Ask A Scientist Molecular Biology Archive

name         Sandra F.
status       educator
age          40s

Question -   Hello.  I thought that the definition of species was a
population that can interbreed.  Why, then, are dogs (Canis familiaris),
wolves (Canis lupus & rufus), and coyotes (Canis latrans) considered
separate species if they can interbreed?  The offspring are not
infertile, as in the case of the mule.  Thank you.
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The inexactitudes of taxonomy and science.

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
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There is always an exception to every rule isn't there?  Dogs and wolves are
so close to each other in evolutionary time, i.e., their common ancestor was
very recent, that apparently they can interbreed and have fertile offspring.
Much of this has to do with the chromosome number and being able to match
each chromosome during meiosis.  Horses and donkeys can produce mules because
there isn't a problem at fertilization but the mule offspring are left with
an odd number of chromosomes.  So when and if two mules tried to mate, at
fertilization, there would be a problem matching genetic information.  The
offspring would fail to develop.

vanhoeck
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Your query is important.  However, one must realize that the definition of "species" is man-made, much like the dog!   The definition is full of holes and certainly seems inconsistent.  You only mention one of many other striking examples, but there is a catch to the definition.....  
One rule of thumb that is generally recognized relates to the domesticated animals, in that, they don't count!   Man has messed up natural selection with artifical selection and the dog would not be found in North America if it were not for their human connection.
It has been found that several other species have been breed with distant relatives that would not normally be found in the same location.  Thanks to man again!  You migfht recall the attempt to develop a cat-dog called a Cog several years ago.
Ok, the definition of species usually includes a statement that refers to "naturally occurring" and is meant to exclude human interference so that is how your question has to be answered.  I hope this all helps some.

Steve Sample
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I am not a geneticist or taxonomist, so you may get a very different answer
from those perspectives, but from this naturalist's view, species are really
constructions of the human mind, just like language and other systems of
thought, that allow us to talk about and make some kind of sense of the
world. Through the process of adaptation and evolution relationships between
organisms are constantly in flux, so that some "species" are in the process
of splitting, while some populations that had become separated may be back
in situations where they can interbreed. "Species" cannot come into being
instantly, so there is always a process - evolution - going on. In other
words, nature is not nearly so neat as we sometimes make it out to be.

J. Elliott
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