Ohhh, the pain. The pain. One of my cherished beliefs is under attack, and I’m doing what we monkeys do when that’s the case. Resisting. Bargaining. Denying.
There are two illustrations of selection — one natural, the other artificial — that I’ve always adored for their explanatory power and elegance.
One is the peppered moth. Peppered moths are light grey with dots of black and brown all over–perfect camouflage for the local light-colored tree bark in 18th century England. A few were completely black, but only a few, because they were easy for birds to spot and eat.
In the 19th century, factory smoke blackened the tree bark in the moths’ range. The black moths were now perfectly camouflaged and quickly became the favored phenotype, while the light grey became visibly delicious. The proportions switched — almost all of the moths in the forest were now black and only a few light grey.
Experiments were conducted to confirm the hypothesis in the mid 20th century. Errors subsequently discovered in those experiments led creationists to trumpet the supposed dethroning of the peppered moth as an illustration of natural selection. But subsequent, better-designed experiments have re-confirmed the original hypothesis to the satisfaction of the relevant experts.
In the book Moths (2002), Cambridge biologist Michael Majerus sums up the consensus in the field: “I believe that, without exception, it is our view that the case of melanism in the Peppered moth still stands as one of the best examples of evolution, by natural selection, in action.”
Sure enough, several other experts in both moths and industrial melanism have also written to reaffirm the peppered moth story as a robust exemplar of natural selection writ small.
But there’s another selection story I adore — and that turn of phrase tells you all you need to know about my vulnerability on this one. It’s the story of the heikegani, a crab found in the waters of the Inland Sea in Japan near Dan-no-ura.
The sea was the site of a major battle in 1185 between Heike and Genji warriors. The Heike were trounced, and the survivors are said to have thrown themselves into the sea in disgrace.
In telling the story of the struggle, an epic called the Heike Monogatari refers to a species of crab in the Inland Sea as reincarnations of the Heike warriors defeated at the Battle of Dan-no-ura. And no wonder — the shell of the crab includes markings that evoke a scowling samurai warrior. And I don’t mean “evoke” like Ursa Major evokes a bear (psst, it doesn’t). I mean the crab looks like a scowling samurai warrior.
In the original Cosmos series, Carl Sagan offered the heike face as an example of artificial selection.1 Fisherman in the area have known the legend for eight centuries. During that time, if the nets pulled up a crab with markings resembling a human face, even mildly so, the fisherman — understandably loathe to disturb the spirit of the samurai — would throw it back. Crabs with less facelike markings would end up dipped in butter. The more facelike, the more likely it would be tossed back in with a girlish scream, free once more to fornicate with others of its uncanny ilk.
Eight hundred years of this and you’ll find yourself looking at some pretty scream-worthy samurai crabs.
What’s most awe-striking about this is the fact that unlike other examples of artificial selection — dog breeding for example — the selective pressure exerted by the fisherfolk is wholly unintentional, but still works. It combines random variation and decidedly nonrandom selection in a way that mimics natural selection incredibly well.
I happen at the moment to be putting the finishing touches on a new seminar (this one based on Raising Freethinkers) to be offered for the first time at UUC Atlanta on January 11. While polishing a section on helping kids understand evolution, I remembered that I didn’t just have moths to work with, I also had crabs. Ahem.
But in Googling for images, I came across the last thing I ever wanted to see: a sturdy, possibly even convincing attempt by a reputable scientist to debunk the hypothesis, claiming that the crabs are seldom kept and eaten regardless of markings, and that nearly identical markings are found on fossil crabs. And some other stuff.
Now the only worthy response to this news is Oooo, truth beckons, let’s follow this lively gent wherever and to whatever abysses he shall lead, lest we miss the chance to glimpse our precious reality more clearly!
Instead, I recoiled. Nooooooo, I thought. Bad man. Stranger danger.
I may have mentioned that I love the story, love the elegance of the hypothesis. I want it to be true. It is too beautiful to not be true.
I KNOW, I KNOW. Don’t lecture me, people. This is confessional literature here. These are the moments that make me empathize with religious folks who are disinclined to lift the veil on their own favorite bedtime stories. Once in a while, I feel their pain.
1Though Sagan got it from a 1952 article by biologist Julian Huxley.
Postscript: When Erin asked for “something new” as a bedtime story last night, I told her the tale of the heikegani, from battle to Cosmos. But when I reached the hypothesis, I did the right thing: “Some scientists think it looks like a face because…” The caveat made it no less cool to her.