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Now, there's an alternative species concept, called the phylogenetic species concept.

It says that these phylogentic species are the smallest diagnosable cluster of organisms within which there is a parental pattern of ancestry and descent. Now, what that means is that it has all three key elements of a definition. Smallest is the level at which you rank, so those populations that are diagnosable are ranked as species. And they're diagnosable. There are some characters that tell you that this population is different from that. We don't say how many characters there should be or what kinds of characters they should be, but they need to be constant characters or relatively so. You need to be able to say yes, that's different from that, that's different from that.

Finally, there's also reproductive cohesion. Reproductive cohesion in this instance is this phrase "parental pattern of ancestry and descent." All that means is males and females are not going to be placed in separate species. There's reproductive cohesion.

This concept is not about individual organisms. We are all genetically different from one another, we are not in separate phylogenetic species, because that phrase about reproductive cohesion says this is a population phenomenon we're dealing with. We're trying to take populations and decide, are they one species or two species or three species.


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