One of the most common questions asked about how speciation works (not asked here, though, grumble grumble. Go ahead and comment, people!) is how it is possible for a new species to arise. After all, when the first member of a new species shows up, it has nothing to breed with, right?
The following material is flagged Green Level. It is intended to reflect material that the author believes to be a matter of consensus among experts in the field. This belief may be incorrect, however; and as the author is not an expert and does not have an expert fact-checking the article, errors may creep in.All (or most, anyway) of us were taught that every living thing belongs to a species, such as H. sapiens, G. gorilla, or E. coli. Everything in the same species can breed with anything else in the same species (barring things like sex differences), and with nothing of a different species. Things within the same species are similar, things of different specie are just too different. If we were to draw a diagram of species, with each species represented by a circle and overlapping circles representing populations able to breed, it would look like this:
Except that this is a lie to children. It is an oversimplification, exchanging truth for comprehensibility.
It is a description of how things generally work, that omits important details for the sake of being easily understood.
If you were to look at a human being on all scales of resolution, from the anatomical to the cellular to the genetic to the quantum, no part of the human would be stamped with the words "Homo Sapiens". And there is no generation in the future where a hypothetical stamp would magically change to "Homo Successor", barring radical genetic engineering.
In fact, there is no magic date before which humanity, or what would eventually become it, was always H. florenseis and afterward was always H. sapiens. There is a generation before which nothing was H. sapiens, if we define H. sapiens as anything that would biologically compatible with the humans that exist today if somehow preserved, but that generation did not suddenly transition to H. sapiens. Rather, if we redefine H. sapiens as anything compatible with the first generation to be compatible with modern humans, the generation before was H. sapiens, even if modern H. sapiens would not be compatible with it.
In short, the question stems from an artifact of the mental models we use. We see each creature as being stamped with one species, even though this only crudely approximates the underlying reality. And as such, we see species as something that can be broken cleanly into separate, quantized chunks, when in reality it looks something more like this:
(And please, don't ask me about different chromosome numbers. I don't quite understand how it works, but apparently chromosomes are, to a degree, arbitrary divisions within the genome and the place where one chromosome stops and another starts, if it even works that way, can be rearranged.)