What is the definition of a species according to the biological species concept?

Quantifying species diversity requires developing a definition of a species. We would not expect all members of a species to be identical, so we must consider what magnitude and types of differences between individuals would lead us to consider them members of different species. A species concept is, therefore, a working definition of a species and/or a methodology for determining whether or not two organisms are members of the same species. In this section, we will consider three species concepts that are commonly used by scientists. All species concepts have limitations and work better for some organismal groups than others. Each section below considers the definition of the species concept, its assumptions and limitations, and examples of groups for which that species concept does, and does not, easily apply.

The Biological Species Concept

By far the most well-known species concept is the biological species concept, which was proposed by evolutionary biologist Ernst Mayr. The biological species concept states that a species is a group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. Reproductive isolation can occur in several ways (which we will discuss in detail later in the quarter); individuals of different groups may not mate with each other, their mating may not produce offspring, or the offspring produced may not be viable (able to survive) or fertile (able to reproduce).

In some cases, the biological species concept is straightforward and easy to apply. For instance, the western meadowlark (Sturnella neglecta) and the eastern meadowlark (Sturnella magna), both shown in Figure 1, respectively inhabit the western and eastern halves of North America. Despite the fact that their breeding ranges overlap throughout many upper midwestern states, including Michigan, Wisconsin, Illinois, Iowa, Missouri and Minnesota, the two groups do not interbreed. The courtship songs of the males of each species are distinctly different and females of each species respond to the songs of the males of their own species, leading to strong reproductive isolation between the two groups despite a high degree of similarity in appearance.

Figure \(\PageIndex{1}\): Males of the western meadowlark, Sturnella neglecta (left) and the eastern meadowlark Sturnella magna (right).
Images from Wikimedia Commons1,2

In many cases, however, the biological species concept is difficult or impossible to use. For example, the definition’s focus on interbreeding means this concept cannot be applied to asexual organisms, such as bacteria. Additionally, it can only be applied to groups for which detailed reproductive data are available, or at least obtainable. It is therefore impossible to apply the biological species concept to long-extinct species for which reproductive data do not exist and can no longer be obtained. It can also be difficult to apply the biological species concept to groups for which little is known about their reproductive biology or behavior. In these cases, extensive research would be required to determine the degree to which individuals do or do not interbreed with other groups before being able to clearly identify species boundaries. Additionally, some groups have complex patterns of reproductive connectivity and isolation, such as the Ensatina salamander group shown in Fig 2. Systems like Ensatina are termed ‘ring species’ because they form a ring around a particular geographic barrier, in this case, California’s Central Valley. As the group spread around the valley, populations maintained reproductive connectivity (ie. interbreeding) with nearby populations, but developed reproductive isolation from geographically distant populations. In the diagram below, interbreeding is shown as a gradation in color between two populations and reproductive isolation is shown as a solid line. For example, E. oregonensis (in red) can interbreed with E. picta (orange), E. xanthoptica (yellow), and E. platensis (pink), but none of the other species. Similarly, E. xanthoptica (yellow) can interbreed with E. eschscholtzii (green) and E. oregonensis (red), but none of the other species. In this case, the biological species concept leads to nonsensical and contradictory conclusions: E. oregonensis, E. xanthoptica, and E. eschscholtzii are members of the same species, but E. oregonensis and E. eschscholtzii are different species.

More philosophically, the biological species concept also considers species at only a single point in time (the present) and ignores the evolutionary and ecological processes that shaped reproductive isolation between groups.

The Lineage Species Concept

Due to the limitations of the biological species concept described above, other species concepts have been developed. The lineage species concept defines species as groups of organisms that share a pattern of ancestry and descent and which form a single branch on the tree of life (Fig 3). This concept focuses more on the evolutionary history that has shaped the species as we see it today, and increasingly relies on genetic data to assign individuals to species. The lineage species concept resolves some of the problems of the biological species concept since it can be applied to asexual species and those for which detailed reproductive behavioral data are unavailable. It’s reliance on genetic data makes it also difficult to apply to long-extinct species; however, recent advances in genetic analysis have allowed scientists to extract DNA from recently extinct organisms such as Neanderthals and wooly mammoths. Scientists using the lineage species concept must still consider what type and magnitude of genetic differences, and in what portions of the genome, constitute different species and must employ modern computational tools to manage the increasingly large datasets produced in genetic analyses.

