Im not saying them make a transition to an open circulatory system im saying they IN GENERAL, will evolve to better suit their particular environment, and of course you dont get different species mating, thats why i talked about the branches on the tree, once a new branch/species is formed it wont join back to the others, just grow and expand, and possible grow new branches ( ok i get it, not the most full proof analogy, but the best i could come up with)
Ill just write a little story in order to illustrate what evolution actually is.
Within a species, there is always going to be variation, some have pale skin, some have dark, some are tall some are short, some have large feet some small ...etc. ( you get the point
)
so lets think about the origin of the Giraffe,
REMOVING god from the equation, as we are ( for the moment) talking of my arguement, not yours.
Giraffes did not always have long necks, these creatures lived on the savanna planes, Giraffes eat tree foliage, but.... without their long necks, the food is often out of reach.
Taking this into account, many giraffes ( especially the short ones ) will starve to death, and will in many cases, not survive to mate.
On the other hand, if a giraffe is born who's neck is slightly longer than normal, then that giraffe will have an advantage and will be more likely ( and on a larger scale, will more often ) reach the age of mating, since the same is true of both sexes in this scenario, it is also more likely that the giraffes mate has a slightly longer neck ( you cant think of this as a coincidence, the numbers of animals involved in evolution are so huge that the coincidence is bound to happen quite often )
So, ( enough of the pre-amble ) eventually, (possibly after many generations)
Giraffes will consistently mate with Giraffes, which have slightly longer necks ( like themselves ) this is a genetic characteristic, and is therefore passed on in the gamete. I dont have time to write much more, but you see, this process can continue for many millenia, and has produced the wide range of species we see today. ( the braches on a tree )
hope you can at least accept what im saying as a theory that is at least as plausible as any other that we humans have presently devised.
Best regards,
AMC