The concept of biological species defines species as populations that can interbreed and are isolated from other species reproductively, thus making different groups that are unable to interbreed with such other species. Mayr also defines biological species as “a reproductively cohesive assemblage of populations” (222). One species can be separated from another geographically, for example. It should be however noted that species differ not only in the areas of their habitation but also in behavior (different mating rituals and processes), genes (differences in chromosomes), physiology (body mass, shapes of genitalia, etc.) that isolate them from each other (or, in return, make interbreeding more likely/possible). Thus, species that have similar physical features, mating rituals, genes, and behaviors (like the dog and the wolf or the tiger and the lion) can interbreed, and their offspring are usually fertile (wolfdogs and ligers are the examples of such interbreeding).
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At the same time, the species that differ in these characteristics or features too drastically are rarely able to interbreed and have fertile offspring, if any offspring at all (for example, cats and bears). Another prominent example of such interbreeding that does occur but leads to sterile offspring is the mule. The mule is an offspring of the horse and the donkey, and while these species do have a common ancestor, their differing number of chromosomes (64 in horses versus 62 in donkeys) does not allow them to have fertile offspring. The mule is sterile and has 63 chromosomes (Yilmaz et al. 450). Thus, differences in behavior, genes, and physiology often define whether species can interbreed or not.
Paleospecies are “species defined from fossil evidence, often covering a long time span” (Jurmain et al. 107). Scientists can observe the reproductive behavior of different species that are modern and not extinct, but it is impossible for those species that are researched only through their remaining fossils (i.e., those that are extinct for a long period of time). When fossil groups are studied, the variations between them are specifically taken into consideration. It is critical to understand whether these variations are intraspecific (differences seen in every biological species) or interspecific (differences between reproductively isolated groups) (Jurmain et al. 107). If we had fossil remains of the wolf and the dog and tried to understand whether these species were different or the same, we would compare the variation between these fossil samples to those present within species of closely related forms (Jurmain et al. 107). If these differences were comparable, we would not split the species. Thus the wolf and the dog would be probably perceived as one species.
It should be noted that the differences between paleospecies are complicated to study because these groups differ not only spatially but also through time. Thus, variations seen in paleospecies often make the distinction between species more difficult exactly due to this separation by thousands or millions of years. The ability of researchers to make distinctions between species (or refrain from making any) depends on the quality and number of fossils that remained from a species (or a group of those). As biologists understand that boundaries between species might be vague or too unclear to identify correctly, it is also possible that the wolf and the dog would be seen as distinct species rather than one. It is likely that their fossils would be compared to other (“modern”) species to understand whether the biological variations of the dog and the wolf indicate that the sample should be split into two different species. Thus, our ability to understand the difference between paleospecies also relies on our understanding of modern species.
Jurmain, Robert, et al. Essentials of Physical Anthropology. Cengage, 2017.
Mayr, Ernst. “A Local Flora and the Biological Species Concept.” American Journal of Botany, vol. 79, no. 2, 1992, pp. 222-238.
Yilmaz, Orhan, et al. “Phenotypic Characteristics of Turkish Mules.” International Journal of Agriculture and Biology, vol. 14, no. 1, 2012, pp. 450-452.