What makes a species a species? In the following, we will discuss the concept of the biological species, then elaborate on how reproductive barriers relate to the biological species concept, and, finally, compare the concept of the biological species to other species concepts.
What Is the Definition of Species According to the Biological Species Concept?
The biological species concept defines species as populations whose members interbreed and produce viable, fertile offspring.
In nature, members of two different species are reproductively isolated. They may not consider each other as potential mates, their mating may not lead to the formation of a zygote, or they cannot produce viable, fertile offspring.
Viable: Able to sustain life.
Fertile: Able to produce offspring.
Let's Discuss Some Examples in Which the Biological Species Concept is Applied
Despite being an unlikely pair to meet, a dog in Canada and a dog in Japan have the potential to interbreed and produce viable, fertile puppies. They are considered members of the same species.
On the other hand, horses and donkeys can interbreed, but their offspring–mules (Figure 1)–will be infertile and cannot produce offspring. Therefore, horses and donkeys are considered separate species.
Figure 1. Mules are the sterile hybrid offspring of horses and donkeys.
How are Reproductive Barriers Related to the Biological Species Concept?
Gene flow is the movement of genetic information from one population of organisms to another. When organisms or gametes enter a population, they may bring new or existing alleles in varying amounts compared to those already present in the population.
Gene flow occurs between populations of the same species but not between populations of different species. Members of a species can interbreed, so the species in its entirety shares a common gene pool. On the other hand, members of different species might be able to interbreed, but they will produce sterile offspring, unable to pass on their genes. Hence, the presence or absence of gene flow can distinguish one species from another.
Reproductive barriers limit or prevent gene flow between different species. Biological species are defined by their reproductive compatibility; we can say that different biological species are distinguishable by their reproductive isolation. Reproductive isolation mechanisms are categorized as either prezygotic or postzygotic barriers:
- Prezygotic barriers prevent the formation of the zygote. These mechanisms include temporal isolation, geographic isolation, behavioral isolation, and gametic barrier.
- Postzygotic barriers prevent gene flow after the formation of the zygote, leading to hybrid inviability and hybrid sterility.
Reproductive barriers help define the boundaries of the species as a reproductive community and as a gene pool and maintain the cohesion of the species as a genetic system. Reproductive barriers are why members of a species share more similarities than they do with members of other species.
What are the Advantages and Limitations of the Biological Species Concept?
The biological species concept provides the most widely accepted definition of species.
An advantage of the biological species concept is that it focuses on reproductive isolation, making it simple and easy to apply in some situations. For example, the western meadowlark (Sturnella neglecta) and the eastern meadowlark (S. magna) look very similar. Still, they are two distinct species because, despite their overlapping breeding ranges, the two species do not interbreed (Figures 2-3).
 Figure 2. The western meadowlark
|  Figure 3. The eastern meadowlark
|
Figures 2-3. The western meadowlark (left) and the eastern meadowlark (right) look similar but are considered two distinct species according to the concept of biological species.
However, in other situations, the biological species concept is difficult to apply. The major limitations of the biological species concept are summed up as follows:
- It is inapplicable to fossil evidence because their reproductive isolation cannot be evaluated.
- The biological species concept defines species in terms of sexual reproduction, so it does not apply to asexual organisms like prokaryotes or self-fertilizing organisms like parasitic tapeworms.
- The biological species concept is challenged by the ability of sexual organisms that freely hybridize in the wild but are able to maintain their cohesion as distinct species.
Because of the limitations of the biological species concept, it is considered a working definition. Alternative species concepts are useful in other situations.
What Are Other Definitions of Species?
There are over twenty species concepts, but we will focus on three: the morphological species concept, the ecological species concept, and the phylogenetic species concept. We will also compare each one with the biological species concept.
The Morphological Species Concept
As defined by the morphological species concept, species are distinguished based on their form and structural features.
Biological vs. Morphological Species Concept
Compared to the biological species concept, the morphological species concept is easier to apply in the field because it is based only upon appearance. Additionally, unlike the biological species concept, the morphological species concept is applicable to both asexual and sexual organisms, as well as to fossil evidence.
For example, trilobites are a group of extinct arthropods with over 20,000 species. Their existence can be traced back to around 542 million years ago. The cephalon (head region) or cranidium (central part of the cephalon) of trilobite fossils (Figure 4) are used to distinguish between species. The biological species concept cannot be used to distinguish them because reproductive behavior cannot be inferred from fossil evidence.
