Dogs, crows and garbage: phenogenetic drift

Scavenger hunts
Monday's the day trash gets picked up in our neighborhood. When I was out running early this Monday morning, I passed a scene I see not infrequently -- a group of crows congregated around a plastic trash bag, its contents spilling into the street and the birds tugging at anything in it that looked appetizing.  As I ran by, two of the birds flew but one brazen bird stayed right where he was, guessing that I was no threat to him.

Crows are very smart, and not just because they've figured out how easily plastic garbage bags are breached.  I've also seen them congregated around trash barrels with the lids knocked off.  I've never seen them actually take the lids off, but it must be they do.  When I was a child it was dogs who got into the trash.  This was before leash laws, when it really was a dog's life.  Dogs we keep as pets now have only ancestral memory to recollect the halcyon days of wandering the streets untethered on trash day.  Now it's a crow's life.

But sometimes it's bears, too.  A mother bear and her three cubs were making the rounds of our neighborhood a few weeks ago, knocking down bird feeders and compost bins, and this isn't an unusual occurrence.  The wilding of America?

This morning's sighting reminded me of a piece in the February 21 New York Review of Books by Russell Baker in which he reviews a book by Jim Sterba, Nature Wars: The Incredible Story of How Wildlife Comebacks Turned Backyards into Battlegrounds. I have not read the book, though now that I'm remembering this, I just might have to. Unhappily for the author, Baker's is one of those pieces that makes you think can skip the book, even as he gives it a fine review.

In any case, Baker describes seeing a pair of foxes mating in his garden situated, he says, just two blocks from the county courthouse in the bustling center of town. The encroachment into human-populated areas of animals who were once sighted only at a distance is becoming increasingly more common.

Sterba ... argues persuasively that events like this foxes-in-the-garden sighting are evidence that humans are losing some kind of property rights struggle with creatures of the wild. He cites an extensive history of resolute and sometimes blatantly hostile real-estate invasion by beavers, Canada geese, wild turkeys, and white-tailed deer, all of which were once assumed to be picturesque and even lovable denizens of the dark and safely remote forest. In-town appearances by coyotes and bears are now commonplace in communities across the country, and trespassers in my own garden, aside from the foxes, have included groundhogs, possums, skunks, feral cats, and one blue heron that ate all the koi with which we thought to beautify the fish pond.

Lucky heron.  As Baker points out, in the last fifty years these animals have discovered that life can be a lot easier closer to town, and food a lot more abundant.

The woods have no garbage cans and dumpsters filled with discarded food, no lovingly tended tomato plants, no ready-to-pluck dahlias and nasturtiums, no tasty, newly planted shrubs. 

Best of all from the animal viewpoint, humans are no longer the same dangerous predators who once pushed the beaver close to extinction and reduced the entire North American white-tailed deer population to a trifling 500,000 scarcely a century ago [there are now 25-40 million]. Sterba believes that this human withdrawal from combative relations with woodland animals is one of the major causes of their proliferation: man as killer has undergone a softening change.

Indeed, the crow standing in the middle of the road waiting for me to run by its food source is evidence of exactly that.

It doesn't matter who does it
But here's the thing -- whoever's doing the scavenging, there's still garbage spread all over the street.  And this is an apt metaphor for an important way that evolution works.  Yes, gene function is often conserved, so that the same gene (Pax6) is involved in photoreception in the eye of fruit flies and frogs and humans, and the human form of the gene can be transplanted into even distantly related species and the eye will still be made.  But it's also true that often a trait is conserved but the genetic scaffolding is very different.  Natural selection might preserve the phenotype, the trait, but it can't see the genotype, leaving it free to vary.

Ken and his then post-doc, Malia Fullerton, published a paper in Theoretical Population Biology in 2000 describing just this, though others have described it as well (reviewed by Brian Hall in his 2003 book, Keywords And Concepts In Evolutionary Developmental Biology.) The effect is called 'phenogenetic drift' to indicate that the trait's genetic basis can change. This is not the same as genetic drift, which is when genetic variation that has no effect on a trait, or at least on reproductive success ('fitness'), changes over the generations, and it's not the same as phenotypic drift, when traits vary over generations but that variation doesn't affect fitness.  This happens, therefore, regardless of whether there is selection, even strong selection, or not.  A prerequisite, or partial one at least, is that many genes contribute to the trait, so that different combinations of variants in these genes can lead to similar traits.

Hall uses the example of proteins that make up the lens of the eye. They can be unrelated among taxa, as long as they let light pass through. Kazu Kawasaki, a very skilled research scientist in our lab, has written a number of papers describing the evolution of mineralization in vertebrates. One of his early papers is on the changing genetic basis of vertebrate teeth. He has found a gene family, that he calls SCPP genes, that varies widely among species, and yet contributes to the formation of mineralized tissues -- teeth and bone -- in all of them.

As Hall concludes his section on this subject, "Phenogenetic relationships are less determinative for more complex traits."  There are many genetic pathways to being tall, or to developing heart disease, or to being a fast runner  And that's before we even throw environmental factors into the mix.  Genetic determinism, which we've blogged about often, is simply too easy to assume, often incorrectly so.  When you look out the window on trash day and see garbage strewn all over the street, don't leap to judgment about who did.  It's only when you find the bear scat or a crow feather that you'll know.  But even then, do allow for the possibility that the neighbor's dog got loose.