Black Jaguars and Colored Marbles

There is a gene knocking around in jaguars that turns about six percent of them completely black.  You may have known this.  People call them black panthers, but they’re really jaguars with a genetic condition called melanism, which is sort of the opposite of albinism.  I stumbled across this while writing my recent article about jaguars.  I spared you the digression at the time, though I don’t know why—it’s not normally my nature to spare my readers a hairpin turn.  Anyway, I set it aside, but a couple of things had gotten my attention about it, and months later they were still bugging me, so I finally gave up on myself and started reading again.

One thing that I couldn’t let go of is that a jaguar’s coat is yellow with black rosettes.  Going from that to solid black is an extremely radical change in coloration, and evolution cares a lot about coloration, especially in ambush predators.  Think about it:  that jaguar spent tens of thousands of years carefully evolving a coloration that is perfect for the dappled sunlight of a tropical jungle—and then one pops out who is solid black?  Are you kidding me?  Why didn’t evolution, which had gone to all the trouble of creating a coat that is perfect for what the jaguar does, immediately select against this trait, and consign it to genetic oblivion?  Why haven’t black jaguars disappeared?

And the other thing that bugged me was the six percent number.  That’s a big number.  It may sound smallish, but the mutation rate in large American cats is only a few hundredths of a percent per gene per generation.  Black jaguars haven’t just avoided oblivion, they’re downright thriving. They’re being selected for.  I wanted to know why.  The first several places I looked casually told me the same thing:  “It confers an advantage,” they said.  Okay, I said.  Okay.

Then why aren’t all jaguars black??

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Melanistic jaguar and normal cub. Sometimes you can still make out the rosettes. By MatthiasKabel GFDL or CC BY-SA 2.5-2.0-1.0, via Wikimedia Commons

Circular questioning is an occupational hazard for me, and the centrifugal force that finally blew me out of this particular loop is a delightful and fascinating topic called polymorphism.  The reason polymorphism exists is that genetic drift exists, and to understand genetic drift what you need to do is go to your closet and grab your big jar of blue marbles and your big jar of red marbles.  Go ahead, I’ll wait.  We’re going to do an exercise, and it’s not about biology, it’s about random number theory.

Put both big jars of marbles on the table in front of you.  Then put a bowl in front of them and label it “Generation 1.”  Into that bowl, put 10 red marbles and 10 blue marbles from the big jars.  Mix them up.  Now put another bowl next to it and label it “Generation 2.”  Close your eyes and grab a marble from bowl 1.  Note its color, then put it back.  If it was red, take a red marble from the big jar of red marbles and put it in bowl 2.  If it was blue, do the same with a blue one.  Do this twenty times.

When you’re done, you might have 10 red and 10 blue marbles in the bowl labeled Generation 2, but you’re more likely to have 8 or 9 of one and 11 or 12 of the other, because that’s the nature of random numbers.  In the short term there is variance.  In the long term there is not, which is why the casinos always fleece you in the end, but in the short term things sort of jump around a lot and are uneven and unpredictable.   Now repeat the process, creating a Generation 3 by looking at the marbles in the Generation 2 bowl.  This time, if you had, say, only 8 blue marbles in Generation 2, the odds will not be 50-50 anymore, they’ll be stacked at 60-40 in favor of the reds, so odds are that the number of blue marbles will decrease again in the next generation, and then those odds become worse yet.  Finally—and it may only take a handful of generations—one trait disappears, and is lost forever.  That’s genetic drift.  Even if natural selection absolutely, positively does not give a hoot whether you are red or blue, one of the traits will drift out of existence, just for mathematical reasons.

That’s what I thought should have happened to black jaguars.  If their numbers weren’t sent to zero, they should have at least drifted there—or at least down to the mutation rate, which is as close as you get to zero in these affairs.  Either that, or they should have fixated, which is the scientific term for becoming the new normal and going to 100 percent.  Black jaguars have done neither.  Their numbers are staying pegged at a fraction—a fairly sizeable fraction—of the population.  What gives?

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It’s not just jaguars: A melanistic eastern gray squirrel (Sciurus carolinensis). By Marc Steensma, CC BY-SA 3.0, via Wikimedia Commons

Evolution is delightful stuff.  At its heart it is so simple that a three-year-old can understand it.  Take two lumps of matter and give one of them the ability to replicate itself, and guess which one you’ll have more of in a little while?  Down at its core, evolution is that stupid-simple.  (And by the way, you’ll notice that nothing about that description requires oxygen, carbon, liquid water, an atmosphere or moderate temperatures—that’s just how things ended up on this planet.)  But simple though the principles may be, in the course of the evolution of, say, 8.7 million species plus or minus 1.3 million, which is the current estimate of species on this planet—well, odd things can happen.  One of them is polymorphism.

