It's not too commone but it does happen that two will be able to grow that closely together.
In a way, it's a form of natural selection, although it selects for trees that retain enough leaves, and/or enough energy reserves, to survive a defoliation event. Most can come back for being completely denuded, although twice is harder (the problem with Gypsy Moths a few years back was that they'd come back again and again, devouring the young leaves until the tree was exhaused and died). But any sort fo environemtnal pressure is a form of selection. Even with the Texas BLuebonnets (basically, wild native lunpines) in my fron planting beds - the seeds that survive teh "Winter" (such as it is here) and grow quickly, and flower, before the heat hits (about this time of year) are the ones that survive and make next year's seeds. A lot of the seedlings that sprouted never quite made it. Anyone with a garden (who doesn't automaticly pull up, or otherwise kill, any and all seedlings) can easily observe the process. You'll see it with oyur trees - the ones that can grow quickly enough so as to escape the shade of he larger/older trees will survive, andthen have the chance to flower and set seed.
It cracks me up when I hear or read people say that "natural selection no longer occurs" - what a crock of nonsense.
Prob., like most people, brainwashed from the day of birth with all sorts of crap as to how things like social climbing and owning the maximum possible amount of miscellaneous stuff are supposedly of Paramount Importance and supposedly constitute The Meaning Of Life. It seems to me that this is why gurus, for example, have such an aura/mystique about them - to most people, even the simplest idea that ignores social bullshit and mindless materialism is a completely new and revolutionary idea.
One of the *advantages* of being socially "retarded" is that I never "properly" learned how to shut out perceptions, and appreciation, of the "stupid little things" like nature and the environment. When I learned about things like Eastern Philsophies, Zen koans, and the like, it was not a new thing, but an affirmation of mcuh that was, to me, patently obvious. What society totally sucks at is teaching children (and adults ;) ) how to *balance* the different aspects of human nature (or at least, the *capabilities* of human nature). We are social beings, and we are material beings existing in a material universe, but we're also intellectual beings, natural beings, emotional beings, and, dare I say it, spiritual, or at least philosophical, beings. Cultures in general tend to emphasize the social first, and the matrerial second, with the emotional nature third in importance, and the intellectual, natural, and spiritual/philosophical aspects being lumped together into a very, *very* VERY distant third.
This is, it seems to me, also why so many people totally reject a statement such as "nature can teach us a lot" - the call it "mystical claptrap", because they reejct the science behind it, and reject the non- social and non-material parts even of their own being. Meanwhile, I think this is also why it takes many people a very long time to recognize, and then start to develop, these other aspects of their being: they basically have to come across these things, learn about them, by accident - the culture offers not one iota of guidance, because the majority of sociocultural institutions, including religion, are completely and totally anthropocentric, to the extent that tehy actually seek to divide humans from nature, and designate nature as "evil". In reality, nature is often hostile, and fundamentally indifferent, but not "evil", which includes a premeditated intent to do harm.