Current evolutionary theory claims that all life is historically descended from a single starting organism or group of organisms (but see reasonable dissenters here). If all species are ancestrally related, there are probably not any distinct boundaries between species if we take a really long view (from the viewpoint of hundreds of millions of years).
Yet, looking at things over just hundreds of years, species are almost completely divided into what seem to be natural kinds of life: an ohia lehua tree seems a very different kind from a tiger shark, though both are alive.
Where species boundaries break down in the present is when we look for ways to determine when one kind of ohia tree is the same species as another ohia tree. Often, as soon as we establish criteria for a particular species of tree, we find a tree which fits just outside our definition. At such times, is this a new species, or just a variant within the species?
Simple essentialism about ohia species does not work.
What kind of universals can be used, then, for species? Vague ones. If physics holds a core of unavoidable uncertainty -- for example, we cannot sometimes know where a particle going at a known velocity is located -- why should we expect biology to have any absolutely working definition of an empirical concept such as where the line between ohia lehua species can be located?
Vagueness is not just an artifact of our language. It is a feature of the world we see.
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