v1.3.0 / chapter 3 of 28 / 01 jan 09 / greg goebel / public domain
* At the outset, Darwin knew his theory needed more work and knowledge, and was gratified to see that new fossil discoveries did much to buttress his ideas. However, he clearly recognized the weakness of contemporary knowledge of the mechanisms of heredity. He would never know that an Austrian monk named Gregor Johann Mendel had taken the first big steps towards resolving that difficulty not long after the publication of THE ORIGIN OF SPECIES.
* Thomas Huxley was not merely an aggressive bulldog in his fight against Darwin's enemies; he was also careful to make a persuasive case for Darwin's ideas. He published EVIDENCE AS TO MAN'S PLACE IN NATURE in 1863 to point out the clear physical relationships of the structures of men and apes, with the book providing a famous -- or infamous, depending on one's point of view -- illustration: the skeletons of the known apes, carefully arranged to hint at a progression to the skeleton of a man.
In proclaiming the downfall of the wall between humans and animals, however, Huxley was careful to point out that did not mean the fall of all morality as well, that a description of the nature of human beings said little about what was right or what was wrong. Did the natural origins of humans, he asked, make the strivings of a philanthropist or a saint futile? On the other side of the coin, was "mother-love vile because a hen shows it, or fidelity base because dogs possess it?" He was far-sighted in making the distinction, since there would be some who would suggest that Darwinism justified a rejection of traditional ethics, and many more critics who would accuse Darwinism of being a fundamentally unethical doctrine.
Sir Charles Lyell also bolstered the case, at least to an extent, by publishing a book titled ANTIQUITY OF MAN in the same year, 1863. It made a case that humans had been around since well before the beginning of recorded history. The evidence was fairly slight at the time, but remains had been found of ancient humans, their flint and bone tools, and their settlements, partly dated by the butchered remains of long-extinct animals that they preyed on. One particularly interesting find had been the discovery in Germany in 1856 of remains of people who were clearly human but whose features were well outside the norm of those of modern humans. They were named "Neanderthals" after the locale of their discovery.
Lyell's book was popular, but Darwin was upset that it did not endorse natural selection. Lyell conceded the evolution of humankind to an extent, but didn't feel that Darwinian natural selection could account for human intellect. Lyell defended himself in a letter to Darwin: "I have spoken out to the utmost extent of my tether, so far as my reason goes, and farther than my imagination and sentiment can follow." In hindsight, Darwin was being, as his custom, fussy: ANTIQUITY OF MAN was not all that hostile to Darwinism, and because of Lyell's authority and good writing skills, did much to bolster evolutionary ideas.
Huxley was more committed to the cause, further building up the brief for Darwinism by helping bring to light new data in the fossil record. In 1861, workers at a limestone quarry at Solnhofen in Bavaria found a fossil of what looked like a reptile, except that the fossil included a distinct outline of feathers: it was clearly a bird. It was given the name Archaeopteryx. A small dinosaur named Compsognathus was found at the quarry as well, and in 1868 Huxley published a paper describing the two as demonstrating elements in a bridge from reptiles to birds.
The Archaeopteryx fossil didn't have a head, but Huxley suggested that the beast actually had teeth. In 1872, the American paleontologist O.C. Marsh (1831:1899) found two more toothed birds from a later era, Ichthyornis dispar and Hesperonis regalis from later strata, suggesting Huxley was on the right track -- which was proven in 1877, when a second Archaeopteryx fossil was discovered at Solnhofen that did have a head.
An important "missing link" had been discovered. Not only did Archaeopteryx have teeth, it also had claws on its wings -- fully articulated hand structures, not the simplified structures found on the wings of a few modern birds -- and its breastbone was undeveloped compared to that of modern birds, meaning its chest musculature was also undeveloped and that it likely wasn't capable of any more than occasional dashes into the air, like a flying fish. At least six Archaeopteryx fossils are known to our time, a fairly good sample but one that illustrates just how rare fossils are. Older birds have been found since its discovery, including a 21st-century find of a proto-bird that had both front wings and small wings on its rear legs. The significance of Archaeopteryx for evolution is so great that some critics have insisted the fossils are frauds, but the fossils have been tested using modern mineralogical analysis techniques and have proven genuine.
