In biology, a species is a group of organisms that interbreed and produce fertile offspring. But, the definition is controversial and complicated. It’s also important, because protecting a species basically involves identifying its existence.
- One definition of a species is that it is a set of organisms that breed and produce fertile offspring. But, scientists do not agree on a single definition.
- Taxonomists estimate there are between 3 million and 100 million different species on Earth.
- Most species have not been discovered and described.
- In taxonomy, the order is kingdom, phylum, class, order, family, genus, and species.
For the most part, organisms that breed and produce fertile offspring are members of the same species.
But, the definition of a species is not the same today as in the past, plus it continues to change. Originally, biologists identified a species based on visible physical characteristics, like fur/feathers/scales or eggs/live-bearing for animals and vascular/nonvascular or monocot/dicot for plants. But, morphologically similar organisms don’t necessarily breed and produce viable offspring, much less fertile ones. Further, some organisms reproduce asexually. Then, you have organisms that theoretically can mate and have fertile offspring, but don’t. Reasons include geographical isolation or mate selection criteria that exclude certain characteristics.
Recent definitions of a species take phylogenetics into account. Here, the genetic similarity or dissimilarity between organisms helps taxonomists decide whether they are members of the same species or not.
The horse, the donkey, and the mule provide a good example of how the species definition works. All three animals have a similar appearance and can mate with one another. If two horses breed, their offspring are fertile horses. If two donkeys breed, their offspring are fertile donkeys. However, if a male horse and a female donkey breed, their offspring is an infertile animal called a mule. Mating a female horse and male donkey produces a hinny, which usually is not fertile. Mules and hinnies are not members of a species because they can’t reliably produce fertile offspring.
The Canis genus includes examples of different species and illustrates one of the issues with the usual definition of the word. Some of the members of the genus are Canis latrans (coyote), Canis rufus (red wolf), Canis lupus (gray wolf), and Canis familiaris (domestic dog). Members of Canis are genetically similar, have 78 chromosomes, and can interbreed and produce fertile offspring. Yet, genetic analysis reveals DNA characteristic of each group. Also, while the different canids can breed with one another, it’s uncommon.
Mating between two dissimilar parents or hybridization sometimes occurs with animals and is common with plants. Sometimes hybrids are not fertile, so they do not constitute a new species. Other times, a hybrid establishes itself in a niche and reproduces in isolation from its parents, generating a new species. Hybrid speciation is when two different species mate and their offspring become a new species. The red wolf appears to be a hybrid species arising from the coyote and gray wolf.
Scientific Name of a Species
The scientific name of an organism gives its genus and species name. For example, Panthera onca is the scientific name for the jaguar, while Panthera pardus is the name of the leopard. Here, Panthera is the genus, while onca and pardus identify the species. The first letter of the genus is capitalized, while all letters of the species name are lowercase. Italicizing scientific names in printed literature is the norm. When the precise species is unknown or a discussion includes a mixture of them, the abbreviation is “sp.” or plural “spp.” (e.g., Canis sp.).
How Many Species Are There in the World?
We don’t know how many species there are in the world, but scientists agree that the number of unidentified species greatly outnumbers the ones described to date. Currently, over 1.2 million species are cataloged. Taxonomist estimates of the total number of species of Earth range anywhere from 3 million to 100 million. However, some of the studies leading to these estimates are highly controversial. One notable analysis estimates there are approximately 8.7 million eukaryotic species, of which about 2.2 million are marine organisms (oceanic). This estimate, by Mora et al., extrapolates the number of species based on a linear relationship between the numbers of higher taxa (kingdom, phyla, classes, etc.).
The Importance of the Species Definition
Identifying and describing species is critical for conservation and biology because it determines the assessment of the abundance of an organism is in an ecosystem. This, in turn, drives whether or not a species gains legal protection from extinction. A classic case where the definition of a species led to debates involves the northern spotted owl. The owl hybridizes with the barred owl and the California spotted owl (not endangered or protected). If the northern spotted owl is a unique species, then it warrants special protection. But, if it is a variation of a California spotted owl, then it does not.
- Lewin, Ralph A. (1981). “Three Species Concepts”. Taxon. 30 (3): 609–613. doi:10.2307/1219942
- Mallet, James (1995). “A species definition for the modern synthesis”. Trends in Ecology & Evolution. 10 (7): 294–299. doi:10.1016/0169-5347(95)90031-4
- Masters, J. C.; Spencer, H. G. (1989). “Why We Need a New Genetic Species Concept”. Systematic Zoology. 38 (3): 270–279. doi:10.2307/2992287
- Mora, Camilo; Tittensor, Derek P.; Adl, Sina; Simpson, Alastair G. B.; Worm, Boris (2011). “How Many Species Are There on Earth and in the Ocean?”. PLOS Biology. 9 (8): e1001127. doi:10.1371/journal.pbio.1001127
- Wheeler, Quentin; Meier, Rudolf, eds. (2000). Species Concepts and Phylogenetic Theory: A Debate. Columbia University Press. ISBN 978-0-231-10143-1.