The Reluctant Mr. Darwin (23 page)

What accounts for the “internal predisposition”? Neither Eimer nor Hyatt nor Cope nor anyone else ever offered a mechanism to account for how this amazing process might work. But it seemed to offer them some satisfaction that Darwin didn't. Eimer's second big volume appeared in 1897, just before he died, under a wonderfully tongue-twisting German title,
Orthogenesis der Schmetterlinge: ein Beweis bestimmt gerichteter Entwickelung und Ohnmacht der Natürlichen Zuchtwahl bei der Artbildung
, which is translatable as
Orthogenesis of Butterflies: A Proof of Definitely Directed Development and the Weakness of Natural Selection in the Origin of Species
.

A person might well ask: If it's Definitely Directed Development, then directed
by what
? Not by God, so far as Theodor Eimer and other orthogenesists were concerned, and not by the imperatives of adaptation.

Saltationism embodied the view that evolution proceeds by leaps. Darwin had explicitly rejected that idea in
The Origin
, citing what he considered a reliable old maxim:
Natura non facit saltum.
It was true, he wrote, that nature makes no leaps, because natural selection “must advance by the shortest and slowest steps.” Huxley had disagreed, believing that nature does indeed move by smallish jumps, and he worried that Darwin had burdened his theory with an unnecessary difficulty. During the late 1880s, a British zoologist named William Bateson came to share Huxley's dissatisfaction with Darwin's gradualism, especially after tossing aside his laboratory approach in favor of fieldwork on the steppes of central Asia. Since species are discontinuous, one from another, Bateson argued, the variations from which species are produced might be discontinuous, too. He went still further: Discontinuous variation
is
evolution. Natural selection isn't necessary, Bateson thought, if variation occurs in big, sudden leaps that sometimes yield new species. The Dutch botanist Hugo De Vries reached the same conclusion about the same time, based on his study of discontinuous variations in the evening primrose,
Oenothera lamarckiana
. De Vries put an old word to new use for such sudden, major changes, calling them “mutations.”

By the end of the 1890s, natural selection as Darwin had defined it—that is, differential reproductive success resulting from small, undirected variations and serving as the chief mechanism of adaptation and divergence—was considered by many evolutionary biologists to have been a wrong guess. It was interesting in its historical context, they conceded, as the pet idea of the man who had opened the world's eyes to evolution. Possibly it did play some small, secondary role. Or possibly none. There were too many damning arguments against it, such as Jenkin's about blending inheritance and Thomson's about planetary age. There were too many newer ideas, such as saltationism, and older ones, such as Lamarckism, that carried stronger intuitive appeal.

But something was missing from all the alternate theories, as it was missing from Darwin's own: a clear understanding of how inheritance works. During the last years of the century, for one instance, Hugo De Vries began writing his evolutionary opus,
Die Mutationstheorie
, with bold notions about the abrupt origin of new species but little appreciation for the routine dynamics of heredity and incremental change. When his first volume was nearly complete, a colleague sent him a small packet, with a note: “I know that you are studying hybrids, so perhaps the enclosed reprint of the year 1865 by a certain Mendel, which I happen to possess, is still of some interest to you.” Turns out, it was of interest to everybody.

39

One of Darwin's great strengths as a scientist was also, in some ways, a disadvantage: his extraordinary breadth of curiosity. From his study at Down House he ranged widely and greedily, in his constant search for data, across distances (by letter) and scientific fields. He read eclectically and kept notes like a pack rat. Over the years he collected an enormous quantity of interconnected facts. He looked for patterns among those facts but was intrigued equally by exceptions to the patterns, and by exceptions to the exceptions. He tested his ideas against complicated groups of organisms with complicated stories, such as the barnacles, the orchids, the social insects, the primroses, and the hominids. Gregor Mendel was a different sort of scientist, with a different cast of mind. He lived in a monastery and studied peas.

