Speed of Speciation

     I'd like to start this out by saying that I think defining a speed at which speciation can occur may inevitably be futile. Not just in the fact that it most certainly depends on the context of each genus/ancestral species in question, but also in that our defining of species is arbitrary even in our most specific of definitions. Because, as always, nature doesn't give a flying hoot about what is a species and what isn't. But of course, our conceptualizations of species is important in the way that we as humans steward and manage populations and communities of organisms, so while it may be arbitrary it certainly can serve a pragmatic use.

   Anyways, this discussion on speciation speed will be something of a paper review, A paper by Hendry, Nosil, and Rieseberg from 2007 did a review seeking to understand an ecological perspective of evolution involving both contemporary evolution and ecological speciation. Most importantly, I think, they examine how adaptive divergence causes reproductive isolation (their definition of ecological evolution); this being a cornerstone to understanding species in a most objective way possible. An important note is that this study doesn't consider reproductive isolation to be zero gene flow between two populations but rather a significant reduction in gene flow. Just as a comment, I think this is very important because it goes back to how we as humans can't seem to solidly define reproductive isolation, at least most of the time.

    They first note how speciation seems to occur very first when their is a single locus or group of pleiotropic loci determining mating between individuals: a so called "magic trait." In these cases ecological separation can occur within well less than 100 generations. Dispersal rates of populations also seems to play a potentially important role: new populations colonizing new habitats are likely to experience bursts of adaptation. Selection against migrants in these newly colonized environment is also important as it significantly reduces inter-population gene flow.

    The next major thing looked at is mate choice. Disruptive selection can encourage assortative mating which can lead to reproductive isolation within a couple hundred generations. Likewise, adaptive speciation, where competition for limited resources forces evolutionary branching, can cause a similar mechanism of reproductive isolation. Reinforcement, where two divergent populations come into secondary contact, can also rapidly evolve mating preference to avoid maladaptive assortment of mating. Hybrids are looked at in this paper, although it seems that at the time the way they interacted with gene flow wasn't very well documented and didn't prove useful to this paper's goal, as the results don't seem very conclusive.  They do mention how hybrid species can quickly become reproductively isolated when they colonize new habitats and quickly adapt to it.

    Plants, especially, seem to have an immense capacity for speciation through dispersal of hybrids. A certain species of sunflower, Helianthus anomalus, based on genomic evidence, seems to have evolved within about 60 generations. Invertebrates, specifically insects, may also have a quick divergence due to hybrids as shown by a fly species, Rhagoletis sp. The authors claim that reproductive isolation may be almost immediate when a hybridization introduces a phenotype that automatically separates potential mates. In a certain species of salmon, Oncorhynchus nerka, after only 14 generations significantly reduced gene flow had occurred between two locally adapted populations.

    So, all in all, it seems that the rate of speciation can be quite rapid in the grand-scale of things, especially at a more microlevel and with certain softened qualifications for reproductive isolation. At the very least, within 100 or so generations seems to be somewhat of an "average" consistent across all the examples looked at in this review. I think it's also worth noting, that once reproductive isolation occurs (after 100 generations or what have you), then a more "recognizable" speciation is apt to take place as new selective pressures and lack of gene flow create ideal conditions to reproductively diverged populations to adapt and evolve different characteristics.

Resource:

(Hendry, Nosil, and Rieseberg, 2007): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605086/