Are different dog breeds different species? Why or why not?

They are the same species.

Explanation:

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.

the answer to your question is:

c. complementary base-pairing