Is there a difference between “Divergence” and “Speciation”?
To give more context, is it possible for two populations to diverge without becoming new species? Yes, we are close to the weeds here about the meaning of species.
For example, we can imagine two populations diverging (genetically and/or phenotypically), but still hybridizing at times at a low rate. We can imagine that conditions change and they start hybridizing a lot more, fully rejoining into the same population before a new species ever forms.
It seems that divergence and speciation are separable concepts. Am I missing anything here?
No, you’re right. They are. You can get divergence without speciation. As long as gene-flow is maintained you can define members of a population as the same species, and you can have the entire population diverge increasingly over time under conditions of relaxed selection or w/e, so everyone goes from being more to less genetically similar over time.
This plunges us into the mire of species concepts. For the phylogenetic species concept, separate species are defined if there is any diagnosable difference between populations. For the biological species concept, there must be a genetic isolating mechanism that would keep the populations from merging if in sympatry. Allopatric populations will inevitably diverge, and will probably after a fairly short time become different phylogenetic species. But they won’t necessarily be unable or uninterested in interbreeding if they become sympatric.
Speciation (of the biological sort) can fail to happen after considerable genetic divergence, or it may happen after no easily detectable genetic divergence, perhaps as a result of a few mutations at a few loci.