How does mimicry evolve?

I think one key to coming to grips with those feelings that "evolution doesn't seem credible" is that there are gazillions of insects and millions and millions of generations for these mutations to occur.

Getting back to the question of "How does the bug know that orange and black stripes will benefit him," the answer is, he doesn't. Some random mutations make the bug green and brown, some random mutations make the bug blue and black, and some random mutations make the bug orange and black. The former two color schemes don't give any advantage, and they soon die out. But as the orange and black mutant lives a nice long life unhassled by birds, he passes his orange and black genes on to the next generation. His offspring that are black and orange also have an advantage—they live long lives and raise up many offspring.

The great majority of random mutations die out, but the few that offer some advantage stay in the gene pool and even come to dominate it.

--Stephen

Stephen Cresswell
Buckhannon, WV
www.stephencresswell.com

Yes, a good point and I agree with it.

But why does such a mutation come about in the first place? I mean, you have 2 totally different species, albeit somewhat closely related (we're not talking spiders mimicing snakes). They are somewhat in the same... err lineage (genus, family you know what i'm getting at i hope)

So, anyway you have this arachnid and then you have an ant. Well, surely the ant-mimicing arachnid lives in a ant-populated area. and somehow the gene comes about where the spider begins to mimic an ant..

But why? I Like i can't help it - i have been drawn to figure this answer out... why does the spider attain this trait? And how?

I'm trying to wrap my brain around this (for who knows what reason). But where does that mutation come from - does the spider KNOW how to pretend to be an ant somewhere? What triggers that mutation to make the spider use its front legs like antannae?

Something has to tell the spider's genes "hey, trying acting like an ant next time!"

I mean, how come a spider doesn't attempt to imitate a beetle or a bird or whatever.. do you see what i'm getting at? yes i'm weird but i can't help to want to figure out the finer details...

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?), but that mutation didn't prove to benefit the spider, and it "died off."

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.

"...pretty good evidence that evolution, at least sometimes, moves in leaps and bounds rather than the classic view of constant minor changes."

I assume you're thinking of the concept of punctuated equilibria. 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 30,000 or 50,000 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? That seems to put a hole squarely in the middle of the mimicry/evolution theory to me. 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., 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.

The newt/garter snake story is a little more complicated. 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.

First remember its not a conscience decision made to, say, evolve fins instead of wings (*random example*) but a matter of need/benefit, fin like wings provide better water mobility/propulsion to a penguin then flying wings for gaining food, thus flying wings loose out to fin like wings over MANY successive generation.

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.") As Karl says above, if the predator is a generalist, with many prey species, there is not much pressure to get around the defenses of one type of prey.

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.

Any time you have change, populations which are isolated from each other change in different directions. Eventually you get different races, then subspecies, then species, etc. The insects adapt to the the hosts in their specific area and thereby become different themselves.

The result is a great diversity of plants mirrored by a great diversity of insects, which helps to keep taxonomists very busy.