Genes as Words or as Subroutines

Genes as Words or as Subroutines

The analogy between genes and words is useful, as we have just seen, but there is a better one that we are now equipped to understand, thanks to the interlude about computers. In his masterpiece, The Ancestor’s Tale, Richard Dawkins (2004) gives credit to another brilliant writer on evolution, Matt Ridley, for pointing out the deep similarities between genes and software subroutines in his book Nature via Nurture. I would not ordinarily insert such a long quotation of somebody else’s work in a book of mine, but I found that my attempts at paraphrasing this passage invariably sacrificed clarity and vividness for a smidgen of originality, so, with his permission, I am passing along the unmutated version as it appears in Dawkins’s book.

Most of the genome that we sequence is not the book of instructions, or master computer program, for building a human or a mouse, although parts of it are. If it were, we might indeed expect our program to be larger than the mouse’s. But most of the genome is more like the dictionary of words available for writing the book of instructions—or, we shall soon see, the set of subroutines that are called by the master program. As Ridley says, the list of words in David Copperfield is almost the same as the list of words in The Catcher in the Rye. Both draw upon the vocabulary of an educated native speaker of English. The difference between the two books is the order in which those words are strung together.

When a person is made, or when a mouse is made, both embryologies draw upon the same dictionary of genes: the normal vocabulary of mammalian embryologies. The difference between a person and a mouse comes out of the different orders with which the genes, drawn from that shared mammalian vocabulary, are deployed, the different places in the body where this happens and its timing. All this is under the control of particular genes whose business it is to turn other genes on, in complicated and exquisitely timed cascades. But such controlling genes constitute only a minority of the genes in the genome.

Don’t misunderstand “order” as meaning the order in which the genes are strung out along the chromosomes. With notable exceptions, . . . the order of genes along a chromosome is as arbitrary as the order in which words are listed in a vocabulary—usually alphabetical, but, especially in phrase books for foreign travel, sometimes an order of convenience: words useful in airports; words useful when visiting the doctor; words useful for shopping, and so on. The order in which genes are stored on chromosomes is unimportant. What matters is that the cellular machinery finds the right gene when it needs it, and it does this using methods that are becoming increasingly well understood. . . .

In one respect the analogy with words is misleading. Words are shorter than genes, and some writers have likened each gene to a sentence. But sentences aren’t a good analogy, for a different reason. Different books are not put together by permuting a fixed repertoire of sentences. Most sentences are unique. Genes, like words but unlike sentences, are used over and over again in different contexts. A better analogy for a gene than either a word or a sentence is a toolbox subroutine in a computer. . . .

The Mac has a toolbox of routines stored in ROM (Read Only Memory) or in System files permanently loaded at start-up time. There are thousands of these toolbox routines, each one doing a particular operation, which is likely to be needed, over and over again, in slightly different ways, in different programs. For example, the toolbox routine called ObscureCursor hides the cursor from the screen until the next time the mouse is moved. Unseen by you, the ObscureCursor “gene” is called every time you start typing and the mouse cursor vanishes. Toolbox routines lie behind the familiar features shared by all programs on the Mac (and their imitated equivalents on Windows machines): pulldown menus, scrollbars, shrinkable windows that you can drag around the screen with the mouse, and many others. [Dawkins, 2004, pp. 155–156]

All this helps us understand why it is so easy for an expert to recognize the genome of a mammal. It has the mammal toolbox, which in addition to its specialized mammal-making tools also includes tools from the reptile toolbox, and the fish toolbox, and even the worm toolbox. The oldest tools in the kit are shared by all living things, including bacteria.