Huxley was only getting started. In the late 1850s, a French paleontologist named Jean Albert Gaudry (1827:1908) had discovered a fossil of an ancient three-toed horse in Greece. Evolutionists suspected that it was a link to a five-toed ancestor, and in the early 1870s, Huxley and a Russian paleontologist named Vladimir Kovalesky had managed to construct a rough evolutionary tree of horses. O.C. Marsh then went on to find four-toed and five-toed horses in fossil beds in the United States. Huxley wrote Marsh to register sheer joy at the finds, and Marsh replied that the evolutionary trees of birds and horses would lead "the doubting brother" across the gap to accept evolution.
In fact the bird and horse trees remain classic proofs of evolutionary theory, though to no surprise they have been enhanced over a century and a half, revealing more complicated evolutionary trees, more heavily laden with branches than once thought. Incidentally, it has become common in modern times to find out that evolutionary sequences become more complicated over time, but as far as Darwinism is concerned, this is not a source of discouragement: the older notions were not incorrect because they went too far, it was because they didn't go far enough.
* Darwin, in his methodical fashion, was not quick to push the limits of his ideas, and in fact for a decade after the publication of THE ORIGIN OF SPECIES he focused on specialized works for naturalists, one of the more interesting being FERTILIZATION OF ORCHIDS, published in 1862. Like his work on barnacles, his study of orchids provided more resources for his ideas on natural selection: orchids feature a wide range of elaborate mechanisms for ensuring that pollinators visiting the flower get their fair share of pollen, but remarkably these mechanisms were all clearly modifications of the ordinary parts of flowers.
Darwin finally took on the issue of human origins in THE DESCENT OF MAN, published in 1871, following it up with THE EXPRESSIONS OF EMOTIONS IN MAN AND ANIMALS in 1872. Neither book had the impact of THE ORIGIN OF SPECIES. As far as DESCENT OF MAN went, there wasn't much fossil evidence available in the 1870s on human ancestry, and all Darwin could do was cast about for other justifications in the evidence, mostly in terms of the similarities of humans with other animals. He did so thoroughly, leaning heavily on his earlier ideas for sexual selection, but in a somewhat overstuffed fashion that had a diffuse impact compared to the focused argument of THE ORIGIN OF SPECIES.
EXPRESSIONS OF EMOTIONS was even thinner, an obsessive examination of emotional expression in animals and humans -- spending pages, for example, discussing shrugs or other gestures -- and did not do much more than expand on the case that he made in DESCENT OF MAN that emotions were not unique to the human species and had their roots in the dictates of natural selection -- as did the human sense of morals. It paved the way for later evolutionary studies of behavior. The two books provoked even greater disgust from disbelievers and Darwin's brief remained controversial.
Darwin shrugged and went back to producing specialized works for naturalists, one of the more interesting being INSECTIVOROUS PLANTS, published in 1875. It was another interesting set of case studies for evolutionary science. Such plants lived in boggy areas where there was little nitrogen in the soil and so obtained from the bodies of insects, but used a variety of tactics to catch them. Venus flytraps imprisoned them between jawlike pairs of leaves; sundews caught them with stalky heads covered with gluey hairs; and pitcher plants drowned them in little basins, lined with slick or bristly sides to make escape difficult.
He never published another major work up to his death of heart failure in 1882. He had planned to be buried in Down, but such was his notoriety that his body was grabbed by the authorities to be buried in Westminster Abbey. The pallbearers included Hooker, Huxley, and Wallace.
* Where Darwin proved particularly weak was in his failure to understand the mechanisms of heredity, and in fact Darwin's arm-waving around the subject is one of the most exasperating features of his writing. He came up with a theory he called "pangenesis", which involved all the disparate parts of the body -- eyes, ears, toes, tongue, and so on -- generating hypothetical descriptive elements, which he called "gemmules", which were collected together and passed by a parent on to progeny. From the modern perspective, the notion sounds dodgy, to put it mildly. That may be due to the unfair benefit of hindsight, but even T.H. Huxley suggested to Darwin that he was on the wrong track, hinting that it sounded dodgy then as well. Darwin would go to his grave never knowing that the fundamental concepts he was groping for were nailed down only a few years after the publication of THE ORIGIN OF SPECIES.
Gregor Johann Mendel (1822:1884) had been born in Moravia, then part of the Austro-Hungarian Empire and now part of the Czech Republic, as Johann Mendel. He was a member of one of the many German-speaking communities that were common in Eastern Europe until they were uprooted by mass expulsions in the wake of World War II. His family was of limited means, but he still managed to get a university education, pursuing his passion for the sciences.
