EVOLUTION, PLAIN AND SIMPLE (Part II)

THE BACKGROUND OF EVOLUTION

 

EVOLUTION, PLAIN AND SIMPLE

By Robert George Sprackland, Ph.D.

The Background of Evolution

Evolution is an ancient idea - Modern science tackles evolution-

Lamarck and acquired characters - Darwin and Natural Selection -

An ancient earth from the geologists - The fossil record - Genes are the "stuff" of evolution - Genes and Natural Selection married in the "New Synthesis"

Charles Darwin did not invent the theory of biological evolution and it is not new, for we have records of evolutionary ideas that predate the ancient Greeks, into early India's history. The sacred books of the Vedic tradition of early India mention the development of simple life forms into more complicated "higher" forms over time. Many cultures have incorporated some similar view of life evolving, either physically, spiritually, or both.

The 18th century Swede who is credited with founding our system of biological nomenclature, Carolus Linnaeus, was a devout Creationist and a believer in a literal distinction between lower and higher life forms, a belief reflected in his most famous book's title, Systema Naturae or the "Ladder of Nature." He was never a believer in any sense of evolution, and felt that a creator's hand was seen in the complexity and beauty of life in its many forms. But in giving us a framework of classification, Linnaeus also grouped organisms by similarities that suggested they had something in common through kinship. Though a sense of kinship could be easily seen and discussed-there are legions of features to link "birds" into a single group, for instance-neither Linnaeus nor his contemporaries dared put forward that these similarities were the result of common lineage. Kinship and modification needed a champion, and the resolutely Christian literalist establishment of the 1700s was not about to provide one.

However, with the turn of the nineteenth century came a scientific description and mechanism for evolution in a simple and understandable form. It is at that point that we can profitably start our review of the history of the concept of biological evolution.

Lamarck's Acquired Knowledge of Biology.

In the first years of the nineteenth century France was a major western power. In 1789 the French cast off their monarchy and established a revolutionary quasi-republican government. The new government wasted no time in declaring war on and conquering neighbor states, from which it quickly extracted the valuables, including huge natural history collections. These biological materials generally were sent to the National Museum of Natural History in Paris, where many remain to this day, and served as perhaps the largest museum collection of plants, animals, and fossils of that time. Among the curators at the museum was a minor nobleman, spared an appointment with the guillotine because of his kind nature and excellent popular scientific writings. (I believe that many academics would do well to learn a lesson here. Academics are to often given no credit or are penalized by promotion committees if they write textbooks or "popular" science articles. See Loewen [1995] for details of the problem as it affects history professors.) He was therefore able to spend long hours studying the embarrassment of riches his warrior nation provided, and in so doing decided that the prevailing views were wrong on why there were so many different kinds of species. We know him today as Jean Baptiste de Lamarck (1744-1829).

Contemporary with Lamarck were the politically and socially powerful biologists of the time, who publicly (if not always personally) held to the view that fossils represented the expired species produced during the flood made famous by Noah and his ark. During the later years of the seventeenth century, scientists noted many peculiarities with the fossils. For example, below a certain level of rocks no mammals can be found. Fishes and seashells frequently were discovered on mountaintops. Geologists were starting to understand that rock layers reflected ages of the earth, and the fossil pattern gave strong evidence to a history with not one, but many earlier extinctions. No less an authority than the Compte de Buffon (1707-1788), considered the predominant and founding biologist of his day, proclaimed that God had merely experimented with different species over time. Being frequently displeased, He invoked a series of floods and started creation once again. Surprisingly, few contemporaries objected to an all-knowing, all-perfect god who had trouble being perfect the first (or second, or third, or...) time He created. The hypothesis of multiple creations, following a multiple of cataclysmic events, seemed instead to glorify the power at the disposal of God. Buffon's somewhat scientifically-supported hypothesis, termed catastrophism, held sway for more than half a century, and many biblical literalists still cite those outdated and long disavowed ideas as solid evidence for a narrow interpretation of the biblical accounts of Genesis.

