In the classification of living terrestrial organisms, the bottom classes are known as "species". There is a species class for all the horses and another species class for all the squirrels, and so on. Speculation has it that there are 50 to 100 million different species of organisms on planet earth. We humans have assigned names to a few million species, a small fraction of the total.
It has been argued that nature produces individuals, not species; the concept of species being a mere figment of the human imagination, created for the convenience of taxonomists who need to group similar organisms. Biologists can collect feature data such as gene sequences, geographic habitat, diet, size, mating rituals, hair color, shape of skull and so on, for a variety of different animals. After some analysis, perhaps performed with the aid of a computer, we could cluster animals based on their similarities, and we could assign the clusters names, and the names of our clusters would be our species. The arbitrariness of species creation comes from the various ways we might select the features to be measured in our data sets, the choice of weights assigned to the the different features (e.g., should we give more weight to gene sequence than to length of gestation?), and to our choice of algorithm for assigning organisms to groups.
For myself, and for many other scientists who use classification, there can be no human arbitrariness in the assignment of species (1). A species is a fundamental building block of the natural world, no less substantial than the concept of a galaxy to astronomers or the number "e" to mathematicians.
The modern definition of species is "an evolving gene pool." As such, species have three properties that prove that they are biological entities.
1. Unique definition. Until recently, biologists could not agree on a definition of species. There were dozens of definitions to choose from, depending on which field of science you studied. Molecular biologists defined species by gene sequence. Zoologists defined species by mating exclusivity. Ecologists defined species by habitat constraints. The current definition equating species with an evolving gene pool serves as a great unifying theory for biologists.
2. The class "species" has a biological function that is not available to individual members of the species; namely, speciation. Species propagate, and when they do, they produce new species. Species are the only biological entities that can produce new species.
3. Species evolve. Individuals do not evolve. Evolution requires a gene pool; something that species have and individuals to not.
Species bear a biological relationship to individual organisms. Just as species are defined as evolving gene pools, individual organisms can be defined as set of propagating genes living within a cellular husk. Hence, the individual organism has a genome taken from the pool of genes available to his species.
The classification of living organisms has worked a true miracle, by breathing life into the concept of species, thus expanding reality.
 DeQueiroz K. Ernst Mayr and the modern concept of species. PNAS 102(suppl 1):6600-6607, 2005.
- Jules Berman (copyrighted material)
key words: classsification, ontology, species, speciation, jules j berman