Why do species Split into two or more often than two merge into one?

Question

sorry last minute resit exam.. and i am still not sure about this

Answer 1

Two species cannot undergo exactly the same course of (retro)evolution at all the genetic loci to end up as a single species. This is because apart from the GxE effects that affect evolutionary processes, there are physico-chemical stochastic processes that are purely dependent on numbers and chance interactions and can hardly ever be the same in two species over an evolutionary time-scale to end up into the identical one species.

Answer 2

To put it more simply than the person above me: it is rare that two species inbreed to create a new specie, but relatively common genetic drift affects two populations of the same specie differently, eventually causing them to evolve into new species.

Populations of the same specie can live in very different environment, say at different side of a mountain range, causing different environmental pressure on their genes. This will cause different genes in the two populations to be favored, and with time enough change can occur so the populations will no longer mate, and two different species will have formed. But for two species to merge, they first have to be physically able to mate, something that is quite rare. They then have to have the right number of chromosomes so a viable match can be made. And thirdly, they must want to mate. So it just takes a lot more to make two species merge, than it takes for populations in the same specie to slowly evolve in different directions. Hope that helped!