August 21, 2013 7:54:37 AM
To the delight of creationists, astronomer Fred Hoyle proclaimed about the formation and the evolution of life in 1983, "The chance that higher life forms might have emerged in this way is comparable to the chance that a tornado sweeping through a junkyard might assemble a Boeing 747 from the materials therein." Hoyle's derisive statement of probabilities has been decisively debunked by biologists, but he tapped into just the right vein to promote distrust: we do not like randomness (even if it is not so extreme as Hoyle declared), and many feel uncomfortable with the formation of life or its modification being powered by random processes. Randomness is at the heart of evolution, in ways that biologist John Tyler Bonner reviews in Randomness in Evolution (Princeton University Press). He does not just give a little primer on the way that random events cause evolutionary change; he also puts forth his own view of how randomness affects some branches of species more than others, specifically that randomness affects tiny creatures more than it does larger ones.
"All of evolutionary change is built on a foundation of randomness," writes Bonner. "It provides the necessary material for natural selection which then does indeed bring forth the order our inner mind so actively craves." The most well known aspect of randomness in evolution is mutation, the chance change of some DNA base. There is no way to argue that such changes are directed toward a certain end, although it may seem simply impossible that the random changes can produce a hummingbird or a wisteria. Indeed, most mutations will have little, perhaps no, effect. Some will be harmful to the organism or make it impossible for the organism to develop even into birth or seed. And a few mutations will produce a modification that will give the organism a tiny improvement over all its fellows, an improvement that makes it slightly more likely the organism will achieve success, and if it does, it will spread its genes (along with the beneficial mutation) into the next generation. Bonner describes these basics, and mentions but does not detail the mathematics and biological studies that confirm these processes at the heart of evolution.
Evolution started somehow even before tiny, one-celled creatures existed, and its most obvious spectacular processes have been to go from small to large, from simple to complex. A huge interplay of developing anatomical systems are required, for instance, to produce a moose. All of these chains of processes have to play out with exquisite timing, step by incalculable step. Mutations that occur will most likely result in an embryo that never makes it to term, especially if the change manifests itself early in development. Those that show up later may become part of a living, mature organism, and the changes will be under the sway of natural selection. Smaller organisms, however, have few developmental steps. A random change might affect not just the morphology of the organism, but perhaps the organism's overall function.
In this way, there might be generated huge numbers of different small organism forms, especially given how quickly one generation succeeds another. The forms could be enormously varied. The cover of Bonner's book shows some of the famous drawings by Ernst Haeckel of radiolarians brought back from the voyage of the HMS Challenger. They are protozoa that possess fantastically diverse mineral skeletons that look like bizarre variations on the Platonic solids. It used to be that biologists argued that these differences were adaptive; perhaps different skeletal forms meant greater strength. "It has even been suggested to me," writes Bonner, "that each shape is a selective response to a specific predator, or a specific niche, but if so, where are those thousands of predators and niches?" Instead, Bonner says that most of these varieties are neutral; they are all pretty good at getting along in their own way, and the baroque changes of skeletons are so contrived because they do not really make a difference to the organism's survival. That sort of variety of morphology through randomness could never be expressed in larger organisms.
Naturally, Bonner pulls examples from slime molds, his focus of research for sixty years. (His delightful memoir Life Cycles begins, "I have devoted my life to slime molds.") These peculiar creatures might be an illustration of what it takes for animals to become multicellular. They are a particular form of soil amoeba found all over the place. The amoeba eat bacteria, and while food is abundant, they do this independently. But when resources dry up, they send out chemical signals, attract each other into a mass which forms what is called a slug, a gathering which has a hind end and a front end and which migrates to the top of the soil. When it gets there, some of the amoeba make themselves into a stalk and some make themselves into spores atop the stalk, and the spores await dispersal by some spider or woodlouse lumbering by. One of the lessons Bonner draws from them is that early forms of slime molds still exist today; they are the sorts of "living fossils" that are more rare among larger creatures. He says that one reason they have continued to exist is that they are small, and because they are small, their morphology is little affected by natural selection, although their biochemistry has probably changed a great deal.
Size also affects sex. In particular, Bonner reflects upon the ability of some organisms to alternate a sexual with an asexual cycle. Almost all larger forms of life have a strictly sexual cycle, with two individuals combining genes to make a next generation. Smaller creatures, like Volvox or aphids, however, are able to make generations asexually, and they do this as the seasons change. When there is abundant food, it is a good strategy to do away with sex, make lots of identical copies, and spread; then when winter approaches, the creatures can have a sexual generation that has the advantages of a recombination of genes. Larger animals cannot do this; their maturation takes more time than can respond to annual cycles.
In many ways, Randomness in Evolution extends the ideas Bonner put forth in another book, Why Size Matters. He does call upon some understanding of biological principles, but his style is forthright, informal, and humorous. His reminder that not every trait has a biologically adaptive function is a welcome lesson, as is his self-deprecating description of his ideas as just another "just-so" story. His current volume contains more speculation (reasoned speculation, to be sure) than his others, and it may be a call to the biologists who take over from him to do more research to confirm or to refute the often surprising ideas here.