It turns out that black jaguars do indeed have an advantage, but it’s not because they’re black, it’s because they’re weird.  They’re not what the prey are looking for.  The prey are looking for a yellow animal with black rosettes.  So this works for them, and evolution selects for it, and they do better than their neighboring, normally-colored jaguars—until they become too numerous and the prey start to wise up.  Then they don’t have the advantage of being non-typical anymore, and the normal jaguars start to out-perform them again, because they actually do have the superior coloration for the job.  So evolutionary forces are acting to keep both forms in existence.  That’s polymorphism.  It acts to keep the black form in existence because it’s an extremely cute trick when no one’s expecting it, but the same forces keep black jaguars from fixating, because black jaguars are actually a little easier to spot if you know what you’re looking for.

There is a swallowtail butterfly in Africa called Papilio dardanus (with the delightful common name of the flying handkerchief) that has fourteen different polymorphic forms.  For almost a century scientists thought they were all separate species.  In this case it is mimicry—they are mimicking various toxic species of the Danainae family of butterflies, which is the monarchs, so that the predators will leave them alone (it’s called Batesian mimicry).  It works until they become too numerous and the predators start figuring it out, and then evolution will start favoring a different, more obscure form.  In the end, natural selection has ended up keeping fourteen different morphs in play, all in numbers that hover just below that critical level of commonness where the predators wise up.  And here’s a fun aside:  only the females have all these forms.  Evolution doesn’t seem to care as much about the males (I’m always feeling a little like a second-class citizen when I research evolution), so the males look more or less the same throughout their range. And what’s even more fun is that some of those female forms don’t mimic other species, they mimic the males of their own species.  I’m not making this up.  Scientists think it’s to keep the males from accosting them constantly to mate with them.  It turns out that the battle of the sexes was not invented by us—the males and females of these butterflies have a profound difference of opinion about when and how often they should have sex, and the battle lines are genetically encoded and enforced by evolution.  The male might stand the best chance of getting his genes into the next generation by mating with the female single-mindedly and repeatedly, but the female’s health gets a little bit threatened by all that pummeling, so evolution has favored a more chaste attitude on her part.  The result is opposing evolutionary forces—and opposing sexual attitudes—in the same species.  Evolutionary scientists call it sexual conflict.

Polymorphism goes on in our own bodies.  Did you ever ask yourself why the sickle-cell anemia gene still exists?  It has a habit of killing its carrier, so you’d think it would have been un-selected into oblivion, but the reason it hasn’t is that it also gives the owner a resistance to malaria, and while sickle cell anemia is pretty nasty, malaria can be absolutely devastating to populations of folks in the tropics.  Besides, you only get sick from sickle cell if you’re unlucky enough to get the gene from both parents, while the resistance to malaria is yours regardless.  Again, there are opposing selection forces at work for and against the same gene.  Malaria won.

The Broad Street Pump in London, with its famous missing handle. By Justin Cormack, CC Share-Alike Generic 2.0

Cystic fibrosis is a devastating genetic disease that has touched my family personally, and the reason that gene is still around is that it also gives its owner a resistance to cholera, and cholera killed tens of millions of people in Europe from the time we started to build cities right up until 1854, which is when Dr. John Snow demonstrated that it was all about contaminated water, and he’s a pretty good yarn too.  He investigated an outbreak in London by walking from door to door interviewing families of victims and putting dots on a map.  He narrowed it down to a well on Broad Street, and convinced the city council to remove its handle.  By that single famous act of removing the handle of the Broad Street pump, he changed history, launching the creation of urban sanitation systems, and, while he was at it, inventing modern epidemiology.  That shoe-leather process of putting pins in a map is how it’s still done to this day.  Now that chlorinated water supplies have largely wiped out cholera in the developed world, some scientists think (there is argument, of course) that the gene is now purely detrimental and is probably being pressured out, but these things take time when your generations are twenty years long.

Polymorphism is the reason we have blood types.  You would think that the type with the best survival rate would have long ago prevailed, but it’s way more complicated than that, and in fact, we are nowhere near to a full understanding of the polymorphism of blood types, and why four different ones have been kept in play, but it’s undoubtedly about disease, and also about geography.  For example, type A people are more susceptible to smallpox, type O people are more susceptible to bubonic plague, and in China, India and Russia, where there have been epic pandemics of both those diseases, there are a whole lot of type B people around.

So that’s the story on polymorphism, with only a couple of short detours into epidemiology and random number theory.  Nature just loves diversity, that’s all there is to it, and this is just one way that it is constantly acting to keep us all different.  As the sixties band Savoy Brown said an extremely long time ago, “Now wouldn’t it be a real drag if we were all the same?”

Now you know.