Mimicry, the fascinating art of imitation, is all around us in the natural world. There are endless examples of both animals and plants that mimic other living creatures or inanimate objects, in a variety of different ways.
One example with which many people are familiar is the monarch and viceroy butterflies. It has long been believed that these butterflies were an example of Batesian mimicry. Batesian mimicry is when a species mimics the warning signals of another species without having the characteristics that make it undesirable to their shared predator. Think of it like hiding your favorite chocolates in a bag of raisins. Those chocolates still taste as delicious as ever, but, thinking that only raisins are inside that bag, many people may walk right on by.
Not long ago, scientists believed the viceroy was an attractive meal for birds, but had evolved to mimic the colorations of the toxic monarch to deter predators. A couple of scientists set out to remove the wings from viceroys, monarchs and queens, and feed the wingless butterflies to birds. The theory was that if viceroys actually tasted better than monarchs, they would be eaten far more often than the monarchs or queens when their warning colors were removed.
What the scientists found, however, was that the birds spit out the viceroys as often as they spit out the monarchs. These studies indicated that the monarch and viceroy butterflies, long considered as examples of batesian mimicry, are actually expressing Mullerian mimicry. Mullerian mimicry is when two species share similar anti-predator characteristics in this case, inedibility and co-mimic each other.
Again, not what you wanted. While all the red pieces were candy, neither had the characteristics you were hoping for, but the rest of the colors did. As a result, you quickly learned to avoid the red ones. This is considered a mutualistic relationship, as both benefited from an accelerated learning curve of their predators. Photo above shows a coral snake on the left, a milk snake in the middle and one type of false coral on the right.
Another astonishing form of mimicry is Emsleyan or Mertensian mimicry. In this form of mimicry, a deadly prey mimics the warning signs of a less dangerous species. A good example involves the milk, coral, and false coral snakes. I'm haven't really studied this stuff in detail, but I think it's supposed to work in small steps. So the first mutation would make a spider slightly more ant-like, and that trait turns out to be advantageous to the spider. Then a subsequent mutation makes the spider even more ant-like, and that succeeds too.
And so on. Perhaps there was a mutation that made the spider a tiny degree like something else a bird? I think this is pretty much right. However, the effects of mutations aren't always small - a single change can have quite a large effect if it occurs in the right place. There is a fair amount of debate about how this works among evolutionary biologists, but there is pretty good evidence that evolution, at least sometimes, moves in leaps and bounds rather than the classic view of constant minor changes.
I'm not sure what the benefit of ant-mimicry is for spiders, but I've always assumed it was for protection since ants can have powerful stings and bites, and the protection of nest-mates. That is to say long periods of relative stasis in a species, followed by relative short periods of rapid change, sometimes leading to speciation.
The thing to remember about this is that the leaps and bounds may take tens of thousands of years, and that's in rapidly reproducing species. Stephen Gould and Niles Eldridge, who proposed this concept, studied snails, and saw changes as suddenly as 30, or 50, years, I believe.
Those leaps are called "hopeful monsters" by some. One mutation can have a large effect on development. Richard Dawkins has written several books explaining some of these mechanistic aspects of evolution. Another general comment on this thread: I think the main agent for driving visual mimicry in insects must be birds. They have excellent eyesight, and small birds are constantly gleaning foliage for edible insects. There's been a fair amount of research on this--I'm thinking of the studies of the Monarch mimicry complex.
Once birds learn that a given insect pattern is associated with a bad taste, they learn to avoid any insect that looks like that. Voila, mimics are protected. I think it's clear, too, that even a small resemblance provides some protection. Birds are constantly on the move and have to make quick decisions about what's edible and what's not. So if that bug doesn't resemble an ant when seen closely, it may still gain some protection if it resembles an ant from one meter. Patrick Coin Durham, North Carolina.
Okay, maybe a Viceroy will mimic a Monarch butterfly so that it won't get eaten by birds. So maybe they evolved this trait over a long period of time. So, how come the bird just doesn't evolve itself to where the bad taste no longer tastes bad? Or, at least it does to my way of thinking. If everything's always evolving, how come everything isn't always evolving? Actually some birds develop tolerance to the toxins and eat Monarchs with gusto 1.
We must remember that most creatures are better adapted to previous conditions than present ones and have to keep adapting. Patrick beat me to it while I was thinking about it and presented all the arguments better than I could have done.
This process goes by the name of "evolutionary arms race" as he said. It is also called the "Red queen mechanism" in reference to Alice in Wonderland: "it takes all the running you can do, to keep in the same place". It goes on everywhere, all the time, in every imaginable way: the milkweed needs to evolve stronger toxins, the Viceroy has to become a better mimic, while the Monarch doesn't want to be identified with the mimic, etc.
Isn't evolution fascinating? I suspect that most "evolutionary arms races" reach some kind of equilibrium. That is to say, at some point the cost of a mutation must outweigh the benefit. Different newt populations have a wide variety of levels of the toxin tetrodotoxin, the same toxin as found in fugu.
And different snake populations have a variety of different reactions to the toxin. I have some links on my Taricha granulosa page. Well, some milkweeds have higher toxicity and some generations of monarchs acquire different levels of toxins as a consequence. I vaguely remember reading an article about different levels of predation. It goes on and on. Always interesting. If the monarchs were the only food source then evolving a tolerance of the taste would be possible, or the birds would die off from starvation.
However since there are masses of much better tasting foodstuff flying around there is no pressure to develop a tolerance. I'll add, too, that one thing that may put a brake on these evolutionary processes is the cost of the traits involved.
Producing toxins requires energy that could otherwise be used for growth and reproduction. Development of metabolism to remove toxins by the predator likewise has a cost to the other functions of the organism. Presumably, an equilibrium is reached between costs and benefits in each case. This is why, perhaps, "everything is not always evolving. Now the synchronized evolution of pairs of species is studied a lot by evolutionary biologists, and is called co-evolution.
That Wikipedia article gives at least one example of an "arms race" between predator and toxin-defended prey: Co-evolution also occurs between predator and prey species as in the case of the Rough-skinned Newt Taricha granulosa and the common garter snake Thamnophis sirtalis.
In this case, the newts produce a potent nerve toxin that concentrates in their skin. Garter snakes have evolved resistance to this toxin through a set of genetic mutations, and prey upon the newts. The relationship between these animals has resulted in an evolutionary arms race that has driven toxin levels in the newt to extreme levels. I believe that another good example of co-evolution is Batesian mimicry complexes, where non-toxic or less toxic species such as the Viceroy evolve to look like the toxic Monarch.
There is probably pressure on the Monarch to develop coloration distinctive from the Viceroy, because the reduces the effectiveness of the warning colors--and the race is on.
Again, the race has costs for each racer, so these may slow down, or even stop, the race at some point. Of course, it is hard to tell how fast the race is running by looking at one snapshot.
A great many plant species have phytochemicals that make them unpalatable or unhealthy for at least some insects to eat.
Insects evolve changes to their metabolism that neutralize the phytochemicals in the types of plants they eat. As the plants evolve new phytochemicals in response, the insects adapt, and so on. Adaptations to deal with the profile of phytochemicals in one type of plant may not help much with those of other plants, so the insect becomes very host-specific.
The adaptations can be like a key that opens the specific door, but won't work for even very similar locks.
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