The embryological processes by which beaver genes shape beaver tails are not known in detail, but we know the kind of thing that goes on…Genes, in each of the different kinds of cells in a beaver’s tail, behave as if they “know” where they are … There are formidable difficulties in working out how they “know”… but we understand in principle how these difficulties are overcome; and the solutions, like the difficulties themselves, will be of the same general kind when we turn to the development of tiger feet, camel humps and carrot leaves.
They are also of the same general kind in the development of the neuronal and neurochemical mechanisms that drive behavior. Copulatory behavior in beavers is instinctive … In beaver gene pools, genes survived whose phenotypic effects on the brains, the nerves, the muscles, the glands, the bones, and the sense organs of generations of ancestral beavers improved the chances of those very genes passing through those very generations to arrive in the present.
Genes “for” behavior survive in the same kind of way as genes “for” bones, and skin. … Genes are “really” or “directly” responsible only for proteins or other immediate biochemical effects. All other effects, whether on anatomical or behavioral phenotypes, are indirect. But the distinction between direct and indirect is vacuous. What matters in the Darwinian sense is that differences between genes are rendered as differences in phenotypes…
…[definition of phenotype from OED:] “A type of organism distinguishable from others by observable features.” The key word is distinguishable. [goes on to discuss eye color, alleles, etc.] … Gene differences cause phenotypic differences. Gene changes cause phenotypic changes. In Darwinian evolution alleles are selected, vis a vis alternative alleles, by virtue of the differences in their effects on phenotypes.
The beaver’s point is that this comparison between phenotypes can happen anywhere along the chain of causation. All intermediate links along the chain are true phenotypes … There is no such thing as the “ultimate” link in the chain: no final, definitive phenotype. Any consequence of a change in alleles, anywhere in the world, however indirect and however long the chain of causation, is fair game for natural selection, so long as it impinges on the survival of the responsible allele, relative to its rivals.
Now, let’s look at the embryological chain of causation leading to dam-building in beaver. Dam-building bheavior is a complicated stereotypy, built into the brain like a fine-tuned clockwork mechanism… [It] is hard wired in the brain. …
Only beavers have this kind of brain clockwork. … *t must have evolved in slow degrees in ancestral beavers. It evolved because the lakes produced by dams are useful. It is not totally clear what they are useful for, but they must have been useful for the beavers who built them, not just any old beavers. The best guess seems to be that a lake provides a beaver with a safe place to build its lodge, out of reach for most predators, and a safe conduit for transporting food. Whatever the advantage it must be a substantial one …
Like any other useful adaptation, the dam-building clockwork in the brain must have evolved by Darwinian selection of genes. … But which is the phenotype? At which link in the chain of causal links shall we say the genetic difference exerts its effects? The answer, to repeat it, is all links where a difference is seen. [brain “wiring,” cellular chemistry] But also behavior … is a perfectly respectable phenotype. … And, by the same token, the consequences of that behavior are also allowable as phenotypes of genes. What consequences? Dams, of course. And lakes, for these are consequences of dams. Differences between lakes are influenced by differences between dams, just as differences between dams are influenced by differences between behavior patterns, which in turn are consequences of differences between genes. We may say that the characteristics of a dam, or of a lake, are true phenotypic effects of genes, using exactly the logic we use to say that the characteristics of a tail are phenotypic effects of genes.
Conventionally, biologists see the phenotypic effects of a gene as confined within the skin of the individual bearing that gene. The Beaver’s Tale shows that this is unnecessary. The phenotype of a gene, in the ture sense of the word, may extend ouside the skin of the individual. {example: birds’ nests}