Let’s not get ahead of ourselves.
Dogs don’t climb very well, so if a chicken was able to flap high enough to reach a low branch, it might be safe. Now just as you might be able to run slightly faster than some of your friends, or swim slightly faster. Or climb a little better, purely due to a genetic throw of the dice, then some chickens are slightly better at gaining a little more height than others.
So the dogs find it easier to catch and kill those who are not so genetically endowed with whatever gene, or combination of genes, that enables some of the chickens to reach lower branches.
So those that are killed do not have a genetic dispostion to flap high enough to escape being eaten and those that survive do.
All sound pretty reasonable so far?
And over time we find that as most of the surviving chooks have a genetic make-up that allows them to flap a certain distance into the air, those that breed have offspring that also enables them to do the same. Just like if your parents were good runners, then there is a reasonable chance that you will be a better than average runner.
And over time, because the chooks without the genetic make-up which allows a better chance of survival do not in fact survive, they are removed from the genetic pool.
Now is there anything at all that I have said so far that is unreasonable?