundaries between human and non-human species is important for understanding how pathogens jump from animals to people and for developing effective public health strategies.