Evolving due to food

[David George]

Pitohuis are brightly coloured, omnivorous birds. The skin and feathers of some pitohuis, especially the Variable and Hooded Pitohuis, contain powerful neurotoxic alkaloids of the batrachotoxin group (also secreted by the Colombian poison dart frogs, genus Phyllobates). It is believed that these serve the birds as a chemical defence, either against ectoparasites or against visually guided predators such as snakes, raptors or humans. (Dumbacher, et al., 1992) The birds probably do not produce batrachotoxin themselves. It is most likely that the toxins come from the Choresine genus of beetles, part of the bird's diet. (Dumbacher, et al., 2004).It is also believed that they aquire toxins as they eat garbage.Ifrita is another bird that secreyes toxins in its feathers. This does mean that these birds can survive better than its relatives.

[alextemplet]

Wow, David, you really know a lot of stuff. I had no idea that there were poisonous birds but that's really interesting. Can you provide a source where I can research it further?

[damien james]

I don't get it David. Can you explain? How is this evolution due to food?

[AstusAleator]

he's saying that these birds have built up a chemical defence by coevolution with a prey that provides the chemical.

[David George]

I think AstusAleator has provided you the correct and simple meaning.

[damien james]

But where is the genetic change? Maybe I'm not reading it right?

[alextemplet]

Genetic change is hard to understand, especially since DNA is almost never preserved for more than a century or so. However, we know that coevolution occurs, a common example being flowers evolving to only be pollinated by one species of insect or bird, so obviously genes have to evolve to produce that system. In the case David mentioned, one genetic adaptation might be the birds evolving resistance to the poison. In fact this may have first evolved only for the birds to eat the poisonous animals, and the birds themselves becoming toxic may have only been a nice little side effect. Evolution due to food, as David put it.

[damien james]

Oh, so the birds being resistant to the toxins is the genetic change. But this is not coevolution though, but still nice to know.

[AstusAleator]

The birds becoming immune to the toxin, and then developing the necessary glands to isolate and transfer the toxin to their feathers would be the genetic/biological change. This adaptation could NOT have happened without the frogs, therefore it is coevolution.

[damien james]

If beetles that bird eats had evolved and genetically changed in sync with the birds, it would have been co-evolution. But they didn't, it's just evolution. Unless you arguing that all evolution is co-evolution, then that is different matter.

[AstusAleator]

It's possible that the bugs evolved a more potent toxin to attempt to stop predation by the birds. Despite that, though, think of it this way. These birds are likely the only predators of this species of beetle, because any other predator would succumb to the poison. So, as the sole predator (probably) the bird will have a large influence on the evolution of the beetle. If the birds have been feeding on these beetles long enough to evolve the necessary structures to transfer the toxin to their feathers, then I don't doubt that the beetles have had their own specific adaptations.
Co-evolution doesn't necessarily mean that they can't live without eachother, or that they both need to exhibit a dependence on the other. It just means that they have had a direct relationship over evolutionary time, and during that time one or both of them has had adaptions driven by that relationship. It can by characterized by an interplay:
Bug is mildly toxic -> Bird and other organisms become immune to toxin and eat bug -> bug becomes more toxic -> bird becomes only predator immune to poison -> process is amplified by a few more stages and then the bird develops its toxic feathers, relying on the beetle for the toxins.

The example of certain plants only being pollinated by specific species of insects is also a good example of coevolution that fits this model.

[damien james]

If you make <b>assumption</b> that beetle has evolved to become more toxic over time in response to bird predation, then there is co-evolution. But if this is only assumption and evidence does not support, then this is not good example of co-evolution. For co-evolution, evidence has to be that genetic change is reciprocal.

Also I do not see evidence that bird is only predator of beetle. And I never said that co-evolution means species cannot live without each other. Symbiosis, mutualism, and standard interaction is not same as co-evolution. True reciprocal adaptation is hard to find in nature, and this does not seem to be case here.