What is a Species? The Biological Definition Explained
The Core Idea: The Biological Species Concept
The most famous definition of a species comes from biologist Ernst Mayr in 1942. He proposed that species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." Let's break that down.
Imagine two groups of animals. If individuals from these two groups can meet, mate, and produce healthy babies that can themselves have babies, then those two groups belong to the same species. The key word is fertile. If the offspring cannot reproduce, like a mule (the offspring of a male donkey and a female horse), then the parents are considered different species.
This reproductive isolation can happen in several ways, acting as barriers before or after mating:
| Barrier Type | Description | Example |
|---|---|---|
| Habitat Isolation | Species live in the same area but different habitats. | One snake species lives in water, another on land. |
| Temporal Isolation | Species mate at different times of day or year. | One orchid species blooms in spring, another in fall. |
| Behavioral Isolation | Species have different courtship rituals. | Fireflies use unique flash patterns to attract mates. |
| Hybrid Inviability | The hybrid embryo dies before birth. | Crossing different goat species often results in miscarriage. |
| Hybrid Infertility | The hybrid offspring is healthy but cannot reproduce. | A mule (horse + donkey) is sterile. |
When the Definition Gets Tricky: Limitations and Other Concepts
While the Biological Species Concept is very useful, it doesn't work for all life forms. What about organisms that don't have sex? Or fossils? To address these issues, scientists have developed other ways to define a species.
1. Asexual Reproduction: Many organisms, like bacteria and some plants and fungi, reproduce by simply making copies of themselves (asexually[2]). Since they don't "breed together" at all, the BSC doesn't apply. For these, scientists might use the Morphological Species Concept, which defines species based on their physical shape, size, and other structural features.
2. Fossils: We can't test if a Tyrannosaurus rex and a Triceratops could produce fertile offspring! For extinct species, paleontologists rely on comparing the physical structures of fossils.
3. Ring Species: This is a fascinating exception. Imagine a chain of neighboring populations encircling a mountain. Population A can breed with population B, B with C, C with D, and so on. However, when the chain closes, population Z cannot breed with population A, even though they are connected through a series of compatible populations. The Ensatina salamanders in California are a classic example of a ring species.
4. Hybridization: Sometimes, different species do interbreed and produce fertile hybrids. This is common in the plant world. For instance, many of our food crops are the result of natural hybridization.
Classifying Life: From Domain to Species
A species is the most specific level in the biological classification system, also known as taxonomy. This system, started by Carl Linnaeus, organizes life into a hierarchy of groups. The scientific name of a species is a two-part name (binomial nomenclature) consisting of the genus and the species identifier. For example, humans are Homo sapiens.
| Rank | Human (Homo sapiens) | Tiger (Panthera tigris) | Bald Eagle (Haliaeetus leucocephalus) |
|---|---|---|---|
| Domain | Eukarya | Eukarya | Eukarya |
| Kingdom | Animalia | Animalia | Animalia |
| Phylum | Chordata | Chordata | Chordata |
| Class | Mammalia | Mammalia | Aves |
| Order | Primates | Carnivora | Accipitriformes |
| Family | Hominidae | Felidae | Accipitridae |
| Genus | Homo | Panthera | Haliaeetus |
| Species | Homo sapiens | Panthera tigris | Haliaeetus leucocephalus |
Species in Action: Observing the Concept in Nature
Let's look at some concrete examples to see how the species concept works (and sometimes doesn't) in the real world.
Example 1: The American Black Bear and the Grizzly Bear. These are two distinct species: the American black bear (Ursus americanus) and the grizzly bear (Ursus arctos horribilis). They look different, behave differently, and typically live in different habitats. While rare, they can interbreed in the wild to produce fertile offspring called "grolar" or "pizzly" bears. This recent hybridization is often linked to climate change pushing grizzly bears into black bear territory. This challenges a strict BSC interpretation, showing that speciation is an ongoing process.
Example 2: Dogs, Wolves, and Coyotes. This is a classic example of the BSC. All domestic dogs, from Chihuahuas to Great Danes, are considered the same species, Canis lupus familiaris. They can all interbreed and produce fertile puppies. Furthermore, dogs can also interbreed with gray wolves (Canis lupus) and produce fertile offspring, which is why dogs are classified as a subspecies of the gray wolf. However, dogs and coyotes (Canis latrans) can produce hybrids (coydogs), but these hybrids are often not fully fertile, reinforcing their status as separate species.
Example 3: Darwin's Finches. On the Galápagos Islands, Charles Darwin observed many species of finches. They all evolved from a common ancestor, but on different islands, they adapted to different food sources, leading to variations in their beak shapes. These finch species are largely reproductively isolated from one another due to differences in song and behavior, making them a powerful example of how new species form (speciation).
Common Mistakes and Important Questions
A: Not necessarily. Look at dogs! Different breeds look very different but are the same species. Conversely, some species that look almost identical (called cryptic species) cannot interbreed. Appearance can be misleading, which is why the breeding test is so important.
A: No, this is a common confusion. A "breed" is a variety within a single species, created by humans through selective breeding (e.g., dog breeds, cattle breeds). Members of different breeds can still interbreed. A "species" is a broader, natural category where interbreeding with other species is not possible or does not produce fertile offspring.
A: Yes, our understanding can. As we learn more about an organism's DNA (using the Phylogenetic Species Concept[3]), we might discover that what we thought was one species is actually two, or vice versa. Species themselves also change over millions of years through evolution, a process called speciation.
The definition of a species as a "group of organisms that can breed together" is a powerful starting point for understanding life's diversity. The Biological Species Concept highlights the critical role of reproductive isolation in maintaining the integrity of species. However, nature is complex, and this definition has limitations when applied to asexual organisms, fossils, and ring species. This is why scientists use a combination of concepts—biological, morphological, and phylogenetic—to classify life. Understanding what a species is helps us document biodiversity, study evolution, and make crucial conservation decisions to protect the unique forms of life on our planet.
Footnote
[1] Biological Species Concept (BSC): A species concept that defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but are unable to produce viable, fertile offspring with members of other such groups.
[2] Asexual Reproduction: A type of reproduction by which offspring arise from a single organism and inherit the genes of that parent only; it does not involve the fusion of gametes.
[3] Phylogenetic Species Concept (PSC): A species concept that defines a species as the smallest group of individuals that share a common ancestor and form a single branch on the tree of life.