i dont think a single random seedling would make a difference, cold hardiness probably comes from years of cultivation

The answer to your question is ........it is possible....but it is extremey unlikely. Remember that plants which have tropical growing needs have adapted to those conditions over countless of generations over thousands of years and maybe even longer. Changes as drastic as going from Tropical to Sub-tropical or even to temperate would generally only occur over the same countless of generations in the future as plants die off that can't hack the changes in temps while those that can deal with the changing climate pass their seed onto future generations, changing the ability to survive in the cold more and more as generations pass. Of course, genetic engineering either through natural or man made processes can speed up the process. Some examples would be Atemoya....a cross between the low land sugar apple and the higher, cooler growing Cherimoya. By hybridizing this way one might expect some crosses to be very sugar apple like, yet be more cold insensitive than sugar apples generally are. I have two jakfruit crosses from the Fairchild breeding program. Both are crosses with Tabouey as one parent. One is crossed with Black Gold and the other is with J-30. The J-30 cross died completely back to a stump with a frost that we had.....while the Black Gold cross, growing right next to it, didn't even burn a leaf. If we kept crossing the most cold tolerant varieites over thousands of generations, eventually we could get a jakfruit that might stand being grown in the cold...that might even turn deciduous. Who knows......

That being all said.....am I growing ultra tropicals such as Durian and leaving them out in the 35 degree cold to see how they fare? Yes......and surprisingly, it has survived two seasons (albeit warmer then normal ones without hard freezes). Do I have the seedling sub- tropical Durian that I have been dreaming about? Well.....one can dream.

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Yeah, human or natural selection is the way to adapt plants to cold or any other type of environmental stress. However, cases like the atimoya and the limequat for example, typically produce results much more quickly, because one of the parents provides cold hardy genes to the resulting plants. Same thing happens when the tropical avocados are crossed with the subtropical Mexican varieties, we get added cold hardiness very quickly...

Now, plants like mangos do not have cold hardy variants, so the process of increasing cold hardiness will probably take thousands of generations and lots of luck, if going the traditional selection way. That is where human genetic engineering will play a big roll in the future... The University of Georgia is studying guavas with the hope of incorporating cold hardiness genes into them, for example.

The best thing to do is probably to make sure your plants are as healthy as possible and have the least number of stressors, so they can handle the cold. I have seen well cared for plants live through a freeze without noticeable damage, while less healthy plants (of the same variety) die to the ground...

I am keeping my fingers crossed!

Speaking from plenty of experience, generally the limit to what a tropical plant can survive in the winter is zone 8, and that is really pushing it. It might theoretically be possible for a tropical to survive in zone 6 with a very extreme microclimate. I'm not talking about anything normal for the average person; you'd have to go way out of your way and be an expert at knowing how to create microclimates. Some fruits are subtropical, and can survive in zone 8 without too much difficulty, but the more tropical ones can be very difficult to even grow in zone 9. Even pomegranates start having some difficulty in zone 8b/7a.

In zone 6, I'd tell you to give up all hope, you definitely need a greenhouse. But for someone who is extremely knowledgeable and creative, it might theoretically be possible with a lot of engineering and specially designed surroundings.

If you don't know what you're doing, and are not willing to give the plant a huge amount of specialized care, probably tropicals are not a good idea in anything less than zone 10.

You might want to investigate all the different types of rare fruits. A few of them can be surprisingly hardy and grow fine in zone 8. I know there are a few cold-hardy banana varieties that are capable of fruiting in zone 8, although they're not very good and are not very productive in that climate. A mature Brewster or Emperor lychee tree might be able to take zone 9 or 8 (fruit might have trouble ripening though). There are some pomelo-like grapefruit varieties that can ripen in zone 8. Satsuma mandarins can sometimes be hardy down to zone 8.

your talking about 2 different things just to be clear...

If a papaya has 400 seeds and you plant all of them, they all will have slightly different characteristics,

The gene for cold tolerance may or may not be effected. you might have to plant 10,000 seeds to get a variation that will withstand 2 or 3 degrees better than the parent... but then it may taste better or worse also.

There are a lot of species which are sub-tropical.

each species has its own tolerance, and variations within that tolerance.

I live in New Orleans which gets below 32F one or 2 times in the winter usually down to 29 or 30F, but in colder years (every 6 or 10 years) it may get to 23F.

my house, the cement, plants around the main plant etc... can provide a micro-climate of 3 to 4 degrees (i find)

so, i prefer plants that can have a tolerance of 25ish.

the ones i picked so far are...

loquat, White sapote, Goumi, pomegranate, mulberry, jujube, acerola, muntingia, fig, mango and papaya.

mango and papaya being the more sensitive. papaya grow very fast, and i replace them every 2 to 3 years anyway.

plants like citrus, loquat, goumi, jujube and mulberry (lots of berries) can handle in the teens or better.

and while things like mango can survive 28F, it may not flower or fruit... that depends more on humidity and rain levels.

and there is the issue of grafting and rootstocks as well. some rootstock performing better than others. some rootstock selected for cold, and others selected for disease control, dwarfing etc...

i grow all the plants listed above from seed by the way, including mango, and they all taste very good to me.