Saturday, March 25, 2017

Essential Differences

Heard a bluebird about a week ago.  There are very few phenomena which can be described indifferently as occurring at different seasons of the year, for they will occur with some essential difference.  Journal, Nov 3, 1853

Henry David Thoreau's attempt to made a detailed seasonal nature calendar was probably doomed.  After my own "years of observation," I am astonished at how variable events can be.  Not only do the early-blooming trees vary year to year by more than a month, these trees have not been varying together, but each following its own mysterious impulses.

Henry David Thoreau is well-known among modern naturalists for his close, systematic, and long-continued observation of nature in his hometown of Concord, Massachusetts.  Among his ambitions was to record the events of nature precisely enough to predict--almost on a daily basis--the flowering and fruiting of plants, arrival and departure of migrating birds, mating seasons, and other events of the season.  Although Thoreau's interests varied from year to year (one year he hatched and followed snapping turtles), he did one concentrated, conscientious "year of observation" around 1852 in which he determined to record every seasonal event.  He was building his own version of the seasonal calendars that were popular at that time.

I have been doing my own "years of observation" in my own little neighborhood over the last three years or so.  The wild swings of the last few springs have convinced me that nature is predictable only within wide limits.  Last year the warm winter brought out the early-blooming trees far earlier than the year before.  This year we had another warm winter--one of the warmest Februarys on record worldwide, and definitely the warmest ever in this region--and I looked for the same trees to bloom at about the same times as last year.  To my surprise, some bloomed earlier, but some bloomed later.  So nature is still less predictable than I'd thought. 

Silver maples began blooming April 4, 2015, but February 29, 2016 and February 23 this year!

Quaking aspens bloomed around April 10, 2015, but March 26th, 2016, and was in full bloom before March 22 this year.

 Red maples bloomed April 15, 2015, but March 11, 2016, and is just beginning to bloom as I write on March 25th.  (Males above, females below.)

But maybe it's better this way.  A predictable Nature would be a boring Nature.  In truth, the closer you look, the more Nature remains full of surprises.

Saturday, March 11, 2017

Reply to William Paley--Father of Creationism

William Paley, 1791.

William Paley was an erudite 18th-century English clergyman who is best known today for weighing-in on what Charles Darwin would call, decades later, "the species question": where do living (and fossil) species come from, and how do they come to be so exquisitely adapted to their place in "the economy of nature?"  Paley wrote, among other things, the elegant and influential book Natural Theology.  In it, he eloquently presents a sophisticated argument for creationism, aka intelligent design.  Even today it is an impressive argument on its face.  He begins by imagining someone encountering a pocket watch on the ground.  Knowing nothing at all about it, he perceives that its intricate, purposeful mechanism could not come about by chance: it must have a designer.  So, then, the still-more-intricate and wonderful mechanism of a living thing, so tuned to its place in the economy of nature, must also have a designer.  Later in his book, Paley similarly discusses in detail the intricate design of the eye, and the clockwork precision of the workings of the heavenly bodies. 

One side effect of Paley's work was that the study of natural history became important to clergymen, who thereby explored God's creation.  Charles Darwin was introduced to Paley as part of his university education, and early on in his own career he found Paley very convincing.

I came across an excerpt from Natural Theology while I was writing a high school biology unit on evolution, and was inspired to write a reply to his argument, in approximately the same style, again using the discovered watch, but also evidence Paley didn't consider, and of course evolutionary theory.  To be fair, Paley wrote Natural Theology the same year Charles Darwin was born, Darwin's Origin of Species came out fifty years later, and the theory of evolution by natural selection did not completely overcome competing theories until another half-century had passed.  (When an undergraduate discovering Darwin's Origin of Species in the university bookstore, I was struck by fellow-feeling: I was born one hundred years after its publication, and was then the same age as Darwin when he embarked on HMS Beagle.)  So Paley held sway on "the species question" for most of a century. 

