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.