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Concept of Species
The gradual changes shown to you in this tour so far, sexual reproduction, gene flow and genetic drift all contribute to a result: species. Because natural selection favours different characteristics in different organisms and thus species are created.
Similar looks is the common basis for identifying individuals as being from the same species. However, there are several exceptions including, a terrier, and bulldog and a golden retriever being very different in appearance. But whilst all dogs can interbreed with other dogs and their species, people cannot interbreed with different species. This ability to interbreed with similarity bring us to a point: there are significant biological similarities which enable interbreeding between these organisms. This interbreeding ability determines species as independent units and thus genetic changes, which occur in single individuals can only spread to other members in their own species.
According to the criterion for deciding species, individuals who cannot interbreed are not considered under the same species. However, the same species in different locations may not be able to naturally interbreed and thus adds ambiguity to the practice. Two separate populations might have been a single specie once but was separated for some reason or another. As time passed, this population diverged to form two separate populations.
Organisms living during different eras also pose a problem. We cannot confirm that humans living now would be able to interbreed with humans living then and thus biologists would classify humans into to species namely, then and now.