The Morphological Species Concept

How do scientists define species in groups where there is limited or unavailable data on reproductive behavior or genetic similarity? Consider, for example, the two trilobites shown in Figure 2.1.4. Both of the fossils shown in the figure are members of the Order Phacopidae, which went extinct during the end-Devonian mass extinction event that occurred approximately 360 million years ago. Consequently, the only evidence that remains of their existence is fossils; there is no behavioral data on reproductive isolation or connectivity with other trilobites and the fossils are old enough that their DNA has degraded beyond our current analytical capabilities. How then, do we define species in trilobites? The morphological species concept is frequently applied in such cases, as it relies entirely on morphology (the physical structures or traits of an organism).

As you look at two photos in Fig 4, you will notice clear morphological differences. The fossil on the right has large spines jutting from both its head and rear ends (in trilobites, these are termed the cephalon and pygidium, respectively). Additionally, the spines along the side of the trilobite’s thorax are much larger and more pronounced in the fossil on the right. These morphological differences lead us to categorize these fossils as members of different species. In this case, the differences are large enough that we even categorize these fossils as members of different genera and families.

Since all organisms have physical traits, the morphological species can be used on any group of organisms on Earth. The major limitation to this species concept, however, is that morphology can be very misleading. Consider, for example, the shark and dolphin shown in Figure 5. These organisms have many morphological similarities in their body shape and coloration; however, we know from genetic analyses and more detailed morphological studies (on internal structures, etc) that sharks are more closely related to rays and other fish and dolphins are more closely related to whales and other mammals. The similarities that we first notice in these organisms are due to the similarities in the marine environment in which both organisms live and not to relatedness between the organisms. Consequently, the morphological species concept is often only used when other species concepts cannot be applied (for example, in the trilobate case) or in conjunction with other species concepts described above.

Linking Species Concepts

Most scientists generally agree that a species is a group of organisms that share an evolutionary and ecological history and that are distinct from other groups. The primary difference in the species concepts described above is the forms of evidence used to quantify those differences and to categorize individuals as members of a particular species. The biological species concept relies on behavioral data and emphasizes reproductive isolation between groups. The lineage species concept relies on genetic data and emphasizes distinct evolutionary trajectories between groups, which result in distinct lineages (branches on a phylogenetic tree). The morphological species concept relies on morphological data and emphasizes groups of physical traits that are unique to each species. These lines of evidence are not mutually exclusive and so multiple species concepts may be used together to define species boundaries. Regardless of the species concept used, not all organisms are easily categorized into distinct groups and so conversations around species concepts, species boundaries, and the evidence used to define them are a dynamic field of evolutionary biology. You will practice defining group boundaries in Lab 1: Discovering Diversity.In this lab, you will categorize organisms into ‘operational taxonomic units’ (OTUs) using the morphological species concept.

Naming Species

Carl Linnaeus began formally naming and categorizing species in 1735 in his publication Systema Naturae. Linnaeus developed the binomial nomenclature system for naming species, which is still used by scientists today. In this system, each species is given a two-word (binomial) Latin name. The first word is the generic name, which is shared among all the species in that genus. The second word is the specific epithet and is unique among species in that genus. For instance, all juniper trees have the genus name Juniperus and each species has a specific epithet that identifies which juniper species it is, such as western juniper (Juniperus occidentalis) or California juniper (Juniperus californica), etc. There are many species that have the genus name Juniperus, and many species that have the specific epithet occidentalis but only one Juniperus occidentalis. As you may have noted in this paragraph, species names are italicized and the genus name is capitalized while the specific epithet is not. The full formal name also includes the last name of the scientist who described the species and the year in which it was named. For example, the common limpets shown in Figure 6 carry the formal scientific name of Patella vulgata Linnaeus, 1758.

Today, scientists have formalized guidelines they follow to choose names for new species. Scientists follow the guidelines of the International Code of Zoological Nomenclature (ICZN) or the International Code of Nomenclature for Algae, Fungi and Plants (ICN). These codes have strict rules for Latinizing names and descriptive terms to use in species names and for resolving conflicts in changes to scientific names. As our understanding of life on Earth expands, revisions to previous taxonomic groupings are often made and these guidelines help track changes and keep renaming or regrouping of organisms consistent.

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