Cottonwood and the River of Time: On Trees, Evolution, and Society

7. Why Sex?

7 / whY Sex ? a s aspen shows, asexual propagation is an eminently successful formula to hang on to life once a tree has gained a foothold in a particular place. and chances are that if this tree and all its suckers made it for a hundred years in that location, this genet has proven its stuff and has every reason to expect a happy existence for another hundred years or more. The varied physical and biotic challenges it has met should adequately prepare it for what is yet to come. So why go to the expense of sex? why not disperse propagules that are exact replicas of its proven genotype? let us first remember what happens in sexual reproduction. at the age of ten years or more, a typical aspen will form floral buds on some of its short shoots. each bud will contain the primordia of an ament, or catkin, composed of many floral discs that in female trees bear a single 52 Why sex? 53 pistil, in males up to a dozen or more stamens. These tissues are all diploid , which means they have two sets of nineteen chromosomes, one set from each of the tree’s parents. what then happens is that the diploid mother cells of the eventual sex cells will undergo a process called meiosis , a sequence of two divisions that (1) halve the chromosome numbers to the haploid level (to one set of nineteen chromosomes), and equally importantly (2) allow the chromosomes to recombine. This recombination not only rearranges entire chromosomes, but also involves the exchange of chromosomal segments and thereby causes a shuffling of genes. as a result, pollen and egg cells each contain a haploid set (each with nineteen chromosomes), but each sex cell will be different from every other one—in fact, thanks to the shuffling process, it is very unlikely that any two of the millions of pollen grains released by an individual tree carry the same genetic package. But, we may ask, why break up a successful genetic recipe that has proved its worth at this location for a good many years? and why then combine it in mating with another such shuffled package from a tree that may not even live in the same neighborhood? or to put it in quantitative terms, why give up half of your genes—a loss of 50 percent of your genetic representation in your offspring—just to let them mix with those of another individual? This conundrum has occupied some of the most fertile minds among evolutionary thinkers. after all, sex is ubiquitous in both plants and animals and also occurs in variant forms among simpler organisms. Clearly it must be successful or we wouldn’t find it everywhere. Yet it comes at a steep price to the individual. what is the explanation? “every year produces a new crop of explanations, a new collection of essays, experiments , and simulations. Survey the scientists involved now and virtually all will agree that the problem has been solved; but none will agree on the solution”—this is how matt Ridley put it in his lucid treatment of the subject in The Red Queen (1993).1 among the many hypotheses offered over the years, the ones focused on forces of natural selection operating within species, especially parasites, seem to be the most persuasive . They posit that biotic factors are likely to be more powerful selectors than the physical environment because they are flexible and able to change in response to an organism’s defenses. if so, organisms that produce varied offspring are more likely to surprise their parasites with a diversity of new problems, thereby leaving more progenies to 54 Why sex? survive in the next generation. But parasites will keep changing in turn, and so the race goes on, never really ending. That is what moved leigh van valen to draw the analogy to lewis Carroll’s Red Queen, who has to keep running just to stay in the same place, when back in 1973 he elevated this hypothesis to a new evolutionary law.2 The compelling aspect of this proposition is that, for any species, continuity is only possible via change, and that past continuity offers no guarantee for perpetuity. in addition, change need not be directional, or progressive, but may be oscillating and fall back on previous forms. So it may well be that parasites are the prime movers in the...