Many people can readily distinguish a sparrow from a pigeon, hawk, or duck. However, to a biologist, the term “sparrow” wouldn’t be sufficiently descriptive. A biologist would seek to identify the specific species of sparrow they are observing. But what precisely constitutes a species, and how is it defined? In the realm of biology, a species stands as the foundational unit of taxonomy, the scientific system employed to categorize life on Earth. Without the concept of a species, numerous scientific endeavors, ranging from wildlife conservation to public health, would face significantly greater challenges.
Officially, a “species” occupies the lowest rung of the taxonomic ranking system, which commences with the domain and progresses downward through kingdom, phylum, class, order, family, and finally, genus and species. These bottom two categories bestow upon each organism its scientific name, such as *Passer domesticus*, the house sparrow, where “domesticus” represents the species within the genus *Passer*. This system was formally established in the 18th century by the Swedish biologist Carl Linnaeus. Historically, distinctions between species were often based on readily observable trait differences—one bird species might possess a blue head, while another species exhibited a red head.
However, in the mid-20th century, biologist Ernst Mayr proposed a more rigorous definition, known as the biological species concept. “He promoted this idea that species are reproductively isolated units, so populations that interbreed together,” explained Dr. [Insert Name], an evolutionary biologist at the University of Minnesota. In essence, house sparrows mate exclusively with other house sparrows, and Eurasian tree sparrows (*Passer montanus*) reproduce solely with other Eurasian tree sparrows, thus maintaining these populations as distinct entities. Nonetheless, certain limitations arise with this definition. Many species can and do interbreed with other species—including *Passer domesticus* and *Passer montanus*. Additionally, some species (including numerous microbes) reproduce asexually, meaning a single individual replicates itself. Therefore, categorizing these tiny organisms into “reproductively isolated units” proves largely ineffective.
Consequently, in the 1980s, an ornithologist currently at the American Museum of Natural History, introduced the “evolutionary species concept,” which posits that a species represents a group of organisms sharing a set of traits and descending from a common ancestor. Species remained the unique groups at the bottom of the taxonomic ranking system, but this new definition offered greater flexibility. “It’s not just this idea that we’re concerned about reproduction being the thing that defines species,” Reddy told Live Science, “but just sets of characters that would tell us that these have an evolutionary identity that’s different from other lineages.”
Today, genetic studies and DNA sequencing introduce even more nuance to this discussion. And the species isn’t truly the lowest rung in the classification system. While house sparrows and Eurasian tree sparrows are sufficiently distinct to be unique species, for instance, both species are further divided into multiple “subspecies,” or unique subgroups that might inhabit different regions and/or possess slightly varying traits. The boundaries between one species and another, as well as between species and subspecies, are frequently drawn on a case-by-case basis. Sometimes, this process is undertaken by larger scientific organizations, such as the American Ornithological Society; other times, individual scientists assume this responsibility.
To date, scientists have identified and described approximately [Insert Number] species. However, estimates of the total number of species, including those yet to be described, existing on Earth can vary wildly. While some scientists posit that there are likely around 9 million species in total, others believe the number is closer to 100 million, or perhaps around a billion—or even a trillion.
These distinctions may appear arbitrary, but classifying species can be essential for conservation efforts. Protecting the saltmarsh sparrow (*Ammodramus caudacutus*), for instance—whose population has dwindled due to the destruction of saltmarsh habitat in the eastern U.S.—will require a very different approach from safeguarding house sparrows, which are abundant. But perhaps most crucially, classifying individual species simply assists us in comprehending and appreciating the diversity of life on Earth. By grouping organisms that exhibit distinct appearances and behaviors into separate categories, we can study those traits, Reddy said.
