Monarchs and Milkweed question?

Question

Apparently, Monarch butterfly caterpillars are poisonous to most would be predators because of the protection conferred upon them due to feeding on milkweed. If it is correct that Monarch butterflies' survival has been enhanced by this selective adaptation (by their larvae being able to tolerate the toxins in milkweed), and this gave them a competitive advantage in the natural world, then why aren't milkweeds crawling with all sorts of other species of caterpillars gaining similar protection? In other words, why did only Monarch caterpillars hit on this means of protection from predation, when other butterflies and their larvae could have also adapted to milkweed? If the Monarch butterly gained this tolerance to milkweed through a chance mutation, then why did chance favour *only* the Monarch butterfly and not other butterflies..?

Answer 1

The monarch is not the only butterfly that has
benefitted from the protection offered by chemicals in milkweed. There are other butterflies
related to the monarch (the queen, e.g.), at least
one moth, a few longhorn beetles and a leaf
beetle that all feed on milkweed. Presumably
they all have the protection, though not all of them
have been tested for it.

Monarchs do not taste bitter. A predator that has
never encountered one before will happily eat
several of them. However it then becomes ill
soon after and, if not killed, avoids monarchs in
the future.

Not all monarchs are toxic because not all kinds
of milkweed contain the toxic chemicals. It is
probably simply the accident of which insects
developed the mutations that allow them to cope
with the milkweed toxins that determines which
ones can feed on milkweed and which can't
without being harmed.

Answer 2

first of all, its a matter of location. the monarchs are prevalent in areas with milkweed were as other butterflies aren't normally found in areas where milkweed grows. now as for why other butterflies don't use mikweed's toxins as a means for protection: its really a matter of evolution, perhaps the monarch butterflies specifically targeted the milkweed plant to lay its eggs or competition between other butterfly species drove it to be a specialist feeder of a plant with lesser competitive pressure. it really could be any one thing or a combination of things. its just a matter of evolution and the situation that the insect was in. for the monarch, it was adapt or become extinct. thats why its still around today

Answer 3

Because there's a legion of predators of this plant, only happens monarchs incorporate the taste to their tissues, other arthropods learnt to dump the poisons or to metabolise them into food.
If a butterfly species as numerous as monarchs did try to eat the plant, it was selected against, because we only see monarchs as THE BUTTERFLY species predating on milkweed. Not only co-evolution happened here, but also species displacement.