Figure 4. Species of trilobites are often identified using their cephalon or cranidium.
The downside to this approach is that morphological evidence can be interpreted subjectively; researchers may disagree on what structural features can set species apart.
The Ecological Species Concept
As defined by the ecological species concept, species are distinguished based on their ecological niche. An ecological niche is a role that a species plays in a habitat based on its interactions with the resources available in its environment.
For example, grizzly bears (Ursus arctos) are often found in woodlands, prairies, and forests, while polar bears (U. maritimus) are often found in Arctic Seas (Figures 5-6). When they interbreed, they can produce fertile offspring. However, this rarely happens in the wild because they mate in different habitats. According to the ecological species concept, they are two distinct species, even though there is potential gene flow between them because they occupy two different ecological niches.
 Figure 5. Polar bear
|  Figure 6. Grizzly bears
|
Figures 5-6. Polar bears and grizzly bears can produce fertile offspring but are considered two distinct species.
Biological vs. ecological species concept
An advantage to the ecological species concept is that it is applicable to both sexual and asexual species. It also takes into consideration how the environment can influence the morphological development of organisms.
The downside to this approach is that there are organisms whose interactions with resources in their environment are overlapping. There are also organisms that switch to other resources due to external factors. For example, feeding habits can change when food becomes scarce.
The Phylogenetic Species Concept
As defined by the phylogenetic species concept, species are a group whose members share a common ancestor and possess similar defining traits. In a phylogenetic tree, species would be represented by branches in a lineage. A lineage that branches off represents the emergence of a new, distinct species. This approach focuses on the evolutionary history of organisms and often relies on genetic evidence.
Figure 7. This phylogenetic tree shows the evolutionary history of different species of the order Rodentia.
Biological vs. phylogenetic species concept
An advantage of the phylogenetic species concept is that it is applicable to asexual organisms and organisms whose reproductive behaviors are unknown. It is also less restrictive in terms of morphological changes within the history of a species, as long as there is continuity of sexual fertility. It is applicable to both extinct and extant organisms.
The downside to this approach is that phylogenies are hypotheses that are open to revision. The discovery of new evidence could lead to species reclassification, making it an unstable basis for identifying species.
Biological Species Concept - Key takeaways
- The biological species concept defines species as populations whose members interbreed and produce viable, fertile offspring.
- The biological species concept provides the most widely accepted definition of species, but it has limitations. It is inapplicable to fossil evidence, asexual or self-fertilizing organisms, and sexual organisms that freely hybridize.
- Other species concepts include morphological, ecological, and phylogenetic species concepts.
- The morphological species concept distinguishes species based on their form and structural features.
- The ecological species concept distinguishes species based on their ecological niche.
- The phylogenetic species concept is a group whose members share a common ancestor and possess similar defining traits.
References
- Figure 1: Mule (https://commons.wikimedia.org/wiki/File:Juancito.jpg) by Dario Urruty. Public Domain.
- Figure 2: Western Meadowlark (https://commons.wikimedia.org/wiki/File:Western_Meadowlark_(fb86fa46-8fa5-43e0-8e30-efc749887e96).JPG) by National Park Service (https://npgallery.nps.gov). Public Domain.
- Figure 3: Eastern Meadowlark (https://www.flickr.com/photos/79051158@N06/27901318846/) by Gary Leavens (https://www.flickr.com/photos/gary_leavens/). Licensed by CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/).
- Figure 4: Trilobites (https://commons.wikimedia.org/wiki/File:Paradoxides_minor_fossil_trilobite_(Jince_Formation,_Middle_Cambrian;_Jince_area,_Bohemia,_Czech_Republic)_2_(15269684002).jpg) by James St. John (https://www.flickr.com/people/47445767@N05) Licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en).
- Figure 5: Polar bears (https://commons.wikimedia.org/wiki/File:Polar_bear_female_with_young_cubs_ursus_maritimus.jpg) by Susanne Miller, U.S. Fish and Wildlife Service. Public Domain.
- Figure 6: Brown Bear (https://commons.wikimedia.org/wiki/File:Grizzly_bear_brown_bear.jpg) by Steve Hillebrand, U.S. Fish and Wildlife Service. Public Domain.
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