does the theory of evolution have an explanation for inter-species mimicry?

July 17, 2005 11:33 AM

does the theory of evolution have an explanation for inter-species mimicry?

disclaimer: i don't subscribe to any -ism, just trying to find an answer to a question that's been troubling me.
i remember a striking picture i saw as a kid in the national geographic of a caterpillar that, when sensing danger, inflates two "bags" that look like big, shiny eyes, transforming its entire appearance to that of a poisonous snake. before reading the explanation, it just looked like a picture of a snake. (something similar, yet not as impressive, here.)
i read about batesian and mullerian mimicry, but they only refer to similar species mimicking one another. how does science explain, for instance, the unmistakable owl eyes on the wings of certain moths?

posted by Silky Slim to science & nature (22 comments total)

Why should this need an explanation different from the natural selection of any other trait?
posted by jjg at 11:40 AM on July 17, 2005

What jjg said. If you look sort of like something that could eat your predators, then your predators are less likely to eat you. Multiplied over generations, this nets you mimicry of any accuracy you desire.
posted by ubernostrum at 11:49 AM on July 17, 2005

Note that evolution is not the same thing as natural selection.
posted by docgonzo at 11:53 AM on July 17, 2005

docgonzo, what do you mean by that? care to elaborate?

i guess what i know about evolution isn't enough to answer this question to my satisfaction. i've seen the famous example of the possible stages the eye may have taken over the generations; from a recess in the skin, to a photo-sensitive cavity, all the way to a fully-formed eye. i also remember seeing a documentary about a group of scientists who attempted to explain the development of the wings of some flying insect by clipping the wings of various specimens to various lengths, and trying to see if the shortened, tiny wings were still useful on their own, as opposed to a stage in the development of something else. (they came up with fluttering across the surface of the water.)
i guess i'm wondering whether someone tried this theoretical approach to the formation of, say, the moth wing eyes. let's say some generation started displaying a certain blotch or spot on its wings. at an early stage, they probably didn't resemble eyes at all. would a pair of blotches be enough to significantly increase the blotched specimens' chances of survival? let's jump a few million generations ahead. the blotches have evolved into kinda-eyes, but not quite the photo-realistic eyes we see today. would the addition of a few tiny white dots in the black area of the eye make enough of a difference in the survival prospects of that moth to register? somehow, that's just not a good enough answer for me. don't know why.
at any rate, thanks all for indulging me.
posted by Silky Slim at 12:46 PM on July 17, 2005

Evolution is often attributed as having some kind of "intelligent" component. It doesn't.

Evolution is only based on these two principles, really:
1. Random genetic mutations happen all the time, because reproduction is not a perfect 1-to-1 copy act.
2. When a living organism outlives its brothers and sisters, the genetic material it contains propogates to the next generation.

So, what makes your question profound is only the fact that we're looking at the results of combining #1 and #2 for literally millions of generations. This gives surprisingly complex results, even though the underlying framework is surprisingly simple. The mimicry didn't happen overnight, and wouldn't have been recognised as such in previous incarnations.

Just, as sort-of a little side track, you might be interested in playing a little with Context Free, a program to generate graphics using a simple language. Poke around in the forums and play with some other peoples' code. You'll quickly see how beautiful, complex, intricate shapes can be generated from extremely simple rules. Just food for thought, really. I consider it related.
posted by odinsdream at 12:47 PM on July 17, 2005

As odinsdream hints, the theory of evolution has an explanation for everything, because it is purely descriptive. That is, given that evolution is true, then if we see something in biology, however improbable that something is, it must be the product of evolution.
posted by gd779 at 12:57 PM on July 17, 2005

jjg: Why should this need an explanation different from the natural selection of any other trait?

ubernostrum: What jjg said. If you look sort of like something that could eat your predators, then your predators are less likely to eat you. Multiplied over generations, this nets you mimicry of any accuracy you desire.

ubernostrum, you're probably right. But you've committed a fallacy anyway - it's the one Stephen Jay Gould used to refer to as a "Just So Story."

Observed: that caterpillars and moths that have big eye-like spots on them do survive and continue to propagate. Fine.

But then making up a story to explain the observation - that's not part of the theory of evolution, nor does it follow from the concept of natural selection. You'd have to do some extra work to prove that that was true - say, painting eyespots on a population of plain green caterpillars in a field and showing that their loss to predation was reduced, or something like that.

A lot of wrong answers in evolutionary biology have come out of these "Just So Stories."
posted by ikkyu2 at 1:18 PM on July 17, 2005

In line with odinsdream's link, I'm including a (DANGER! DANGER!) self-link. Here's an app I wrote years ago, when I was teaching myself Javascript. You start with random sequences of letters and they gradually evolve into words. I made it for fun -- just a toy -- but a few people have said it helped them get a better grasp on evolution.
posted by grumblebee at 1:19 PM on July 17, 2005

Errr, no, gdd79, that's not true. Or rather it is true, but trivially true (X=X). All of modern biology is descriptive.