Lamarck was not on the same academic or social level as Buffon or his successor at the museum, the Baron Georges Cuvier (1769-1832). Cuvier was an intellectual and social giant, acknowledged by many as the founder of paleontology and comparative anatomy as sciences. He was a full professor in charge of the national museum of natural history and its offshoot labs and the zoo. While Cuvier held the honored position of Professor of Vertebrates, Lamarck had been assigned a lowlier role. He had trained to be an authority on plants, and had written a book on the subject that won him the job offer to go to Paris. But botany was a prestige science, and the obscure young Lamarck was not to be given such a center stage position in highly competitive Paris. Instead, he was relegated to the intellectual no-man's land of the museum's collection of "worms," the animals without bones in their bodies. It was Lamarck's major contribution to his science that he recognized that not all "worms" were closely related, and actually contained a breadth of variety that far exceeded that seen in the better-studied vertebrate animals. Lamarck coined the term "Invertebrata" to replace "worms," based on his observation that the single most defining feature of these creatures was their lack of bony parts. He divided the invertebrates into several phyla, and in essence invented the field of invertebrate biology. In point of fact, Lamarck was also the man responsible for giving his science a name: "biology" is a Lamarckian term (Though Cuvier may have respected and accepted some of Lamarck's premises, his vanity lead him to consistently and publicly denounce the invertebrate biologist, to the point of delivering an insulting eulogy at Lamarck's funeral), as is "zoology."

History has been unkind to Monsieur Lamarck. Biology textbooks have presented little of his work to the new generations, focusing instead on his major flaws of logic. In studying his invertebrates and fossils, Lamarck became convinced that during the course of time simple types of animals evolved into ever more complex and modern creatures. He thus anticipated Darwin's idea by some sixty years. Like Darwin (and unlike the few evolutionary thinkers before and contemporary with him), Lamarck added a mechanism to his hypothesis. It was not enough to report that an event happened, one must also provide a possible explanation for how the event happened, for this is how science is done. For Lamarck, the explanation for evolution was the accumulation of acquired characters. This contrasted strongly from the prevailing explanation that "God did it," a view that sidestepped asking the problematical "how."

What Lamarck had observed was that organisms develop in very individual ways as they mature. Some of us become lanky and thin, while others, under a more rigorous lifestyle, become broad and muscular. Aren't the sons of blacksmiths also likely to become strong and muscular? Lamarck extended this idea to animals, with his flaw best illustrated in the hypothesis of how giraffes got long necks (Actually, Lamarck had very little to say about giraffes and their necks, but scores of subsequent biologists have used the giraffe story to illustrate how Lamarck's idea would account for the long neck) Lamarck assumed that as ground vegetation became scarce, or as competition from other herbivores intensified, giraffe ancestors began stretching their necks in order to reach leaves in trees. He believed that such giraffes could then pass on some intrinsic factor that would produce longer necks to their offspring. Over multiple generations, a long-necked giraffe would finally be produced. Essential to Lamarck's hypothesis were two points: first, organisms strove to make changes, innately sensing the value such a change could confer, and second, these changes were acquired cumulatively. Acquire enough changes and voilà, a new species is born.

Lamarck's idea was never very popular, though the French eventually embraced it in preference to Darwin's natural selection mechanism after Darwin published his epic book. The flaws in Lamarck's reasoning are apparent with the perspective of 20/20 hindsight, in that he did not know anything about genes and the nature of inheritance. We now know that genes are the major factors influencing characteristics of an individual; thus, if a blacksmith has muscular children, it is because the children have probably endured the physical rigors required of following in their father's footsteps in choice of vocation and they share some of his genes that allow a degree of muscular development. But if those same children follow another occupation they may well be considerably less muscular, perhaps even downright scrawny, unless their genetic code favored a muscular physique even in the presence of a comparatively sedentary life. Proponents of Lamarck's model will note that perhaps these children do not desire the traits that are so essential to their father. As for the short-necked "prototype" giraffes, they probably would have starved and died out while "striving" to extend the length of their necks. Lamarck's hypothesis just didn't account for interim survival or unwilling offspring.