At age 21, he became a novitiate at a Catholic Augustinian monastery in Brno, acquiring the name Gregor. His vocation to the monastic life had little to do with religious conviction; the monastery was more or less an agricultural research station, and he saw it as a place where he could live a comfortable existence and pursue scientific studies. There followed a decade of further university studies and various attempts to acquire formal credentials as a teacher. As busy as he was, in the mid-1850s he still was able to begin a set of studies on plant breeding.
The general assumption at the time was that cross-breeding different organisms resulted in a melding of characteristics, sort of like mixing two shades of paint together. This was to an extent common sense: a child with, say, a black father and a white mother would be expected to have coloration and features -- traits -- somewhere between the two. Darwin himself was inclined towards this view, until a Scots engineering professor named Fleeming Jenkin (1833:1885) pointed out that it was lethal to the idea of natural selection: any variations from the norm of a population would be diluted in a few generations back to the norm. Jenkin suggested that if a white man were to be shipwrecked on an isle inhabited by black folk and adopted into the fold, then in few generations the physical traits of that white man on his descendants would simply fade away.
No variation meant no evolution by natural selection. Actually, it was obvious to Darwin and everyone else that blending was a bit too simple in itself to be the answer. For example, children might be brown-eyed or blue-eyed but never any mix of the two, and to really confound things, a blue-eyed child might have parents who both had brown eyes. As an even more dramatic example, any child has both a male and female parent, but the child isn't a hermaphrodite, a mix of the male and female. The child is, barring the very rare accident of nature, always either male or female.
Animal and plant breeders had been performing studies, some of them thorough and exacting, for years and had discovered many interesting empirical facts, but despite their efforts hadn't been able to devise any theory to account for them. In short, the patterns and plan of inheritance -- the "heredity" of organisms -- was a mystery.
As his test subjects, Mendel selected pea plants. They would prove almost ideal for the task, and it wasn't by accident either. Pea plants could be bred under precisely controlled conditions that ensured Mendel knew the parentage of descendant generations, and the plants exhibited a number of clear and specific traits. He first ensured that plants with specific "bred true", that is, each generation of a particular line always had the same traits. In other words, to use a human analogy, given a line of brown-eyed pairings, no blue-eyed children would ever appear to complicate the pattern.
He finally came up with seven of true-breeding traits to tinker with, which took alternative forms, listed below in a slightly simplified way:
He then started crossing the plants. The first observation that he made was that if plants with a specific opposed trait were crossed -- for example, one plant with round peas crossed with one plant with wrinkled peas -- then all the results of the first generation of the cross all had the same single trait, with no mixing of traits. In this case, they would all have round peas. The same principle applied to all seven traits, with crosses yielding yellow pea interiors, or purple flowers, or puffed up seed pods -- but not green pea interiors, white flowers, or deflated seed pods.
Now Mendel crossed the results of each crossing among themselves, and found out something fascinating: the second generation did not breed true, and the way it didn't breed true was distinctive. Crossing the hybrid pea plants with round peas produced a second generation of pea plants, some with round peas and some with wrinkled peas. However, the ratio of the plants with round peas to those with wrinkled peas was a distinct 3:1. The same applied to the second generation of all the other hybrids. This was obviously not a pattern that resulted from chance.
Mendel concluded that the traits were encoded by what he called "factors", sometimes referred to as "Mendelian factors", but now called "genes" -- the usage preferred here even though Mendel never knew of it. For example, pea plants with round peas had a factor that could be labelled "R", while pea plants with wrinkled peas had a factor that could be labelled "W", with the two different or "polymorphic" forms of the gene being later called "alleles". How the genes were actually constructed, Mendel had no idea -- in fact, nobody would know for sure until the middle of the next century -- but he could infer their existence from how the plants bred.
Furthermore, he assumed that each plant had twin genes for the particular trait, and that each parent provided one gene each for that trait to its progeny -- the matching specific genes from each parent later being described as "homologous". The true-breeding plants with round peas had a gene pair "RR", while those with wrinkled peas has a gene pair "WW". Bred among themselves, the true-breeding plants with round peas would produce progeny with round peas since each parent would contribute an "R" gene and the result would be "RR". Similarly, breeding the pea plants among themselves would produce a generation with the gene pair "WW", and all the peas would be wrinkled. The matched gene pairs are now referred to as "homozygous".
