Monday, February 8, 2016

What is the selective advantage for keeping dead leaves in winter?

In my first long walk since the snow, I noticed a young black oak surrounded by leaves and fallen twigs.  Other tree species did not suffer such damage. 

Many oaks are "tardily deciduous"--they hold many of their dead leaves at the end of the growing season when other trees are completely bare.  Both black and white oaks seem prone to this.  A black oak I have been watching for a long time kept many of its lower leaves until early spring, dropping them only just before buds began producing new leaves and flowers. 

I had assumed that this habit of black oaks had some advantage, or at least no disadvantage, but here was evidence that I was wrong.   

 In my experience, oaks tend to keep their dead leaves on younger trees (above),
and low down on older trees (below).  (It might be a function of wind speed at different heights.)

 Last month the young black oak carried a modest load of dead leaves.

 After a recent fall of heavy, sticky snow, the tree had fewer leaves, 
but also lost twigs that would have led new growth this spring and summer.

A white oak holding dead leaves low down in January.

Evolution by natural selection is dead-easy to understand.  So easy that, after Darwin published it in 1859, scientists were seen to slap their foreheads: "why didn't I think of that?"  In short, in the competitive struggle with its fellows to survive and reproduce, any individual that has an advantage will leave more offspring that carry the genes for that advantage.  Over generations, more and more individuals in that population will carry those genes, so that the population has evolved--changed--from what it was earlier.  

Those advantageous genes can make small, incremental changes that add up to big advances--such as the development from a simple "eye spot" of a flatworm to a complex "camera eye" like ours.*  Those changes can generate new species, families, even whole kingdoms of living things from populations diverging gradually from an ancestral type over millions of yearsEvidence of this over the last hundred years or so has become overwhelming: natural selection is the only scientific theory that has successfully explained adaptive change: the more and more successful fitting of organisms to the their place in the "economy of nature."**

The many subtleties, wrinkles and on-going issues that keep evolutionary biologists arguing don't change this basic picture much.  

On the other hand, some characteristics of species do not appear to be adaptive at all.  Leaves of closely-related trees (oaks, for example) may have many shapes, and all appear to be successful.  It is possible, of course, that we simply haven't found the subtle advantage that each shape provides in the subtly different niche of each species. 

But it is also possible for differences to arise randomly by a process called "genetic drift."  In genetic drift, a very small founder population that becomes a new species develops random differences.  Such differences can arise by a "roll of the dice" in very small populations, and then become fixed and permanent.  As long as these differences do not affect the survival or reproduction of individuals, they are not affected by natural selection. 

That is what I had thought the tardily deciduous habit was: a harmless trait that arose by genetic drift.  Now I wonder if this isn't a trait that does affect survival--negatively!

How to explain this?  

First of all, I should say I have no answer to my question, and haven't looked much into the scientific literature yet.  But I know one evolutionary principle that might apply.

One subtlety of evolution is that a characteristic does not have to benefit every individual that carries it in order to be favored--it must only benefit individuals "on the average."  For example, several genetic diseases of humans are maintained in human populations--even though you'd expect them to be eliminated by natural selection, since these genes reduce the survival and reproduction of individuals.  It turns out that such genes may confer advantages such as resistance to other serious diseases.  

For example, the sickle cell trait severely injures those unlucky enough to carry a "double dose" of the gene, but increases resistance to malaria in those who have the more common "single dose" of the sickle cell gene.  This gene confers advantage to the population as a whole, even as it cruelly disadvantages a minority of individuals. Other examples abound.***  The morning sickness experienced by many pregnant women would seem to disadvantage both mother and baby at a time when they need more nutrition, but it turns out to help the mother avoid foods that are harmful to the developing fetus.  A gene that causes a small fraction of male rhesus monkeys to be "out of control" and "mad" almost from birth, has the effect of producing confident, capable, high-status females

Is the "tardily deciduous" habit of oaks one of these factors that confers advantages on average, even though it is harms some individuals?  I have no idea, but intend to look for similar damage to other oaks, and keep an open mind.

*A very nice and highly-accessible article in the most recent National Geographic shows the range of eyes in the animal kingdom, and how even the most complex eye can arise by gradual steps from simpler models.  Some creationists love the eye, thinking it is a problem for the theory, not realizing that Darwin himself pretty much solved the problem over a century-and-a-half ago!  Mor recent research adds depth and detail.

**History records many other theories of evolution, but between the decades of the late 19th and early 20th centuries these were found to fail.  Most also lost credibility in the light of modern understanding of genetics.  Note that I do not include creationism among these failed theories; it is not a scientific hypothesis, because it is not testable.  To be testable (scientific), a hypothesis must make predictions that can be confirmed or disproved by observation or experiments.  Recent attempts to update creationism by removing explicit reference to God (e.g. "intelligent design") have also not resulted in testable predictions.  Therefore these are not science as it is understood today, no matter whether you believe in them or not.

***These last come from a deeply illuminating and thought-provoking book: Evolution for Everyone by David Sloan Wilson.  Delacorte Press, 2007

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