Lamarck provided a view of life that lessened the importance of an almighty deity, and it was a worldview that few were ready to accept. For scientists or philosophers who persisted in publishing such views, society had little tolerance. Jobs were lost, careers ruined, social status reduced. Many of the "heretics" were imprisoned or exiled. Lamarck died a pauper's death; little known, blind and lonely, as his ideas began to slide into the lowest pits of academic respectability. The sad fate of the once popular naturalist must have had a strong influence on the man who would resurrect the idea of organic evolution and provide it with a more respectable mechanism. Can there be much doubt that fears of similar ridicule stayed Charles Darwin from publishing his theory for some twenty years after its inception?

A Mechanism and its Evidence Evolve.

Charles Robert Darwin's (1809-1882) childhood reads like that of many truly brilliant scientists. He was fascinated by the outdoors and animals from boyhood. He knew what he did like, but detested school and was at best a mediocre student. After a mediocre grammar school experience, he was sent to medical school in Edinburgh with the intention of following in his distinguished father's footsteps, but attending surgeries literally nauseated him and he dropped out. Frustrated by his lackluster second son's performance, Charles's father shipped him to Cambridge to study for a ministerial career. Charles spent most of his time hunting, collecting beetles, and conversing with friends. Somehow, he graduated, and was soon to land a position as companion to the captain of a Royal Navy survey vessel headed for South America.

Contrary to most accounts, the Royal Navy did not have an official position for "ship's naturalist." In those days a ship's captain held a position of authority that was virtually absolute, a situation made necessary by being out of touch with higher ups for months at a time. In order to maintain proper authoritarian distance from his men, the captain had to be socially removed from his crew. Even his most senior officers would be treated only in a professional context, though they could share meals with the captain. In order to keep from going stark-raving mad, the captain was allowed the option of bringing a civilian -often a spouse - for social company while at sea. Frequently, these passengers doubled by taking on some unofficial task, such as artist or naturalist. It was for this role that Darwin was summoned to meet with Lieutenant Robert Fitzroy (In one of the more ironic aspects of this story, the Fitzroy lineage seems to have "inherited" notable career success followed by suicide in the male line. Fitzroy's uncle was Lord Castlereigh, the Minister of War during the Napoleonic Wars against France, and he committed suicide shortly after the end of hostilities. Fitzroy rose to the rank of vice admiral in the Royal Navy, and ended his life by slitting his own throat) Though Fitzroy initially vetoed Darwin on the basis of the shape of Darwin's nose, he soon reconsidered and Darwin became the naturalist on board H.M.S. Beagle. Charles Darwin left England an unknown young man with poor career prospects; five years later he would return as a famous and respected naturalist, collector and author.

It has become part of "biology history mythology" that Darwin arrived at the Galapagos Islands and was instantly inspired in recognizing evolution in action. In fact, he was there for a scant three weeks, and though he did much collecting he was rather slipshod in recording where each specimen was taken. By the time he got to the islands he had been away from home for years, and the trip to the islands was preceded by several weeks at sea. By the time the perpetually seasick Darwin got back on solid ground, he may have been less interested in wildlife than he was in the lush South American forests. It was only as Beagle departed from the islands that young Darwin began to understand that the specimens from each island were subtly different, and he did not know which specimens came from which island. Most of the real significance of the Galapagos fauna would be discovered and further explored by twentieth century biologists. Even the famous "Darwin's finches" would reveal their secrets only late in the twentieth century to Dr. Peter Grant of Princeton University and his procession of colleagues and students (An interesting bit of trivia concerns "Professor Grant." The instructor who most molded Darwin while he was at the University of Edinburgh was an invertebrate zoologist, Robert Grant. Grant, who would later be the first head of zoology at University College London, would late adopt intellectual ideas that alienated him from much of the scientific establishment. I find it ironic that another Professor Grant would pick up Darwin's most famous model an demonstrate its veracity more than a century after Darwin first encountered his finches)

But the Galapagos specimens did start him to ask important questions. "How did these animals get to such remote islands?" "Why are they so similar to species from South America, yet so unusually different?" He found cormorants, but they had stubby wings and could not fly. He found large iguanas, but they swam in the cold ocean and ate seaweeds. As similar as they appeared to South American animals, they were also radically and uniquely different. Why? The islands also had large long-billed seabirds called cormorants, similar to species seen on the continent, but the island birds had tiny wings and could not fly. Again, he asked, why? The questions would stay with him for the rest of his life.