Excerpt from Natural Theology
When we come to inspect the watch, we perceive...that its several parts are
framed and put together for a purpose, e.g. that they are so formed and
adjusted as to produce motion, and that motion is so regulated as to point out
the hour of the day; that, if the different parts had been differently shaped
from what they are, if a different size from what they are, or placed after any
other manner, or in any other order than that in which they are placed, either
no motion at all would have been carried on in the machine or none which
would have answered the use that is now served by it...Every indication of
contrivance, every manifestation of design, which existed in the watch, exists
in the works of nature with the difference, on the side of nature, of being
greater and more, and that in a degree which exceeds all computation.
-William Paley

A Reply to Natural Theology
When in awe we step back a bit from the watch, we perceive on the ground round about a great many other glittering cases with gears and springs, of
diverse sizes and shapes and composed of gears similar to those in the watch,
but variously differing in configuration and complexity, some indeed also pointing out the hour, but others fulfilling functions unlooked for, but carried out with action no less perfect.

And we find among many of these mechanisms curious similarities, in which a particular gear or spring is widely shared, though sometimes differently configured and sometimes even differing in function. 

And in the earth we find the rusted remains of still others: some simple, others complex, but all sharing some part of their mechanisms with their contemporaries, and others both ancient and modern.  Each is formed so as to function in some way, whether plain or subtle. We find moreover the least ancient to be most similar to the new and still functioning, and those nearby to be more alike with the rusty nearby, and-- by report-- those far away to have their own similar ancient relatives.

And when we again turn our attention to the first watch, now with mind
altered by seeing the breadth and diversity of the mechanical world, we see
more than we did at first: our eye drawn to certain gears and parts familiar
from one or another of those other devices, yet seeming disconnected and now
curiously functionless.

And finally-- most curious of all-- we conclude that these glittering
mechanisms do not arise ready-made from some designer's bench, but develop and change, beginning simply and similarly but diverging in
form until they reach the function we see today, however alike or different,
and whether simple or complex. How amazing is the ever-increasing diversity
of all this mechanical universe!
-Jeffrey Michals-Brown

These are the very features found in the living world, and which are nicely explained only by evolutionary theory:
1. the diversity, yet order in living things (since they are related by common descent),
2. homology of structure-- especially that which is not related to function (forelimbs of whales, people, horses, bats and birds share ancestral arrangement of bones, even thought that arrangement has been bent to uses as varied as swimming, running, tool-use, and flying);
3. structures evolved for one purpose have been repurposed for new uses; structures that seem "irreducibly complex," such as the eye, are present in many simpler but still useful forms in other animals--showing that appearances can be deceiving.
4. a fossil record in which species appear and disappear through time in an orderly fashion;
5. extinct creatures related to still-living species in the same geographical places;
6. "vestigial" structures inherited from extinct ancestors (whale hip bones, for example) which seem no longer serve a purpose.

Of course, these are the features that were within reach of Darwin and, to some extent, Paley.  Today there are others that were unknown to them:
1. genetics (we understand inheritance, and where the variation that is evolution's raw material comes from--something Darwin was quite confused about)
2. molecular biology, which allows us to compare genes and see the built-in molecular fossils that we all carry in our cells (some of my favorites: practically every living thing "reads" the language of DNA the same way, all use the same energy molecule, ATP, all build and use left-handed amino acids and right-handed sugars); and
3. a growing appreciation of living diversity and biological toughness that Darwin never dreamed of: bacteria living at the edge of space, inside rocks miles underground, and sealed into underground crystals for thousands of years.

Sunday, March 5, 2017

Evolutionary Thinking

Silver maples (Acer saccharinum) beginning to bloom in late February.
Flowers are red; females have finger-like pistils with pollen-receiving surfaces.
Males each have a puff of numerous, white pollen-bearing stamens.

Over the last two weeks I've watched the silver maples coming into bloom.*  Silver maple (Acer saccharinum) is monoeious--having separate male and female flowers on the same tree.  I had noticed that the female flowers mainly opened a little earlier than the male flowers.  This makes sense as a way to encourage outcrossing--reducing chances of a tree fertilizing itself.  (Inbreeding often results in unhealthy offspring.)  Further, the female flowers were mainly low on the tree--which makes sense considering that pollen is more likely to lose altitude than gain it in drifting from tree to tree.  They tended to be at the ends of branches, also

But this year I also noticed that there were many more male flowers than female on each tree.  Why? I wondered.

