Full question: How come some similar animals are different species, while with domestic dogs, wildly dissimilar types are considered different breeds?
-- Z. Kornberg, Jerusalem
Michael Bruford, a professor of biological sciences at Cardiff University in Wales, explains the thought process behind this seeming double standard.
Scientists have been distinguishing between species on the basis of how they look, behave or live since recorded history began. However, two famous scientists stand out in terms of how we perceive species differences today: Carl Linnaeus, an 18th-century Swedish naturalist, and Charles Darwin. Linnaeus was the first person to formulate a single approach for describing species in a hierarchical manner according to their similarity, using his binomial nomenclature of genus followed by species (Homo sapiens, for instance). Darwin was among the first people, and certainly the most celebrated among them, to develop a credible theory on how species evolve (via natural selection). Both of these scientists' insights underpin most of what modern science utilizes when studying species and speciation.
Currently species are still primarily distinguished by their appearance, but it is becoming very clear that looks don't always tell us all we need to know about whether two organisms are different. Many so-called cryptic species exist that, to the untrained eye, look very similar or even identical to another organism--commonly found, for example, in nocturnal mammals such as bats and bush babies. If the two mated, however, they may never be able to produce viable offspring; this, in fact, is the primary criterion for dividing similar organisms into different species. Because of these red herrings--and also because the process of describing species is very long and labor-intensive--scientists are increasingly turning to DNA to assist them in identifying and describing species.
Indeed, there is an endeavor under way at the moment called the Barcode of Life project, which aims to sequence all living organisms for a single gene that is common to them all, to produce a species "bar code." The key is that the sequence must vary greatly among species but not vary much within species. Such a bar code can then be used to identify organisms which may not be easily identified (such as tracing back what primate was the source of mysterious smoked meat in the rainforest) and even to distinguish organisms such as microbes that we cannot see or culture in the laboratory. There has been much debate among scientists about which DNA sequence is best for this purpose, and it is likely that a different sequence will work for each different kingdom of organisms. Currently a small gene found in the mitochondrial DNA of our cells--the cytochrome oxidase subunit 1 gene--is most commonly used in identifying members of the kingdom Animalia; a huge database is already in place for many of the world's animals using this gene.
Domestic animals fascinated Darwin and continue to enthrall those of us who own pets. It is certainly curious how domestic dogs, which we know--because DNA bar coding has told us!--were raised by man from a wild gray wolf (Canis lupus) ancestor, can take on such a dramatic variety of forms. But among dogs, which are well known for their hybrid (or mongrel) varieties, different breeds can mate and have viable offspring, so they are all found under the umbrella of a single species, Canis familiaris.
Dogs are highly unusual in their variation, from the Chihuahua to the Great Dane. (Recently, body size was found to be largely explained by differences in a single gene among dog breeds.) Darwin realized that man can force selection by picking particular individuals for breeding who show a particular characteristic that we want to see in our pets. So humans can accelerate the process of selection dramatically by exploiting the diversity naturally found in domestic forms and homing in on a form that is desirable. Natural selection usually acts more slowly, relying on what Darwin described as "descent with modification"--the chance arrival of new forms through DNA mutation.