From What is a species? by Stephen Jay Gould
But in almost all cases large-scale evolution is a story of branching, not of transformation in a single objective division. One species rarely turns into another by total transformation over its entire geographic range. Rather, a small population becomes geographically isolated from the rest of the species-and this fragment changes to become a new species while the bulk of the parental population does not alter.
reproductive isolation; isolating mechanisms: genetic programs, behaviors
By grasping the objective status of species as real units in nature, we may better comprehend the moral rationale for their preservation. You can expunge an arbitrary idea by rearranging your conceptual world. But when a species dies, an item of natural uniqueness is gone forever.
There are no pieces left when the last dodo dies.