The Questionable Authority: Applications of Evolution 1 - The Erythrina Gall Wasp:
At the moment, there is a new invasive species that is making the news here in Hawai'i: a species of "gall wasp" that has been wrecking havoc on trees of the genus Erythrina in Singapore, Taiwan, and a number of other places was found in a valley on Oahu in April. Since then, it has been found in a large number of other places on Oahu, and has started to turn up on other islands, including Maui, and a number of scientists believe that it poses a serious threat to a culturally-significant endemic plant - the Wiliwili (Erythrina sandwichensis). The threat is being taken so seriously that scientists have reportedly begun to bank Wiliwlil seeds as a precaution in case the extant population is completely lost.
So what does this have to do with evolution?
Individual species do not evolve in a vacuum. They evolve in an environment, and natural selection favors the preservation of variations that increase the chances for an organism to survive within that environment. This is common knowedge. What people sometimes forget, however, is that when we discuss the "environment" that an organism evolves in, we are talking about much, much more than just the climate. The evolutionary environment also includes every other species of organism that has any sort of effect on it. Species evolve within the ecosystem or ecosystems that they inhabit, and they evolve as a part of those ecosystems.
So what happens when people - either intentionally or inadvertantly - introduce a species to a new habitat? That's a broad question, and one where the answer is obviously going to depend on a lot of things - not least, what the new species is, and where it is being introduced. (Biology can be really annoying that way, with the answers to so many questions depending on specific circumstances.)
Since the broad question isn't really answerable, let's narrow it in a way that is tailored to these specific circumstances: what happens if you introduce a pest species (either parasite or predator) into a new, hospitable environment that contains a species that is closely related to the pest's usual target? That is a very complexly worded question, but it has a simple answer. Nothing good is going to happen.
In the case of the Erythrina gall wasp, this is exactly what happened. The gall wasp is not native to Hawai'i, and it did not evolve here. This means that up until now, it has not been a part of the evolutionary environment for any of the native species. As a result, it should come as no surprise that the native species has no natural defences against the gall wasp. At the same time, the gall wasp has now found itself in an environment where it has lots of access to a number of species that it can use, and which completely lacks any of it's normal predators. In short, it's gall wasp heaven out here - at least until the Erythrina are all gone.
For more on how evolution helps us understand invasive species and vice versa, check out TQA, a new kid on the evolution blogging block.