1.2 Definitions of Species

John Ray (1628-1705) made the first systematic attempt to classify living organisms. He used the criterion of the similarity in the form and structure (morphology) of the seeds of organisms to classify the individuals belonging to a species. He argued that one species could never spring from the seed of another (1).

Carolus Linnaeus (1707-78) elaborated Ray's ideas and developed a system of generic and specific names for all known plants and animals. He adopted Ray's criteria of the morphological form and the possibility of producing a fertile offspring to arrange members into the smallest units of taxonomy and called the units species. Linnaeus adhered to the concept of the fixity of species. It states that there existed at that time just as many species as God had created in the beginning.

Linnaeus's species concept stimulated the cataloging of different varieties of organisms by a multitude of collectors and taxonomists. Soon the number of species became enormous, producing a refinement in the Linnaean classification scheme. Linnaeus in his later work found so much difficulty in defining species that be questioned whether it might not be the genera (e.g., oak tree) rather than the species (e.g., white oak) that were separately created. His ideas were challenged by Buffon and Lamarck, who set the stage for Darwin's Origin of Species. Today, although the taxonomic system of Linnaeus is still being used, his scheme of classification has been repeatedly revised.

According to Mayr (2) there are three species concepts currently being used:

1. The Typological Species Concept. According to this concept, if two organisms are morphologically different, they are considered two distinct species. Using this criterion, even two organisms in the same reproductive community that show only slight morphological differences from one another would be two distinct species.

2. The Nondimensional Species Concept. This concept fixes the individuals of a species as those found at a single locality (sympatric) and [31] occurring at the same time (synchronous). The separation of a species from another by space and time is emphasized. However, sometimes the territories of species are not obvious, and ambiguity is introduced in defining local population.

3. The Interbreeding-population Concept. The criterion of interbreeding between two populations is used to determine a species. It considers species as a group of individuals that actually or potentially interbreed with each other. This concept has the advantage of being multidimensional in that populations occupying different geographical regions or living in different time periods are classified on the ability to interbreed. The difficulty, however, of the practical application of "potentially" interbreeding is apparent; nevertheless, this concept provides an operational definition of species.

The most widely accepted current definition of species among scientists represents a synthesis of elements from all three of the above concepts, namely (3):

1. Species are defined by distinctness rather than by difference.
2. Species consist of populations rather than of unconnected individuals.
3. The decisive criterion of the classification of species is the reproductive isolation of populations rather than the fertility of individuals within the population.

Differences between two populations may be very subtle, making it difficult to determine whether both are distinct or the same species. For example, if two members of the genus Drosophila were brought together and they failed to produce fertile offspring, they would normally be classified as two separate species. However, the reproductive isolating barriers both premating and postmating have to be considered.

Premating barriers prevent the mating of two individuals. These barriers may be in the forms of (1) habitat isolation, the different preference of habitat of two populations; (2) seasonal isolation, the difference in breeding seasons; (3) ethological barriers, the incompatibilities in mating behavior; and (4) mechanical isolation, the structural differences in genital armatures that prevent mating. In contrast, postmating barriers prevent gene exchange in the offspring of the two individuals after mating has occurred. They include (1) gametic mortality, the insemination reaction [32] that kills the sperms; (2) zygotic mortality, the irregularities of the development of the zygote that leads to its abortion; (3) hybrid inferiority and sterility, the genetic incompatibilities of the hybrid that either impose a selective disadvantage on the hybrid in mating or cause it to be sterile (4).

The criterion of interbreeding cannot be applied to organisms that reproduce asexually only. Attempts to categorize microorganisms by morphological standards alone have not been adequate. Many other criteria had to be employed in the taxonomical studies of asexual haploid organisms, and no consensus has been reached as to the best solution for these difficulties.
The application of the biological species concept to paleontological collections is also a difficult task. Since fossil specimens cannot interbreed, other criteria are used by the paleontologist to assess the taxonomic status of natural populations that became fossilized. A certain degree of subjectivity has to be invoked in the classification of fossils. Paleontologists have to rely on not only morphological, but ecological, stratigraphical, and distributional evidence to arrive at a probable species identification of fossil organisms.

Notwithstanding all these difficulties, the advantages of having a nonarbitrary definition of a biological species far outweighs its shortcomings, and the biological concept of species is widely accepted as a working definition in classifying the living world.

1. Bedall, B. G. “Historical notes on avian classification.” Systematic Zool. 6:129-36; 1957.
2. Mayr, E. Animal species and evolution. Cambridge, MA: Harvard Univ. Press; 1963: 16-30.
3. Mayr, E. Animal species and evolution. 20.
4. Mayr, E. Animal species and evolution. 92-106.