Silky Slim, there is no such thing as "inter-species mimicry." I'm not sure where you heard this term, but my limited research indicates it is not widely used. What looks like mimicry to you is nothing more than what were once called 'maladaptive traits'. Evolution doesn't that take place in a vacuum and it's hardly a completely arbitrary process. Eveolution takes place in highly specific, complex ecological niches. If you understand this, the existence of mimicry no longer seems so strange. It is no different from the manner in which many poisonous and non-poisonous animals developed bright colors as a kind of primitive "warning system" or the plethora of extraordinary camoflauge that exists in the natural world. In each case you have a species responding strongly to environmental factors in a highly particular manner. (The reason this was once viewed as a "bad" thing is that when the environment changes such highly adapted species will get wiped out). So in the case of a catepillar it's hardly the case of one species trying to imitate another species, but of an animal attempting to display to look like an abstract "poisonous thing" (long, bright colors, big eyes) that is similar to many other poisonous things in its environment. As for the "owl eyes" on moth's wings... never even heard of that... that sounds like folklore. What you call "owl eyes" may just be "big circular dark patches."
posted by nixerman at 1:22 PM on July 17, 2005

posted by Silky Slim at 1:46 PM on July 17, 2005

I don't think anyone is really answering docgonzo's question, and I'm not a biologist, so I'm not really qualified. It's clear how an animal has an advantage if it can mimic a dangerous predator. But it wasn't able to mimic that predator overnight. Before it developed big scary "eyes" on its wings, it had to first develop proto-big scary eyes, which probably didn't look much like eyes at all. Evolution is not smart. It can't say, "hang on, I know these don't look like eyes yet, but stick around for a few thousand generations and they WILL." For an animal to pass on its traits to a future generation, it must be fit NOW. And would tiny discolorations -- which really don't look like eyes (but which will evolve into eye-like markings in FUTURE generations) -- really help an animal survive?

My non-expert guess would be that such traits could piggyback on other survival-necessary features, as-long-as they don't hurt survival. In other words, if a mutation causes some trait in me that doesn't help or hurt me, I might pass on this trait if OTHER traits make me fit enough to survive. Eventually, the piggyback trait might wind up being useful.

Also, it IS possible that even amorphous blobs could be read as eyes (or some other scary trait) to some animals.

Or the blobs are useful for some other purpose and the eye-like look is a side-benefit not necessary for the survival of the trait.
posted by grumblebee at 1:47 PM on July 17, 2005

nixerman - i made up the "inter-species" bit, but "mimicry" is commonly used to explain extreme similarity between different species.
posted by Silky Slim at 1:55 PM on July 17, 2005

grumblebee - interesting. after all, creatures display many seemingly random patterns in their fur etc. maybe these patterns are all shots in the dark towards something that'll end up useful.
(btw, i asked the question, not docgonzo.)
posted by Silky Slim at 1:58 PM on July 17, 2005

If the first proto-big scary eyes have any net beneficial impact on the organism's reproductive success, even if it is very small, then it's likely that that trait will survive. Remember that we're talking about large populations and very large amounts of time -- small differences cascade.

This is a very common question.
posted by xil at 2:08 PM on July 17, 2005

Before it developed big scary "eyes" on its wings, it had to first develop proto-big scary eyes, which probably didn't look much like eyes at all.

I'm not an expert here, but how do we know this? Is it impossible for a butterfly to have randomly mutated what look to us like big scary eyes in a single generation, or some other short span? Why do we assume that it would've had to go through stages to get there, each one more closely resembling eyes? It sounds like you're committing the same fallacy you're warning about.

This comment by nixerman seems to do the same thing:

It is no different from the manner in which many poisonous and non-poisonous animals developed bright colors as a kind of primitive "warning system" or the plethora of extraordinary camoflauge that exists in the natural world. ... So in the case of a catepillar it's hardly the case of one species trying to imitate another species, but of an animal attempting to display to look like an abstract "poisonous thing" (long, bright colors, big eyes) that is similar to many other poisonous things in its environment.

Maybe I'm misreading you, but I don't think the animal is "attempting to look like an abstract poisounous thing" at all.
posted by ludwig_van at 2:26 PM on July 17, 2005

Before it developed big scary "eyes" on its wings, it had to first develop proto-big scary eyes, which probably didn't look much like eyes at all.

Sure they did. Many animals have poor eyesight. Some can barely detect anything other than movement. I can imagine that big splotches looked enough like eyes to help those butterflies survive a little better (because bad-eyesight predators were deterred). Realistic eyes helped later butterflies survive even better (because even good-eyesight predators were deterred).