The monastery was an Augustinian one, in Brno, an ancient town southeast of Prague in what was then part of greater Austria. Mendel's experimental organisms were the common garden pea,
Pisum sativum
, and its close relatives. Luckily for him, the genetics of
Pisum
happen to be simple and straightforward in a way that those of
Oenothera lamarckiana
and many other organisms are not. After eight years of crossbreeding experiments, in which he tracked the inheritance of traits in flower color, leaf size, stalk length, seed shape, and other easily visible aspects of a pea plant, Mendel described his work to fellow members of the Brno Natural History Society. That was early in 1865. His results included several important observations: that some traits are
dominant
, whereas others are
recessive
(Mendel's terminology, adopted from earlier workers); that a dominant trait, when crossed with a recessive, is transmitted intact to the next generation, not in diluted or compromised form; that a recessive trait becomes latent when crossed with a dominant, but appears in full force when crossed with a similar recessive; and that, after a large number of crossings between any pair of dominant and recessive traits, the ratio among offspring will be almost exactly 3 to 1. Crossing red-flowered plants with white flowered plants, for instance, Mendel got 705 red-flowered offspring and 224 white-flowered, for a ratio of 3.15 to 1. Crossing puffy-pod plants with constricted-pod plants, he got a ratio of 2.95 to 1. Round-seeded plants crossed with wrinkle-seeded plants gave him 2.96 to 1. The average from seven experiments was an overall ratio of 2.98 to 1, registering a mystifying consistency that couldn't be coincidence.

The implications were huge. With these experiments, Mendel had shown that heredity functions by way of indivisible, particulate units, only two in each case, and not (as Darwin and others believed) by way of cumulative masses of tiny elements afloat in the blood. He had demonstrated that each parent contributes just one hereditary particle, not a profusion of them, for any given trait. His 3-to-1 ratio reflects the four different ways that two parental particles can be combined in a second-generation individual, given that each parent might contribute either a dominant particle (call it
A
) or a recessive particle (call it
a
) for the given trait:
AA, aa, Aa, aA
. Of those four possibilities, three (
AA, Aa, aA
) will result in manifestation of the dominant trait, while only one (
aa
) will produce the recessive trait. Mendel had outlined a central law of heredity and pointed toward the concept of the gene. He had also suggested the modern distinction between phenotype (what the organism shows) and genotype (what the organism carries). He had punctured the illusion of blending inheritance.

Like the Darwin-Wallace presentation to the Linnean Society, Mendel's lectures made no great impression at the time. A year later, published in the Brno Natural History Society's journal under the modest title “Experiments in Plant Hybridization,” they made no great impression again. Mendel himself arranged for about forty reprints to be sent to botanists and other scientists who might have been interested; but the interest didn't flare. His paper lay almost entirely unnoticed and unused for thirty-four years. Why? Was he too far ahead of his time? Yes, in the sense that he offered answers to questions that hadn't yet been clearly enough asked. Was he ignored by the scientific community because of his monastic isolation and obscurity? Yes, that didn't help either—Brno wasn't London, and its Natural History Society was an improbable venue for announcing a major scientific breakthrough. Was he disadvantaged by the fact of having published only one notable paper, not a body of interrelated work? Somewhat. There's no single reason, just a handful of contributing factors, to account for this accident of neglect. You could say that Gregor Mendel was too modest and unassuming to call attention to himself. That he was unlucky. That biology itself was unlucky. That he made the fatal mistake, for his follow-up studies, of shifting from peas to a more complicated group of plants, the hawkweeds. That he got distracted from further plant experiments by his election as abbot of the monastery. Anyway, for all the response Mendel's article evoked, at least during his lifetime, he might as well have buried his forty reprints in the garden. Then, in 1899, a copy of his paper was mailed to Hugo De Vries. It may have been one of those original forty that Mendel himself had hopefully cast forth.

