4.2.2 Defining a Species

While species is a word that most people are familiar with and comfortable using, just what determines a species is incredibly difficult to define. At the most basic level, a species comprises a group of organisms with shared characteristics that distinguish them from other groups. Most scientists distinguish a species based on behavior, genetics, and/or morphology. Species definitions are the basis for scientific names. The common name of a species, on the other hand, is usually based on general physical characteristics noted by a culture or local population. Common names are also referred to as folk taxonomy or ethnotaxonomy (classifications influenced by culture, etc.). There is a growing interest among anthropologists and the scientific community in preserving Indigenous classifications of the natural world and connecting them with scientific classifications.

Decisions related to classification often involve tremendous taxonomical controversy, especially within the field of biological anthropology. There are more than 20 distinct species definitions, or ways of categorizing or distinguishing one type of organism from another. Below are the four most common definitions of a species.

Biological Species

The biological species definition states that a species is a group of interbreeding organisms that are reproductively isolated from other groups of organisms. Reproductive isolation means that members of a species are not able to mate successfully with members outside their species. Gorillas, for example, cannot successfully breed with Pan paniscus, the bonobo. The biological species definition uses the ability to interbreed as its foundation because successful mating leads to gene flow, or the movement of genetic material from one population to another.

Ecological Species

The ecological species definition emphasizes the role of natural selection in maintaining species boundaries. This concept is based on the idea that gene flow is neither necessary nor sufficient to maintain species boundaries. Instead, natural selection plays an important role in maintaining the boundaries between species. In nature, species boundaries are often maintained even though there is a substantial amount of gene flow between species. Gene flow between species generally occurs at places called hybrid zones, areas of overlap where two species are known to successfully breed. A classic example of a hybrid zone occurs on the island of Sulawesi in Southeast Asia, where Macaca maura (the moor macaque) and Macaca tonkeana (the Tonkean macaque) are known to have successfully interbred for more than 150 years. Despite this, the integrity of the two distinct species has been maintained.

Phylogenetic Species

The biological species definition is based on breeding behavior, specifically whether species are capable of mating with one another. This foundation is problematic when trying to identify species over time. It is hard to know whether two fossil specimens were capable of interbreeding. It is also difficult in the fossil record to distinguish between interspecific variation (differences between members of two different species) and intraspecific variation (variation within a species). Imagine finding the bones of two individuals, one five feet tall and the other six feet four inches. Identifying whether these individuals were members of two different species (interspecific variation) or representative of the normal variation within a given species would be extremely challenging.

These problems are addressed by the phylogenetic species definition. The phylogenetic species definition states that a species can be determined by shared possession of one unique characteristic. For example, imagine you found a group of fossil leg bones. In order to decide if they were from the same species, you would need to determine if they had a trait in common that only these fossil leg bones possessed. If the bones all possessed trait A and this trait was not found in any other species already identified, then you would have a new species, and all of the fossil leg bones could be placed in that species.

Mate Recognition Species

The mate recognition species definition states that a species is a set of organisms that recognize one another as potential mates. A classic example of a group of species that can be distinguished using this definition is American crickets. Within a single habitat in the United States, there might be over 30 different species of crickets. Each species of cricket is known to produce a distinct song. Despite all these different species living side by side, the female cricket of each species will only mate with a male after hearing the male sing her species-specific song. The song, and the female recognition of it, constitutes a mate recognition system. This is analogous to the biological species definition in that the song acts as a reproductive isolating mechanism.