There are a number of species of insects that use a foul taste for defense. I watched part of a TV program recently that showed one such species with a baby alligator as a predator. The narrator said that after the alligator eats a couple of the bugs, it will learn its lesson and eat no more. Altough the scene may have been staged for TV, they showed the 'gator killing one of the bugs.
Obviously the defense is good for the species as a whole, but how did natural selection come into play in the development of the foul taste, if a couple bugs are killed before the predator learns its lesson? The foulest tasting bug has no better chance of survivng to pass on its genes than any of the other bugs.
I'm sure biologists have contemplated this and written about it. Maybe it's even been discussed here at EVC before. But I haven't come across the answer yet. Who has plausible speculation?
The most obvious explanation, and one that Fisher came up with in the 30s, is that it is propagated through a form of kin selection. While as you say the bug which gets eaten doesn't get any benefit from its being eaten the other bugs in the area do. If we assume that the other bugs in the area are likely to be closely related to the eaten bug, and therefore more likely to share the same genes for noxious taste, then the protection from its noxious taste enhances the survival of those related bugs relative to other unrelated bugs in different regions.
So if you have a whole brood of dozens of foul tasting bugs in an area with a few predators and each predator learns to avoid them after 1 or 2 samplings then losing a moderate proportion of the brood could confer protection on the rest.
As to the very inital development of the foul taste, it depends. We might assume that populations of bugs naturally vary in their tastiness and predation acts as a strong form of directional selection favouring less tasty bugs.
Alternatively we might posit a novel exceptionally untasty mutation in an individual which would need to survive independent of its tastiness before it could mate and produce a brood in which tastiness could provide some group protection. Of course if we allow that predators sometimes taste noxious prey without killing them then the chances of a rare de novo noxious mutation propagating become much better.
I've been thinking about this, and it seems that while the foul taste alone would provide a limited benefit (until it was very widespread), a foul taste AND an identifying mark (one which predators would recognise) would provide a whole lot more.
Sure and the whole concept of the evolution of noxious taste is usually bound up with the development of aposematic signals for letting predators know you are noxious. And from then of course onto various forms of inter and intraspecies mimicry and evolutionary cheating strategies.
The reason it does not make sense is that you are thinking of a simplistic explanation of evolution. While it is easy to imagine a single organism being born and exposed to selection pressures, it really works with larger populations and over larger periods of time. For instance:
What would happen is a population of bug would develop into several bloodlines, each with its own mutation. Perhaps some would be slightly larger, or a little bit yellower, or somewhat bad-tasting. Predators would still be willing to eat the bad-tasting bugs, but not as much as they would like the other bugs. This means that there is a clear pressure toward tasting nasty, and also being distinguishable as being an icky one.
Predators would be looking for a way to tell the foul-tasting bugs apart from the normal ones, so those that interbred with the yellow strain might survive better than the standard color scheme. Once predators link that color with tasting bad the selection pressure would be toward tasting worse and being more vividly marked. Normal-tasting yellowish bugs might even be more likely to survive despite tasting OK, and become one of the many "fake" poisonous species.
Large populations with inherited mutations exposed to slight selection pressures over long periods of time isn't as dramatic as a single organism being eaten or not, but it is more accurate. The key is that even the foul-tasting bugs would get eaten to some extent, but not enough to wipe out their entire bloodline.
Even more intriguing to me is Mullerian mimicry, where other bad tasting beetles will evolve to look similar to existing ones.
Well, isn't that kinda the same as Batesian? I mean, if you look like a bad tasting bug, you are much less likely to get eaten, even if you don't taste bad. Now, if you do taste bad, yet look completely different, there's no way to tell you taste bad, except after being eaten. So, it's better to look like each other, whether bad tasting or not.
The difference is in the honesty of the signal. If you are all signalling honestly it is better for everyone, if only some are signalling honestly then they are weakening the protection of the signal for everyone including the honest signallers.
The basic effect is the same but the underlying evolutionary strategy, mutualism or parasitism, is what makes the difference.
I mean, if you look like a bad tasting bug, you are much less likely to get eaten, even if you don't taste bad. Now, if you do taste bad, yet look completely different, there's no way to tell you taste bad, except after being eaten. So, it's better to look like each other, whether bad tasting or not.
Yes, it is, that being why it evolves. I just like it, and there's some really neat traces of regional variation in warning patterns, where the variation between different species in one region is smaller than the variation in the same species across different areas. So you get these (apparently) reasonless cross-species parallel drifts.
kin selection or just evolution in sub-populations?
hi Wounded King and Mr. Jack,
Message 3: Kin selection. Message 4: The most obvious explanation, and one that Fisher came up with in the 30s, is that it is propagated through a form of kin selection. While as you say the bug which gets eaten doesn't get any benefit from its being eaten the other bugs in the area do. If we assume that the other bugs in the area are likely to be closely related to the eaten bug, and therefore more likely to share the same genes for noxious taste, then the protection from its noxious taste enhances the survival of those related bugs relative to other unrelated bugs in different regions.
What we see is a mutation that makes a bug taste less palatable for predation. Initially this type of mutation is neutral, as there is no way for predator/s to distinguish the palatable from the less palatable bugs, and so many such mutations can spread by neutral drift within a population.
As the mutation/s becomes more predominant within a sub-population (or in the whole population) then we can see a response to the taste in bug selection by the predator/s and subsequent selection for the populations of bugs with less paletable taste, allowing the less palatable taste to become predominant and even reinforced by later mutations for even less palatability.
We are seeing more and more instances of mutations that are initially neutral being spread by neutral drift, and then becoming subject to selection, and this could easily be a similar situation.
Another group of organisms that exhibit this noxious taste ability are the poison dart frogs of south america.
quote:Poison dart frog (also dart-poison frog, poison frog or formerly poison arrow frog) is the common name of a group of frogs in the family Dendrobatidae which are native to Central and South America. Unlike most frogs, these species are active during the day and often exhibit brightly-colored bodies. Although all wild dendrobatids are at least somewhat toxic, levels of toxicity vary considerably from one species to the next and from one population to another. Many species are critically endangered. These amphibians are often called "dart frogs" due to indigenous Amerindians' use of their toxic secretions to poison the tips of blowdarts. In fact, of over 175 species, only three have been documented as being used for this purpose (curare plants are more commonly used), and none come from the Dendrobates genus, which is most characterized by the brilliant color and complex patterns of its members.
As far as I know, these frogs do not use their toxins to poison prey (the way some snakes and lizards do), and that it is only defensive.
We can also discuss the evolution of color in such populations, once selection pressure for any camouflage or blending into the background to reduce predation has been removed, but that's getting off topic.