In the meantime, a German zoologist named August Weismann had developed his own theory of inheritance, containing several strong ideas. One of those ideas was that heritable traits pass from generation to generation by way of molecular material contained in the nuclei of cells. A second was that, contrary to Lamarckian and neo-Lamarckian belief (including Darwin's own Lamarckian misconception), acquired characteristics are not inherited. Never. In no cases. Not possible, according to Weismann. He argued that the
germ plasm
(the cell line that eventually produces gametes, reproductive cells, such as eggs and sperm) stands isolated from the
soma
(the rest of the body), and that it can't be altered within an individual by neck-stretching, weightlifting, blacksmithing, cave dwelling, drought, severe cold, or any other activities or environmental conditions affecting the body. The soma is what changes with habit or stress, Weismann argued; the germ plasm remains untouched; and changes to the soma aren't heritable. More clearly than Mendel (without having read about Mendel's peas), he saw the distinction between genotype and phenotype. Building on recent insights in cell biology, he also recognized another important phenomenon: that the haphazard intercrossing of chromosome branches, during the process of cell division to form gametes, results in chromosomal recombination. That is, tangling, breaking, and reconnecting. In sexual reproduction, this intercrossing continually generates a richness of possible combinations, and therefore an abundance of variations among offspring—even among offspring of the same parents. Biologists today realize that such recombination of existing genes, along with outright mistakes of gene duplication that create wholly new gene forms (now known by De Vries's term, mutations), are the main answer to the lingering question that clouded Darwin's work and his successors' for decades:
What is the source of variation?
Mutation and recombination supply most of it.

Mutation produces new variants of existing genes. Recombination generates variation by splicing together new gene combinations from one chromosome to another. In the process of meiosis (double cell division, producing gametes), normal and mutant genes on their normal or recombined chromosomes are parsed out into the reproductive cells. This egg gets an
A
, plus
BCdEF
. That egg gets an
a
, plus
BCDEf
. Another egg gets a
mutant-a
, plus
bcdeF
. Shuffle, cut the deck, add some jokers, shuffle again. Insofar as mutation and recombination are accidental processes, variation is undirected by need or purpose. Natural selection acts upon it. Mendelian inheritance prevents the results from being blended away.

40

It's not my intention, so near the end of this little book, to try to quick-step you through all the major episodes in the later history of evolutionary biology. That would take me too far beyond my allotted length and too far out of my depth.

If it
were
my intention, I'd be obliged to describe how the saltationists seized on Mendel's rediscovered ideas, thinking that particulate inheritance supported their arguments against natural selection, but they were wrong; how Weismann's concept of the isolated germ plasm led to a strict view of natural selection as the sole evolutionary mechanism—a view more Darwinian than Darwin himself—that came to be known as neo-Darwinism; also how Thomas Hunt Morgan's research on fruit fly genetics and Richard Goldschmidt's notion of the lucky mutant (or, as he called it, “the hopeful monster”) brought saltationist thinking well into the twentieth century; and how saltationism eventually faltered and dissolved in the face of brilliant new work in mathematical genetics, mostly by R. A. Fisher, J. B. S. Haldane, and Sewall Wright, showing that Mendel's particulate inheritance actually supported Darwin's selection theory rather than confuting it. Having mentioned Sewall Wright, I'd offer at least a parenthetical explanation of his concept of
genetic drift
, a random process that becomes very important in small, isolated populations and (as some biologists believe) may be largely responsible for speciation events. I would also remind you that the discovery of radioactivity by Henri Becquerel, at the end of the nineteenth century, furnished a decisive rebuttal to William Thomson's cavils about the age of planet Earth (its internal heat source now better understood) and recast the estimates of elapsed time, allowing Darwin all the eons necessary for evolution by natural selection. Most important, I would need to outline an intellectual event known as the Modern Synthesis, during the 1930s and early '40s, in the course of which George Gaylord Simpson (a paleontologist), Theodosius Dobzhansky (a geneticist), Julian Huxley (a wide-ranging biological thinker, grandson of Darwin's friend T. H. Huxley), Ernst Mayr (a naturalist and systematicist), and several other influential biologists, building on the work of Fisher, Haldane, and Wright, unified Mendelian genetics with Darwinian selection and established a synthesized theory of evolution, roughly as it's accepted today. I say “roughly,” of course, because even their Modern Synthesis is no longer modern. In the past sixty years it too has been critiqued, modified, added to, and otherwise improved. I would be duty-bound, in addition, to touch on some latter-day developments and modifications, such as Ernst Mayr's hypothesis about genetic revolutions among insularized populations, Niles Eldredge and Stephen Jay Gould's concept of punctuated equilibria, Motoo Kimura's neutral theory of molecular evolution (along with Richard Lewontin's response to it), the thinking of George C. Williams and Richard Dawkins on selfish genes, Edward O. Wilson's provocative overview of sociobiology, Stuart Kauffman's fascinating suggestions about self-organization emerging from complex genetic systems, and much more. Whew. But no, I won't try to do all that. Not here, not now.