After returning to England and settling into his new life, Darwin started keeping a secret notebook, about how species "transmuted" from one into another over time. Prior to his voyage, convention accepted that the earth was a mere 6,000 years old, a figure derived by Bishop Ussher based on study of the generations of people listed in Genesis. Ussher had made intricate calculations to support a date for the creation that would be in the year 4004 B.C. (Here we see not only evolution, but also geology coming under attack. In time, this "young earth" idea of Biblical literalists will threaten to silence scientific evidence about drifting continents, origins of planets, and the Big Bang theory of the origin of the universe. In short, creationists drew a line that obliges them to object to all scientifically derived discoveries, as each reinforces another to some degree). At about the same time as Darwin's voyage began, Britain's most eminent geologist, Charles Lyell (1797-1875), published the first of a three volume text in which he explained why the earth must be older - far older - than a few thousand years. Two generations earlier, Scotland's James Hutton (1726-1797), founder of scientific geology, had made detailed observations of erosion, volcanism, and other earth-changing forces of nature. Hutton concluded that such forces, given enough time, could account for the deepest seas and highest mountains, a view summarized by his famous quote, "the present is the key to the past." Time, it seemed, could account for as much as divine intervention. Later, re-examining the geology of Europe, Lyell suggested that erosion, volcanoes, and other natural occurrences were indeed molding rivers, building mountains, and creating islands today. Many of the processes were slow, but if given enough time - far longer than any human lifetime - one could hypothetically observe these acts take place. Lyell calculated how long some events would take to occur, and this led him to a remarkable and heretical thesis: the earth was at least hundreds of thousands of years old, possibly even millions. Lyell's idea aimed to crush Buffon's catastrophism, replacing it with the similarly unwieldy term "uniformitarianism." This idea freed Darwin from trying to restrain his animals and plants from evolving into innumerable diverse species within only a few millenia. Given millions of years, could we begin to comprehend how much variety had arisen, how much had been lost? Lyell gave Darwin the time needed to propose as a scientific hypothesis that organisms could evolve into myriad new forms without invoking the need for a deity or supernatural forces. Note well that Darwin did not disprove the existence of God, nor that God started life on earth, nor try to advance either idea, but he did provide an explanation for life that made the existence of God irrelevant, and that upset (and upsets) a great many people. In short, science had made the necessity of a deity unnecessary to the origin and modification of life (though it did not "prove" nor suggest that there is no God). Science and religion had begun the final phase of their long and contentious divorce proceedings.

One of Darwin's and Lyell's later critics was eminent British physicist Lord Kelvin. Kelvin used his knowledge of heat and thermodynamics to produce calculations that the earth could not be so old as Lyell surmised because it would have lost heat at too great a rate. Darwin fretted over the scientific giant's calculations, but it wasn't until long after Darwin died that improved calculations took into account the heat produced by the earth's radioactive core. Indeed, radioactivity was all but unknown in Darwin's time, but many newer calculations show that the earth's age is indeed in the 4 billion plus year old neighborhood.

So what, precisely, is Darwin's theory of evolution? Unlike Lamarck and other philosophers who advanced the notion that life had evolved, Darwin provided the first scientific mechanism to explain how it evolved. In a massive series of observations that are both empirical and logically sound, Darwin explained how organisms are changed by selective breeding by man--termed "artificial selection"--and how similar but less discriminating actions were instituted in and by nature. There are four cornerstones to the Darwinian theory:

First, organisms tend to produce more offspring than can be expected to survive to maturity. Few of the millions of salmon eggs laid, or dandelion seeds planted, or elephants born will develop into reproducing adults. Human females are born with somewhere between 250,000 and 300,000 eggs at birth, but very few women bear more than two or three children (with Mrs. Johann Sebastian Bach, with 19 children, being a notable exception). No matter where Darwin looked, he saw that the potential number of young always greatly exceeded the number that survived to reproductive age. Why?

