system
Given the following post, respond as if you're commenting on Reddit:
human
I can understand how natural selection for traits works. But how do complex processes (such as metamorphosis) come about through evolution?

My friend and I were discussing evolution and we don't understand how processes that are so complicated and dependent on sequential events come about in the first place. From the molecular level (e.g. metabolic pathways) to the physiological level (e.g. metamorphosis), are there any general principles to explain this?
gpt
This is probably better suited to ELI5.

If you look at insects as a whole, the extent of the change between nymphs and adults (and the duration of pupation/metamorphpsis) is highly variable.

Some animals do not undergo metamorphosis at all.  Some pupate but are not significantly changed and it essentially serves as a method of hibernation through cold and/or dry seasons.   

Which establishes the baseline adaptation upon which all other changes become available and radical differences between nymph and adult form can begin.

After a time, some of these species would be present in areas where the hibernation aspect was unnecessary, and a new range of changes becomes available.

And, eventually, you arrive at today where folks assert that intermediate steps are an impossibility.