Can't Get There From Here
In Evolutionary Biology, distinctions can be made between traits that are plastic (i.e. have the capacity for short-term, reversible change) and traits that are fixed (i.e. do not change, or if they do change, do so in an irreversible fashion). Plants for instance, may be able to change their growth habit in very different ways depending on where and/or when they germinate and grow. And yet, biologically speaking, they may be the same species, or genetically even the same individual. A plant grown in shade may change its growth form to maximize available light absorption (by increasing leaf surface area), and when transplanted into open sun may change in the opposite way, reducing surface area to avoid damage from overexcitation of its photosynthetic machinery.
Some traits however, are not so easily mutable. Take pentadactyly, for example. As long as 360 million years ago, the earliest tetrapods (animals with four limbs) had as many as eight "fingers" (or toes). We now known that the five-finger/toe arrangement we see throughout the animal kingdom first appeared 340 million years ago. For 340 million years we've seen tetrapods with, more or less, five fingers and toes. Some traits are so deeply ingrained into the developmental plan that any evolutionary change which doesn't have an immediate penalty or may have a positive effect is fixed.
I've been there I know the way
When I was young, my dad yelled at my mother and I a great deal. There's a great deal of psychology and history behind it all but to make a long story short, he was frustrated with the turns his life had taken, and that left him with an exceedingly short fuse. He had little time, patience, or tolerance for people or things that didn't agree with him or go his way.
I didn't have any means to fight back - any uprising was seen as an immediate excuse to increase the severity with which my father would try to enforce his will. He demanded that I show only happiness around him - anything less was forbidden. The fact that he used it to feed his Martyrdom Complex was the icing on the cake. So I trained myself to completely internalize all of the hatred, and anger and frustration and animosity I built up for myself over the years. I pushed it down, compacted it, distilled it, poured into a little botttle and left it in a dusty corner of my subconscious where I'd hopefully forget about it. Of course, the bottles would pile up, cracks in the glass from the contents inside would occur and an explosion would result, necessitating a release. But better to do it in all in one shot, I thought, than to continually subject myself and those around me to a continual, prolonged misery.
I don't remember when I started doing this - I must have been maybe 8 or 9 or so, sometime between the point where I vomited all over the dining table when I force fed myself in terror over what my parents would do to me if I didn't finish my supper and when I first started cutting myself (only very occasionally mind you, but it was still nevertheless deliberate). But somewhere long long ago in the past, I adopted this strategy to deal with the times I was sad and angry with my friends. In a way, it wasn't very different. Like my dad, some of my friends had ended up developing an idea that I could only be happy around them and never be anything less. Or, they had an image of who I was and looked down on me for not living up to that ideal. And so I further refused to talk about whatever it was that bothered me about someone or something someone had said or done.
Around 15 years later I still find myself stuck in old habits. The silent building up of an acute sense of rage and anger to the point where I would explode in momentary bursts of omnidirectional vitriol. It isn't a sustainable strategy - some day I'll have to make that evolutionary leap to something that works better. But like many evolutionary changes, that leap is really only a series of very small baby steps aggregated together over a very long period of time. As in biological evolution, in the course of human experience, that kind of change doesn't happen in the blink of an eye. The change is ongoing, unceasing. Like that open source project you may have read about on slashdot, our version numbers seem to be increasing in ever smaller steps - from version 0.98 to 0.98.1, to 0.98.2. Looking at how human personalities grown and change with time, human beings are always perpetually works in progress. And I realize now, I'm no different.