Uncle Charlie wrote: I would like to know why or what caused the difference between Old and New world tarantulas defense techniques? Same with the venom potency between Old and New worlds. Cant find any answers on google or im just not searching in the correct place. ...
An in-depth answer to your question would require an entire term's, if not year's college level course in the biology of heredity and evolution, not to mention geology, plate tectonics, biodiversity, and biochemistry. Not only do I not have the time, but I'm probably also not qualified for such an undertaking.
Here's a thumbnail sketch of the issues. You'll need to consult Wikipedia for more details on most of the terms and concepts.
A LITTLE FOOD FOR THOUGHT:
Rule #1: A basic concept in evolution is that "natural" differences like climate, geology, and geography are not the only things that drive evolution. Very potent forces are the interactions between the various organisms including the members of one's own species or closely allied species. Thus, while climate and geology may help to drive evolution, so will the ecological system (e.g., desert, savanna, forest, swamp), available food resources, competition for living/breeding space and mates, predation, and even what parasites may be present. Bottom line: Evolution is driven at least as much by biology as it is by the physical world.
Rule #2: The popular concept of evolution is that all organisms evolve to fit hand-in-glove with their environment in some sort of idyllic harmony. In fact, the various kinds of organisms found anywhere have evolved to survive there
IN SPITE OF everything that "Mother Nature" could throw at them. Remember that in nature something on the order of 99% of baby tarantulas die before they can reproduce. Read
The 99% Rule
.
Bambi and James Cameron's
Avatar are fairy tales! Bottom line: Mother Nature is a vicious bitch!
Rule #3: The forces of evolution don't "create" anything. The forces of evolution can only cull out those organisms that can't survive in spite of them. They are destructive forces instead. The principle is called "survival of the fittest," and seems to be one of the more misunderstood concepts in evolution.
Mutation and gene reorganization are constantly producing new combinations of genes, and therefore new characteristics in organisms. In time this produces an overwhelmingly diverse spectrum of different varieties and species. The overwhelming majority of these fail. Only a very few manage to survive the culling action of evolution. Bottom line: By the
The 99% Rule
, you need to be almost perfect, unbelievably lucky, and tougher than nails to survive!
Rule #4: A given characteristic survives not necessarily because it confers some advantage to its possessor. It can also survive if it merely doesn't do anything to get its possessor killed! See my comment in
Tarantula Coloration
. Bottom line: You have a chance even if you only keep your head down, your mouth shut, and keep moving!
You may remember being told some time ago that at one time the New World was joined to the Old World as one huge supercontinent called Pangaea.
About 125 to 150 million years ago
Pangaea
split to eventually create the Atlantic Oceans and the two, separate continental complexes (our modern "Worlds").
Apparently, there were tarantulas or the precursors of tarantulas on both sides of the split, just as there were pre-monkeys, pre-pigs, and a number of other animals and plants. (This matter is badly confused and corrupted by animals' proclivity to migrate back and forth between the two worlds across various land bridges and by other means.)
Spiders in general and tarantulas in particular, however, don't migrate well over transcontinental distances. So, two or more distinct populations arose, partly because of the impossibility of inbreeding/crossbreeding across the oceans (i.e., inability to maintain a relatively uniform gene pool), and partly because of the different conditions that arose on each side of those oceans (different conditions tend to select against different characters).
So, it just so happened that one or more kinds of tarantulas on this side of the widening pond "way back when" possessed a tendency towards bristles that caused an urtication or irritation on smooth, soft skin and especially on any exposed epithelial membranes. Especially mammalian skin and membranes!
Why didn't New World tarantulas develop a more potent venom? Because the developing/evolving, urticating bristles worked at least as well, but had the added advantage of also working at a distance. A tarantula didn't have to come into direct physical contact with a predator to deal a
coup de grace. All that was necessary was to throw a bunch of those bristles into the air where the predator might inhale them or get them into their eyes.
Why did not Old World tarantulas also develop urticating bristles? Luck of the draw. They all got the long straws. They didn't have the opportunity of inheriting a gene or genes for urticating bristles. One of the New World kinds got the short straw. (It's unclear to me if this occurred before or after the continental split. My guess is after.)
Why did Old World tarantulas develop a more portent venom? Not all of them did. (Rule #3, above.) But those that didn't were eaten by predators and failed to pass their mild mannered "Clark Kent" personalities on to any offspring. Only the "vicious little #$%^&#s" with severely debilitating venoms survived and reproduced!
It is important to note that only two or three subfamilies of tarantulas have developed urticating bristles:
Theraphosinae
Aviculariinae
Selenocosmiinae (Only a few, New World species. I think. I can't find any confirmation of this at the moment. Anybody?)
And, the Theraphosinae are by far the best known and most numerous.
Hope this helps. Now, somebody pass the duct tape please!
Enjoy your little 8-legged biology lesson!