However, if the two distinct lines were crossed, all the progeny would have the gene pair "RW" -- a mixed or hybrid gene pair, what is now referred to as "heterozygous". What Mendel realized was that one gene overrode the other; as he put it, one was "dominant" and the other "recessive", terminology that is retained today. (The terminology is a bit unfortunate in that "recessive" seems to imply "inferior", which is not the case, but we're stuck with it now.) In this case, the gene for round peas was dominant, the gene for wrinkled peas recessive.
However, once this generation was crossed among itself, there were four
possible results of the pairing:
RR
WR
RW
WW
In the last case, "WW", the recessive gene can be expressed and the result is
wrinkled peas. That's why, on the average, the second generation had this
clear 3:1 ratio of traits.
Mendel also recorded the effects of crosses on two traits. Given, say, a
line of true-breeding plants with round peas and yellow (Y) pea interiors,
and a line of true-breeding plants with wrinkled peas and green (G)
interiors, then the first generation of the cross would all have the
"genetic" coding of RWYG, giving round peas and yellow pea interiors --
yellow being dominant over green. Inbreeding that first generation would
give the possibilities, along with the traits they produced:
______________________________________
RRYY = RY
RRYG = RY
RRGY = RY
RRGG = RG
RWYY = RY
RWYG = RY
RWGY = RY
RWGG = RG
WRYY = RY
WRYG = RY
WRGY = RY
WRGG = RG
WWYY = WY
WWYG = WY
WWGY = WY
WWGG = WG
______________________________________
9 RY 3 RG 3 WY 1 WG
______________________________________
The actual results reflected this 9:3:3:1 ratio. He completed his work in
1864 and published the results in 1866. It was all neatly done, in fact
astoundingly neatly done, since as the previous history of heredity studies
were full of confounding results, and Mendel had to have enormous insight to
see through them to find out the facts.
The first issue was that, obviously, an organism had a lot of different genes, the sum of them being now called the "genotype", that resulted in a lot of different traits, the sum of them now being called the "phenotype". That's why the blending notion seemed so plausible, with the progeny of two parents obtaining a mix of the genes of the two. Mendel had to be shrewd to sort through the traits of pea plants to find a set that were distinct enough to permit methodical analysis. Worse, as Mendel understood, a specific trait of an organism might be determined by several genes, such traits now being known as "polygenic" -- and on the other side of the coin, some genes might each control several traits, such genes now being known as "pleiotropic".
In some ways, a pea plant was an unrepresentative test subject, because its heredity was unusually simple, but that was what make it useful to help sort through the noise to find the signal. Mendel was actually a physicist by training and, though he didn't have Darwin's sweep of vision, he far surpassed Darwin in mathematical rigor -- Darwin himself admitted that he didn't have a good head for figures. In fact, Mendel's results were so tidy that there were some who would later suspect that his figures had been fudged, but the meticulous way the experiments were conducted and documented suggests simply that Mendel really was that good. The argument is academic anyway, reduced to irrelevance by the fact that all later experiments would show that Mendel's answer, however he got it, was absolutely on target.
* In fact, nobody raised any objections to his work at the time -- for the simple reason that nobody noticed it. In 1867, Mendel became the abbot of the monastery, meaning that he had to increasingly focus on administrative issues. He continued to perform studies on heredity, but not only did he have less time to spend on them, he also selected some poor test subjects. Under the encouragement of the Swiss botanist Carl Naegeli, then a prominent figure, Mendel investigated the hawkweed. It was a bad choice. Pea plants were good subjects since they could be inbred by self-fertilization, but hawkweeds have the perverse habit of "parthenogenesis" -- they could reproduce without being fertilized. Trying to figure out any pattern of inheritance with such a flexible organism was endless difficulty and went nowhere.
Mendel died in early 1884, a respected and influential local figure, but not one recognized by those who knew him personally as a great scientist. His obscurity remains a bit puzzling, since he was scientifically well-connected and his research was in hindsight brilliant. However, not having obtained any stature in his scientific work, during his lifetime the paper would not be read by anyone who grasped its real significance. Naegeli seemed particularly obtuse to the significance of Mendel's research, making the association between the two men a real disaster. It was a belated tribute to Mendel's memory that the paper would, in less than two decades after his death, become recognized for the ground-breaking piece of work that it actually was.
