Many species have bright and conspicuous colors. When these colored individuals are dangerous or distasteful, the pattern is called warning coloration or, as biologists call it, aposematic coloration. These patterns act (and presumably evolved) to cause predators to avoid attacking the individuals, which either taste dreadful or could hurt the predator. Predators (often birds) do indeed learn to avoid the colors after an initial experience with them. You, for example, have learned to avoid insects having black and yellow stripes, so you won’t go near anything that looks like this:
Here are some examples of aposematic colors and patterns in diverse species:
The striped skunk (Mephitis mephitis), which squirts a nasty fluid from anal glands
The poisonous coral snake (Micrurus fulvius)
The ladybug (Coccinella septempunctata); not a bug but a beetle. When grabbed by a predator, they secrete a foul-tasting fluid from their leg joints
The noxious caterpillar of Eupackardia calleta (the calleta moth).
The blue-ringed octopus (Hapalochlaena lunulata). It’s one of the deadliest sea animals, with a venomous bite that’s killed many people.
The yellow banded poison dart frog (Dendrobates leucomelas), which secretes toxic alkaloids through the skin.
Now scientists have shown that lots of these species are indeed toxic or dangerous to predators, and that the predators do indeed learn to avoid the patterns and colors (sometimes, as we’ve discussed before, the predators can even evolve a hard-wired aversion to the pattern). So this system is good for both the aposematic species and its predator. But there’s always been a big problem with this system: how did the coloration evolve in the first place?
You can show from phylogenetic analysis that many of these brightly-colored species evolved from camouflaged (“cryptic”) ones. This lead to the evolutionary problem: the first mutant individual that was toxic but had a new, bright color would call attention to itself. Since no predator has yet learned, since the pattern is new, that individual would be more likely to be eaten than its camouflaged conspecifics, and so there would be a penalty attached to the new mutation. Only when those aposematic individuals were sufficiently common—and thus could be encountered often enough by predators who would remember the pattern and avoid the next aposematic individual—could they enjoy the protection afforded by their color. The problem of aposematic coloration, then, is that at very low frequency it would seem to be disadvantageous and could not invade a population. Only when it gets above a certain frequency would natural selection then act to fix the trait in the species. (This kind of natural selection, in which the advantage or disadvantage of a mutation depends on its frequency in the population, is called frequency-dependent selection.)
Various solutions have been offered to this puzzle, but all of them suffer from problems. I won’t recount these, but want to briefly highlight a paper that offers a possible solution. In the June issue of Evolution, Michael Speed and his colleagues suggest a way to get aposematism off the ground.
The solution is this: although the first mutant aposematic individuals are more likely to be detected by predators, they have another feature that more than offsets this advantage: they forage more openly, enabling them to get resources that their cryptically colored (and presumably hidden) conspecifics don’t share. This could increase the fitness of aposematic individuals above that of their cryptic kin. Speed et al.’s paper conducts numerical simulations, involving “optimization” of the color and foraging traits, that show (as you might guess), that if these foraging advantages (which of course make them more conspicuous to predators) are sufficiently large, they can outweigh the deleterious effects of being seen and attacked by a predator.
Under this model, then, the evolution of warning coloration is associated with a niche shift. Speed et al. show that their model makes several predictions, which preliminary data seem to support:
- Species that are aposematic should expose themselves more often to predators (because they’re out seeking resources) do their non-aposematic relatives. Speed et al. show that there is indeed evidence for this in poison-dart frogs and Lepidoptera: aposematic species forage during the day more often than do their cryptic relatives. And, of course, skunks are perhaps the noisiest and most conspicuous foraging mammals in North America!
- Aposematic species should, because they are able to get more resources, be able to evolve larger body sizes than their relatives. There are preliminary data supporting this in insects and amphibians
- For species with very different life stages, like Lepidoptera, the mobility of a life stage should correlate with its degree of warning coloration. Speed et al. cite some supporting evidence: “Pupal forms of lepidopteran species that are aposematic in larval or adult stages are often cryptic. . . consistent with the fact that they are completely stationary and hidden away from predators.
- Aposematic species should “make use of a greater range of food plants than closely related, but cryptically colored species.” This prediction has not been tested.
This model sounds good, and may well obviate the problems with getting warning coloration off the ground, but there’s one problem. It is an optimization model in which several traits are simultaneously “optimized” by natural selection. But evolution has to work on genes, and a gene-based model of Speed et al.’s theory seems a bit problematic. Which mutation came first, the one for bright color or the one for foraging more openly? (Presumably these two types of traits—behavioral and morphological—are based on different genes.) If the aposematic mutation came first, it would still encounter the problem of being more visible to predators without the compensating benefit of getting more food. If the foraging-openly mutation came first, it would be disadvantageous because it would expose itself to predators more (and if there were an advantage to foraging more openly if you were cryptically colored, the species would already be doing it).
In other words, the theory seems to depend on the concatenation of new mutations for two traits, each of which is individually disadvantageous at low frequencies but which become advantageous when they occur together. But, given the rarity of mutation, this seems to be improbable. Indeed, I know of only a few cases of evolution that involve two mutations that are useful together but individually disadvantageous. (For you evolution fanatics, one example is the evolution of compensatory mutations in the loop structure of transfer RNA.)
This shows a persistent problem with optimization models: what looks optimal from an evolutionary standpoint may not be attainable when you deal with the messy genetic details. And, after all, anything that evolves has to obey the rules of population genetics.
I may be missing something, and perhaps the authors will correct me, for I really want this idea to be true. It fits the facts about nature so well!
Speed, M. P., M. A. Brockhurst, and G D. Ruxton. 2010. The dual benefits of aposematism: predator avoidance and enhanced resource collection. Evolution 64:1622-1633.