You must have selective pressure on genome size for organisms to evolve mechanisms to reduce the "junk". The metabolic cost of carrying around the junk is small. The cost of cleaning up the junk comes from much more frequent accidental deletions/truncations of important sequences. Upending that equation requires massive selective pressure for a smaller genome - maybe something like a tardigrade that gets desiccated regularly? In any case no chance any vertebrate species would have that kind of pressure. You'd need insane offspring counts and short generation cycles to afford the selective pressure price.
The fact that we can tap junk at some future point is probbly just an accidental side-effect... though there is another theory that claims having lots of junk provides some protection against environmentally-induced damage because most of the time it is a junk section that gets damaged. Hows that for the next error protection algorithm: pad the message with mostly zeros so occasional bit corruption doesn't matter. Take that Shannon!
If you want a specific example of this mechanism working: primate 3-color vision. In our two color blue-yellow seeing ancestors the yellow pigment sequence got duplicated, then eventually slightly mutated. That's why the red and green receptors overlap so much yet blue is standing way off by itself. It is high likely this started as a useless duplication and was carried around for a long time before one of the duplicates got mutated.
The fact that we can tap junk at some future point is probbly just an accidental side-effect... though there is another theory that claims having lots of junk provides some protection against environmentally-induced damage because most of the time it is a junk section that gets damaged. Hows that for the next error protection algorithm: pad the message with mostly zeros so occasional bit corruption doesn't matter. Take that Shannon!
If you want a specific example of this mechanism working: primate 3-color vision. In our two color blue-yellow seeing ancestors the yellow pigment sequence got duplicated, then eventually slightly mutated. That's why the red and green receptors overlap so much yet blue is standing way off by itself. It is high likely this started as a useless duplication and was carried around for a long time before one of the duplicates got mutated.