Evolutionary conundrums like the Venus Fly Trap fascinate me the most. But a VFT is orders of magnitude simpler than a spider, so I imagine the answers are a bit simpler to deduce. There are several things I try to remind myself when approaching such a puzzle:
1. Those that can win over those that can't.
2. Mundane features get readapted to new uses.
3. Evolution doesn't happen in a vacuum. Organisms co-evolve systematically. Or, ecosystems evolve as a whole. In the long run, organisms that fit tend to win over those that predominate.
4. Selection gains momentum: static features of organisms reflect and encourage developmental tendencies. To put it another way, the process of evolution doesn't just evolve forms but also evolves mechanisms that accelerate and improve evolution itself. Those organisms which adapt more quickly and more appropriately across successive generations are favored over those which mutate more slowly and in a more random manner (up to a point - it's a balance).
So let's get down with the Fly Trap puzzle!
There is nothing novel about plants being able to absorb nutrients, so it's just a matter of root capabilities drifting into leaf capabilities. There's nothing novel about the production of nectar, and there's nothing novel about plants producing all manner of caustic substances. All these elements got readapted in the VFT.
I would draw history this way for precursors of the VFT:
- Precursors with stickier leaves caught more flies and so ended up with more nutrient-rich soil once trapped insects decayed.
- Precursors with bowl-shaped leaves trapped more insects.
- Precursors that accelerated the decay of insects won over those that didn't, so leaves with more caustic digestive chemicals were selected over leaves with less caustic digestive chemicals.
- Leaves that closed reduced the amount of digestive enzyme required, thus saving even more energy for the plant.
- Precursors that used less energy in the production of sticky nectar were more energy-efficient and thus could produce a greater number of viable seeds.
- Leaves which lay open a certain amount were selected over those which were folded a certain amount... sometimes. Dependent on their shape.
- Leaves which folded in response to touch were selected overall, in a very gradual manner, because openness and folded-ness both brought competing advantages. Eventually, the quick-snapping trap-leaf was "hit upon." (It is likely that precursors had varied leaf types, and even increased variation within the same plant for many generations.)
- Leaves with interlaced spikes were selected over leaves with no spikes, or spikes that didn't interlace.
The specific order of these developments is certainly wrong, but without a doubt all these sorts of selections had to have occurred to give us the modern Flytrap.