What are the primary facts that one must consider
when examining organisms that are obviously closely related in
determining whether or not they should be in a single or separate
species? For example, the blue heron of North America (Ardea
herodias) and the grey heron of Europe and Asia (Ardea cinerea) are
treated as separate species even though there is considerable
argument that they should be treated as a single species. How
should one weight the merits of cases similar to this? Even though
in this example it is obvious that the two species have been
separated by geography (allopatric speciation) but it does not seem
as though that alone should render them into two distinct species.
Well, the species concept is one of the most hotly debated in science.
After all, it is a man made concept. The organisms don't know what species
they are supposed to be in. The wolf and dog are another good example.
Modern genetic analysis nests them together in some cladograms, which some
think means they really are not separate species. And the most commonly
accepted definition is that if they can breed and produce fertile offspring,
they are considered the same species. Wolves and dogs can still
interbreed. It's possible we are watching a speciation event in progress.
Speciation doesn't happen overnight. There has to be something that
prevents interbreeding, such as the fusion of chimp chromosomes to become
one, which some use as evidence of speciation between humans and chimps.
The herons of which you speak may be assumed to be separate species because
it is unlikely they will be able to interbreed because of the distance
between them. One could bring two together and see...
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Update: June 2012