It isn't immediately obvious that there shouldn't be equal numbers of male and female flowers, but there certainly might be an advantage to a ratio other than 1:1.  Let's try a little "natural selection thinking."  Natural selection should drive the evolution of the flower sex ratio to one which maximizes reproduction.  What influences might be at work?

Unlike most animals, plants are typically hermaphrodites--simultaneously male and female--therefore any silver maple can be mother of some offspring, and father of other offspring.  And, also unlike most animals, silver maples depend on the wind to carry sperm (packaged inside pollen grains) from tree to tree.  On the one hand, wind pollination is the chanciest way to have sex: surely not more than a few pollen grains in a million finds its random way to a receptive female flower on the right kind of tree.  (My neighborhood walks cover perhaps a square mile in which I count only a half-dozen or so silver maples).  Therefore a lot of investment in pollen gives only a very low return in offspring.  Does that mean any individual tree is better off with a lot of female flowers? 

On the other hand, it is the female, like in most animals,** that actually has to bear the offspring, and all the costs associated with that.  (Male flowers usually drop off the trees at the end of flowering, but female flowers are only just getting started: they have to grow into the fruit and seeds.)  Having said that, silver maples disperse their young with wings--depending on the same winds that disperse their pollen.  That means silver maple trees don't need to create large, nutritious fruits that will attract animal carriers, the way many plants do.  So it isn't clear which gender has the better return on investment: the male that must produce large amounts of pollen, or the female that must grow the fruit and seeds.

On the other other hand, it does no good for all the trees to have only female flowers: where would the pollen to fertilize them come from??  Natural selection works not for a particular individual, but for the average individual.***  To put it another way,  a silver maple tree is just a gene's way of making more of itself.  In order for this to happen, somehow or other natural selection MUST advantage the reproduction of the average individual: any competing adaptations that didn't would simply lose out in the race to dominate the genes of the next generation.  It seems as though the balance of forces acting on silver maples results in very male-heavy trees--though just why is difficult to say. 

What does the science say?  As a genus, maples have a lot of variety in their flowering patterns, and silver maple is referred to as "labile," meaning that expression of flower types and ratios is flexible.  I found nothing specific to silver maples, nor monoecious plants in general; but dioecious (separate sex) plants have been investigated.  A study of the deioecious alpine herb Rumex nivalis found that, in the presence of a higher density of male plants, offspring tended to be female; but if neighboring males were few, male offspring were favored.  This is a flexible response, rather than one fixed by genes more narrowly.  Another study hypothesizes that sex ratios (again in dioecious plants) may interact with stress tolerance through hormones that influence both these things.   A large comparative survey of 243 species looked at a number of hypothesized influence on sex ratios.  This study found that male-biased species were more common, and more often associated with long-lived plants like trees, and fleshy fruits dispersed by animals (neither of which is true of silver maples), while female-based plants tended to be herbaceous, clonal (often reproducing by runners, etc), have wind-dispersed pollen and plants having sex chromosomes.  This study finds that costs of reproduction, mechanisms of pollen and seed dispersal, and chromosomal sex determination can all play roles.  The short answer as to why silver maple is so male-heavy hereabouts? no one knows.

Silver maples in full bloom, beginning of March.

*Tragically early, just like last year: the delicate flowers and developing fruits will be doomed by days of temperatures in the teens or twenties, like those over this weekend.  Are these warm winters mainly a consequence of climate change, or just a fluke of weather? 

**A conspicuous exception being the seahorse, in which the male holds the babies in a special brood pouch.  Just as intriguingly, most of the gender-related trends in other animals (such as which gender selects the mate) are reversed in seahorses. 

***Which is why it is perfectly possible for natural selection to act against individuals, as long as it benefits individuals on average!  There are nifty examples of this in David Sloan Wilson's amazing book, Evolution for Everyone