Saturday, September 16, 2006

The crab and the Samurai

Antuko, the child-Emperor of Japan, stared in somber silence. The year was 1185, and as he stared in to the ocean from his deck, he knew the end was near. A school of dolphins had approached his ships, and then dived under the ship and disappeared. Had they swum alongside the ships, his clan, the brave Heike, would have survived.

But he knew that victory was imminent for his rivals, the Genji clan. Three thousand of their ships approached his one thousand ships. Each of his ships carried his brave warriors, Samurai loyal to him.

He clasped the hands of his nurse, and they jumped in to the sea. His samurai followed him to the bottom of the ocean. The Heike would now rule the kingdom of the bottom of the sea. The world henceforth belonged to the Genji.

Japenese fishermen would find crabs called the Heikegani, which had shells that looked astonishingly like the mask of a samurai warrior. Thanks to this remarkable co-incidence, the crabs would not be eaten by the fishermen, who believed that the samurai of the Heike continued to live, as crabs at the bottom of the sea. This was my own summary of a folk fable that's still remembered well in Japan, and happens to be a famous examples that's attributed to artificial selection (quoted by Sagan in his Cosmos).

But may not be artificial selection alone at work here.

Artificial selection is a modification of a species by human actions. This results in specific traits being bread in the species. We’ve be at it for thousands of years now, and have bread different species of dogs (chosen for their size or fur or shape) or cats, or cattle or grain, or fruits and vegetables. The list is endless. The process of artificial selection, and the genetic events involved are the same (but often rapidly amplified, “artificially” by us) as natural selection, where organisims retain and propagate certain traits that are beneficial to them and allow them to survive or proliferate better.

With the crabs, the speculation was that the crabs that looked most like samurai masks survived, while those that didn’t look like masks became dinner. This may have created an artificial pressure for the proliferation of only those crabs that looked most like samurai, and due to that, today all the Heikigani look like Samurai masks.

However, this example isn’t really true of artificial selection, and there are much better examples of artificial selection (dog breeding that gave rise to different strains of dogs is much better). There is some truth in the artificial selection story here, but there are very strong other evolutionary reasons why the crabs have those dents on their shells (that just happen to look like samurai masks, when you throw the feet of the crab in there).

To quote:

….. The grooves are external indications of supportive ridges, called apodcmes, inside the crab's carapace that serve as sites for muscle attachment. Elevated areas between these grooves allow for an increase in internal space, so that the various parts of a crab's viscera - gastric, hepatic, cardiac, branchial, etc. -are reflected externally. This is not
to say that these structures are unaffected by selection ......The point here is that
these ridges and grooves occur in nearly all members of the crab family Dorippidae…..there are at least 17 different species of crabs in two families in the
Indo-West Pacific that are similar enough ..., and there are many related species from other far off waters that bear a likeness to a human face. Many Asian countries have names to account for the similarity of such crabs to a human face, such as the Chinese name Kuei Lien Hsieh (Ghost or Demon faced crab), and in several countries the crabs play a prominent role in local folklore, sometimes being considered sacred, with the face representing that of a deceased relative.

This is a very nice example of a mistaken hypothesis based on a final result.

Cause and effect again.

More interestingly though, this example still serves as a beautiful example of evolution by natural selection, that may have been influenced by artificial selection.

You can read all about the crab and its myths here.

I’m just going to use all of this as an excuse to post a picture of a Japanese painting depicting the fable, an exquisite Samurai mask, and the crab itself.


Anonymous said...

Isn't this example in a carl-sagan book?

Sunil said...

Yes it is.......i said as much in this post
"and happens to be a famous example that's attributed to artificial selection (quoted by Sagan in his Cosmos) but may not be so."

Part of the point of the post is to illustrate that scientists may be carried away by their own here....where it really seemed like a strong example of artificial selection. But it's not really....and is more an example of normal evolution and natural selection. Sometimes.......what appears obvious may not be so.

Sunil said...

hp.....thank you for the pointer to "Kwaidan". I'm a big fan of old Japanese cinema (there are a number of posts on that if you trawl the archives) I have little doubt that I'll enjoy it.

It's amazing though how sometimes you talk about something, and then it's everywhere :-)

Anonymous said...

well, they certainly don't LOOK at that appetizing... could it be that they also don't taste very good?

Sunil said...

anonymous.......there might be something to that......

if they weren't tasty, then artificial selection would also work again! But then...that couldn't be used to explain the deeper ridges, since if ALL crabs were not tasty, then the ones with face like shells and those without face like shells would both have equal chances of survival, so there wont be a drive towards one particular look. it was more a case of a mistaken assumption for artificial selection, though it seems more like natural selection at work!

Anonymous said...

Gud one. Are u a writer?

Sunil said...

Kulpreet......I write. Does that make me a writer? :-)

(I'm a scientist).

Anonymous said...

Sunil, if you are a scientist, then how come you think that artificial selection has led to the existence of different dog species ?

It hasn't. Now, a scientist would go back through his publications and correct any errors that he has made. Are you going to do this ?

There are other fallacies in your brief article about the crab, but I won't point them out to you until I see some evidence that you love truth.