A species is a population of animals that possess common charactoristics and freely interbreed in nature and produce fertile offspring." Audubon field guide



or Biological Species Concept. The problem with that species definition is that it doesn't work for many (most?) living organisms. How do you apply this to distinguish two closely related species of asexual organisms, for example? There is no interbreeding.

How do you decide if Cal Kings and Eastern Kings are the same species? Are they potentially interbreeding? Certainly not in the wild.

How do you apply the BSC to determine fossil species?



There are other (hotly debated) species concepts that are more popular, such as the Evolutionary Species Concept, Phylogenetic Species Concept, the Paleontological Species Concept. They all have flaws that can be pointed out. (As soon as anyone reads anyone else's species concept, the first thing everyone says is - What about _______? This concept doesn't work for those species!)



Here is a list of some of the many species defintions that have been penned.



As for Heterodon, it doesn't really matter what concept you apply. The issue is that kennerlyi and nasicus don't seem to interbreed much at the limited areas of contact between the two species. You find good nasicus right up to edge of the range of good kennerlyi. There are occasional individuals that show intermediate characteristics, but generally snakes from the contact zone are either one taxon or the other.



Therefore, we can infer that there must be some sort of isolating mechanism keeping these snakes from interbreeding at the contact zone (even if the mechanism is the historical ranges). kennerlyi from West Texas look more like kennerlyi from south TX and Mexico than they do from snakes from populations of nasicus less than 100 miles away. Clearly they are maintaining some sort of genetic identity.



Contrast that with the large intergrade areas you see between subspecies (black/yellow rats, eastern/florida kingsnakes).



This is largely the basis for recognizing both hogs as independent evolutionary lineages and therefore separate species.
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Chris Harrison

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