Various criteria by which substantial reproductive isolation between populations is assessed.
The concept of species is much more easily appreciated intuitively than it is to define. The root of the resulting conflict is that there is a difference between distinguishing organisms into phenotypic categories, on the one hand, and understanding why organisms tend to be distinguishable into phenotypic categories on the other.
The truth is that organisms are not universally easy to differentiate into separate phenotypic categories. We thus have various species concepts which attempt to provide both rules for distinguishing populations into separate species while at the same time providing some basis for what it means for two populations to in fact represent separate species.
Species concepts include:
See also the pluralistic species concept. Consider too that much of modern systematics is sequence based, which really does not readily fit into any of the above-listed species concepts.
Note that these various species concepts are not mutually exclusive but instead represent different ways of looking at the same question: What is a species?
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