Happy Sci-Day, everyone! This week will be somewhat related to last week's topic, as well as topics that I will be covering related to evolutionary biology in future posts. The topic for today is speciation - basically, the formation of new, distinct species.
The reason this relates to last week's post is because in order to understand how a 'species' is formed, we have to have some idea of what a species actually is. Speciation basically involves the splitting of an original, single population of organisms into two new populations, each of which is reproductively isolated from the other. Through some process, whether that is biological or abiotic, this isolation means there is no gene flow between the two populations, leading to independent evolution in each new population. Over time, mutations build up in each lineage, and they become more and more distinct (This could be purely on a genetic level - there don't necessarily have to be any evident morphological differences).
There are two main modes of speciation, defined by the process that split the original population. The first mode, and the one that is the easiest to identify in paleontology, is called allopatric speciation. In allopatric speciation, the ranges of the two new populations do not overlap - one example would be a large mountain range rising within the range of some species, where populations of that species on one side of the range are isolated from the population on the other side due to geographical factors - the species cannot cross that new barrier and as such the population is split.
A second type of speciation that is a bit harder to examine, especially from a paleontological standpoint, is called sympatric speciation. Unlike allopatric speciation, the ranges of the two new populations are not separated geographically - the two populations at least partially overlap at some part of their range. Since observing sympatric speciation relies primarily on genetic/molecular data, it is not exactly an issue that paleontologists can investigate with long-extinct creatures. For that reason, we will focus on allopatric speciation.
There are two main subtypes of allopatric speciation - these are allopatric and peripatric. The difference is that for peripatric speciation, rather than an existing population being split by the creation of some barrier to gene flow, some portion of a population enters a new area that is geographically isolated from the rest of the population. One very good example of this mode of speciation is island colonization by various organisms. Many islands across the world are formed by volcanism, rising up over many thousands of years to finally break the surface of the ocean. This means that no terrestrial organisms exist there to start out with. Basically, through some process, plants and animals from other islands (or perhaps even the mainland) manage to colonize the new island - since said island is geographically isolated from the original population, over time there will be a divergence.
Out of the two subtypes, allopatric is the most easy to observe in paleontology. This is because we have a good understanding of the movement of tectonic plates through geological time, and thus we can understand how continents moved, mountains formed, sea levels changed, etc. There are mounds of easily observable evidence of this type of speciation throughout organisms in history.
As many of you all know, all of the continents were joined into a single landmass called Pangea during the Triassic period. This meant that any population of terrestrial organisms had relatively continuous gene flow - there were no seas to split up these populations. This is why we see very closely related organisms from Triassic rocks across continents that today are thousands of miles away from each other. An example would be Coelophysis - while Coelophysis bauri is found in the Southwest United States, there are closely related species (sometimes even classified in the same genus) in Africa - this is because at that time, there was little to no isolation.
Throughout the rest of the Mesozoic, the continents began to split up, and we can actually see the effects it had on diversity and speciation over time by looking at species from different stages. In the Late Jurassic, we find species of Allosaurus, Ceratosaurus, and Torosaurus from both the Western United States and from Portugal, because during that time those regions had only just started to split up - the populations had only recently become isolated and as such had not diverged all that much.
Another very cool pattern we see is due to the fact that there was a clear North-South divide in the way the continents split, and this is reflected in the types of Theropods we find on different continents. In the Northern continents such as Asia and North America, Tyrannosauroids dominated as the largest carnivores during the Late Cretaceous, whereas the Abelisaurs dominated the Southern continents at that time. We do not find any Abelisaurs in those Northern continents at that time, nor do we find Tyrannosaurs in the South. However, another note is that we see closely related Tyrannosaurs [and dromaeosaurs] in both Mongolia and Western North America. This is because Asia and North America had only split relatively recently, much more recently than the split that resulted in the North-South divide.
Another interesting thing is that North America was split in two during much of the mid to late Cretaceous by a shallow sea, creating two subcontinents called Laramidia [on the west side] and Appalachia [on the East side]. While we have a rich collection of fossil organisms from Laramidia, there is very little material of that age from Appalachia. Since this sea would have isolated any species that ranged across the continent prior to the rise of sea levels, it is certainly plausible, if not probable, to assume that Appalachia would have been home to organisms somewhat similar to those in Laramidia, but still somewhat distinct. However, until more remains from Appalachia of the relevant age are recovered, we will not know to what extent this is true, if it is at all.
Well, I hope this has helped you learn about the fascinating topic of where species come from! Have a wonderful weekend, everyone!
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