Of course, the pressures work on the predator as well as the prey. The predators with slightly better eyesight who can tell that a eye-winged butterfly is still prey will survive better. Then butterflies with more eyelike wings will survive better, then predators who can see better will survive better...and so on and so on until everyone gets faster and smarter and better eyesight and better camoflauge and better weaponry that destroy everything so we all have to start over.

(Oops--got carried away.)
posted by equipoise at 2:38 PM on July 17, 2005

would a pair of blotches be enough to significantly increase the blotched specimens' chances of survival?

Some study I saw once indicated if a trait increased the likelyhood of surviving to reproduce by a mere %1, it would (on average) spread to fully half the population in less than 100 generations. And as already noted, mimicry does not have to be even remotely accurate to have a significant effect, it just works a little better if it's accurate. Advantages much smaller than 1% likewise are more than sufficient to spread.

The differences don't need to be large to be selected for. Luck is of course muddies the lives of individuals, but over time, tends to even out. You'd be surprised how small an edge is needed to make a difference in the cut-throat world of nature.

Think of the extreme accuracy required by the judging apparutus of the Olympics - a hundreth of a second can be sufficient to out-compete a competitor. And they're only doing it as career - their death doesn't hang in the balance :-)

Keeping the Olympics metaphor, I don't need to be able to outrun a bear to survive a bear attack, I just need to outrun you by a hundreth of a second, and you become the bear food instead of me.

The proto-mimicry moth doesn't need to look like a preditor, it may be sufficient that the bland moth sitting next to it looks ever so slightly more like prey, and so the bird's first meal is that one, and the proto-mimicry moth manages to get away.

And your question is valid - the ability of natural selection to produce mimicry often does not appear to be a Sure Thing at any specific stage of evolution (ie, statistically, it will happen on average), so you might predict that some animals that would benefit from mimicry don't have it, or don't have it very well formed. And a quick look at nature seems to confirm this.
posted by -harlequin- at 3:11 PM on July 17, 2005

thanks everyone. some food for thought here.
posted by Silky Slim at 10:31 PM on July 17, 2005

I'm surprised that nobody's mentioned that this "gradual" theory of evolution doesn't seem to be as useful as that of punctuated equilibrium.

It has been summarized by Gould (1980, pp. 183-4) as follows:

"Large, stable central populations exert a strong homogenizing influence [on the gene pool]. New and favorable mutations are diluted by the sheer bulk of the population through which they must spread....But [in] small, peripherally isolated groups [that] are cut off from their parental stock ... selective pressures are usually intense because peripheries mark the edge of ecological tolerance for ancestral forms. Favorable variations spread quickly...

"What should the fossil record include if most evolution occurs by speciation in peripheral isolates? ... In any local area inhabited by ancestors, a descendant species should appear suddenly by migration from a peripheral region in which it evolved. In the peripheral region itself, we might find direct evidence of speciation, but such good fortune would be rare indeed because the event occurs so rapidly in such a small population."
posted by Aknaton at 11:30 PM on July 17, 2005

Silky you might also be interested in these articles (second link is a PDF) on the thoughts of Vladimir Nabokov on mimicry, 'non-utilitarian evolution', aesthetics, and teleology.

I would make a more coherent and comprehensive post, but I'm exhausted.

Cheers,

trip
posted by trip and a half at 12:41 AM on July 18, 2005

Richard Dawkins (I Am Not an Evolutionary Biologist. He makes a fairly convincing case, but I'm not arguing for that specifically--just throwing out one theory) argues that it wouldn't have to look like big scary eyes all the time--even the first steps toward big scary eyes could scare off a predator in bad light conditions, or from a distance, or (as Equipoise said) if the predator couldn't see that well in the first place.
posted by Jeanne at 3:26 AM on July 18, 2005

docgonzo: I see. Obviously the question was wanting an explanation within the context of, say, Lamarck's theory of evolution, and not the theory of evolution by natural selection which is foundational to modern biology. Right?

ikkyu2: no, it's not a fallacy. Gould's point was that it is incorrect to assume that each and every adaptation we see in the natural world exists for a specific purpose, unless we have credible evidence which warrants the assertion of a particular purpose (which is actually a bad word here; "selection pressure" would be a better term) as explanation for a particular adaptation. This was all within the context of his larger point that we often assume a general purposiveness behind evolution, whether toward the development of human beings or toward the development of "more complex" or "more intelligent" creatures in general, an assumption he spent a good deal of time fighting (and rightly so).

However, in this case I fail to see any harm from guessing that early variations which resulted in some form of eyespots created a selection pressure in favor of such spots, and that this pressure "caused" the accuracy of the mimicry of such spots to grow with time. Such a guess should, of course, never be taken as given and should be empirically tested if at all possible, but that does not, in and of itself, justify a prima facie rejection of the guess.
posted by ubernostrum at 5:33 AM on July 18, 2005