|MadSci Network: Evolution|
Thatís an excellent question regarding a complex problem that is still being explored by biologists. As you point out, we often see a correlation in nature between bright coloration and toxicity. Certain frogs are dramatic examples, but we can see it in many other species.
The most common hypothesis regarding this phenomenon is that bright coloration is a warning to predators; this is called aposematism. This hypothesis predicts that, in the history of a lineage, bright coloration will evolve after or in tandem with (not before) the evolution of toxicity.
Often, hypotheses of aposematism involve the assumption that predators can survive an encounter with a toxic prey item. Perhaps the predator only gets sick from eating the toxic item; or, perhaps the toxic animal tastes bad enough that the predator spits it out before actually consuming it. Individual predators that have had such an encounter can and do learn not to eat prey items that look similar to the ones that made them sick.
If it so happens that predators that attack the toxic prey actually die, there is very strong selection on traits involving a predatorís tendency to attack or avoid those prey. If variation in this trait is at least partially based on heritable genetic differences, population genetics tells us that a gene decreasing the tendency to attack toxic prey should become fixed in the population fairly quickly.
Hereís a simplified scenario of how aposematism can evolve:
Start with a population of drab little frogs that blend in well with the leaf litter on the forest floor. Our frogs are not toxic, and are a favorite prey item of a certain species of snake that lives in the area. Now imagine that our drab frogs acquire the ability to produce toxins. Initially, the snakes continue to eat the frogs at the same rate. Now, however, the snakes get sick and often die when they eat the frogs. If some snakes have a gene that makes them less likely to eat our deadly frogs, those snakes will be more likely to survive and reproduce, passing on that gene. If the toxic frogs also acquire (in evolutionary terms) a bright, conspicuous coloration, it will be easier for the snakes to identify and avoid them, rather than getting them mixed up with some frog species that is still safe and delicious to eat. The fact that toxicity comes first is key here: bright coloration makes the frogs easier to see, but the existing toxicity makes the snakes want to avoid them rather than eat them. This means that aposematism in the frogs, and avoidance of bright toxic frogs by the snakes, can evolve without the predation rate ever getting much (if any) higher than it was initially.
There are some other interesting things that can happen, such as mimicry (many toxic species look similar to each other, and predators avoid them all; also, non-toxic species may look like toxic species and can thereby avoid being eaten). This is a complex issue and I would encourage you to read more on the subject and possibly submit additional questions if you wish.
Shannon DeVaney, Ph.D. Candidate
Dept. of Ecology and Evolutionary Biology
University of Kansas
Additional Reading (just a start):
Try the links in the MadSci Library for more information on Evolution.