In Which We Get Long-Winded About the Minutia of Speciation

    Speciation is...icky. 

    It's impossible to address the speed of speciation without first addressing which definition of species you're using. Oxford defines a species as "a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding." 

    Definition 1. 

    A species is something that needs to be able to be able to breed within the population and exchange genes. Generally this is the accepted definition that is the "scientific" one, tacking on the ending additive that the offspring also needs to be able to do the same (i.e. the offspring also needs to be viable and fertile). 

    What happens, though, when something technically can reproduce, they just...don't? How do you determine which one is the species if two things look holy cow exactly the same but they're two very different things? What if two things can reproduce and create perfectly viable offspring, they look vastly different, and they're in two different parts of the country?

    And that's why I've decided that speciation is icky. 

    For simplicity sake, we're going to stick with the Oxford definition, ignoring the morphological species concept, phylogentic species concept, and anything else that might pop up. If they can reproduce and make viable offspring, they live in the same area or moved there on their own (no one put them there artificially), and can pass on genetic information, then we are going to consider them a species.

    The act of speciation itself can be broken down into three distinct scenarios:

    Allopatric (in which we are also including Peripatric)

    Perapatric

    and Sympatric

    I believe that the speed of speciation will often depend on which mode has come into play as well as the strength of selection required to maintain any species within the given environment. For example, in the case of allopatric speciation, there is a physical barrier separating the two populations; be it a river, mountain, man-made structure, chasm, or anything else. In some way, something is there preventing the mingling of individuals and subsequently the transference of genes. A physical barrier might be seen as just the ting to increase the speed of speciation, or how fast Species B (the population now separated) becomes so different from Species A (the original species) that even if the barrier was removed they would no longer be able to create viable offspring. During the formation of the Grand Canyon there was a separation of the squirrels and other small mammals that resided there, eventually leading to the formation of two distinct species, one on either side. However, looking at the environment of the Canyon itself as well as how long it would have taken for the chasm to become so large that it was virtually impassable, a question stands: what is the actual time frame between Species A splitting to become Species A and B? At what point would it still be possible for someone to transplant squirrels from one side to the other and they would be able and willing to breed with one another? Each side of the Canyon is fairly similar in environment (though, granted, not identical; hence the selection for change in the first place). This would lead me to believe that allopatric speciation is one of the slowest and easily reversible of the three in many cases. 

     Perapatric speciation is similar in many ways to allopatric bar something physical keeping the two populations apart. In this type of speciation the physical barrier is replaced entirely with some sort of divide between two ecological niches. I like to thing of species like the Dark-Eyed Junco regarding situations like this. 

    Juncos and Cedar Waxwings can be found in the same area: both part of Order Passiformes, both preferring lightly wooded areas, etc. 

But, there will never be a case of a Cedar Waxwing and a Dark-eyed Junco suddenly eyeing each other up and deciding to create some sort of Dark-Wax hybrid. Ignoring the more-than-likely morphological preferences, the Juno and the Waxwing are in different areas of the same environment, therefore evolving to eat different types of food and fill a different niche than the other. They might be found in the same forest, but Juncos are often found hopping along the ground, in and out of the tree-line on the edge of a forested area, picking what seeds they can from the floor. Waxwings, on the other hand, are always seeking out berry and fruit bushes, looking for places where the soil and sunlight are good enough to propagate their meals. In this way, there's nothing "physically" stopping them from intermingling and possibly even breeding, but they just...won't. They might meet and cross-paths, but one is more apart from the other. In this way, since selection would be taken into account, I would say this is the middle fastest of the three. There needs to be selection at play to keep the species separate compounding to the minimal amount of distance, but early I would assume there would be a higher chance of hybridization or maintaining species continuity given the populations have more option for "gene flow."

    The final scenario for speciation is sympatric speciation, in which two species divulge from one while still within the same geographic area. I would have to argue this to be the fastest of the three. Since there is no physical barrier separating the two, if selection begins to act upon the population and a separate spices is in the process of emerging, that selection has to be incredibly strong. Something has to have occurred: some sort of mutation or new niche in the area, in order for there to be any selection in a differing way at all. Selection has favored something that has been "deemed beneficial" to fill that new niche or refill an old one and in order for there to be two species instead of one there needs to be an incredible amount of selection. The population is still only one, the individuals are still intermingling, possibly interbreeding at times, there is nothing stopping the flow of genetic material. It needs to act hard and fast. Along with this, since we're assuming there's some reason that selection has begun to act so strongly (though, we accept that it could very well be arbitrary IRL), we would assume there would also be some selection for speed as well (fast mating, fertilization, reproduction, fledgling, etc.) in order to maintain the niche that was "deemed beneficial" and was selected for in the first place. For these reasons, I believe that if there are cases of successful sympatric speciation it would need to be the fastest by nature. 

Links

https://www.nationalgeographic.org/encyclopedia/speciation/

https://examples.yourdictionary.com/examples-of-speciation.html

https://www.allaboutbirds.org/guide/Dark-eyed_Junco/lifehistory

https://www.allaboutbirds.org/guide/Cedar_Waxwing/overview