Because of the second observation, that resources such as food, water, and mates are limited and there is a struggle among and between species to utilize those resources. There is simply not enough food for all offspring produced on the planet to survive. Some creatures will be food for others, and some will succumb to disease or accidents long before they can reproduce. Because members of the same species need identical resources, the competition within a species far exceeds that between species. For example, you do not have to worry about losing your teaching job to a polar bear, or having your company taken over by porcupines at a board meeting.

Third, each individual organism is slightly, often imperceptibly different from all others. Some of these differences may confer an advantage in survival, obtaining food, or finding a mate. Even identical twins are note really identical. The features that make one sister prefer jazz to classical music, or even Mozart over Bach are subtle clues to the distinctness between the twins. If one of the tiny differences allows one creature to succeed where the other cannot, or succeed better (as in producing two offspring that survive into adulthood instead of just one), then that difference is a favorable adaptation. As Darwin phrased it, the best adapted (to an environment) are most likely to survive and reproduce.

Finally, those survivors will pass their advantageous traits along to their offspring, who should then have a selective advantage over other organisms lacking those traits. Given sufficient time and barring accidents such as being eaten by another organism, dying from a disease or accident, or being sterile, those offspring eventually will have passed along a sufficient number of new traits such that a completely new species will be formed. Thus, Darwin did not invent the concept of evolution, but put forward his own explanation of how evolution may have taken place.

Of course, Darwin lacked important pieces of the puzzle about how species came into being. He was unclear just what a species was; he didn't know how long it took to form a species; and most important, he didn't know what specifically nature selected to make an individual better, and allowed it to pass traits on to its offspring. He had figured out the great sculpting mechanism that shaped the evolution of organisms, and simultaneously explained why so much diversity was possible. But his real missing link, the gene, eluded him until his death. On the other hand, Darwin did not present us with a body of data and expect it to be accepted because he was an authority. For almost every point Darwin raised, he offered at least one counterpoint that could potentially "shatter" his theory. He gave his enemies access to every part of his theory that was weak or could be weakened in the face of appropriate counterpoints. So far, these theory-shattering counterpoints have failed to appear or have been reinterpreted in light of better understanding of the earth and life. In contrast, Creationists don't-can't-provide such potentially falsifying explanations, because their constant answer to every "problem" is the untestable statement "that's the way God did/wants it." Such explanations, whether true or not, are not scientific.

Science is as filled with irony and close calls as any other facet of human behavior. As Darwin's Origin of Species spread after 1859, one of Darwin's major caveats -that the fossil record was notoriously poor, but should eventually yield some missing links between species - received a powerful bit of material support. The sculpting mechanism was enhanced by the findings from a quarry in south-central Germany. The Sölnhoffen limestones were being mined to produce the fine material on which lithographic (literally "stone drawings") pictures would be printed. The extremely smooth and fine surfaces were carefully carved, coated with ink, and used to make many of the finest illustrations in books. These stones were very fine-grained, and as a result many intricately detailed fossils were found in the quarry. Barely a year after Darwin's book was published, the Bavarian rocks gave up a remarkable and nearly complete skeleton of what appeared to be a tiny dinosaur. Closer inspection, however, revealed the shocking truth, that the dinosaur had had feathers! Named Archaeopteryx lithographica ("the lithographic ancient-wing"), it was soon hailed by most biologists as a perfect example of a missing link, one between reptiles and birds, precisely as Darwin had predicted. Today, taxonomists classify birds as a branch of the dinosaurs, and that whole group as a part of the vast class Reptilia (Even this inclusive idea that birds and dinosaurs/reptiles are a common group is not as recent as some scientific writers would suggest. The "dinosaur-bird" renaissance of the 1970s, and revitalized in Michael Crichton's novel and movie Jurassic Park was actually first put forward by none other than Darwin's friend and "bulldog," Thomas Huxley in the 1860s). At about the same time as the Archaeopteryx discovery, a skullcap was found that was later to be assigned to fossil human status. Initially believed to be a heavy-browed Cossack from Russia, it was soon determined that the skull's features were somewhat different from those of living humans. It, too, was found in Germany, and bares the name of its discovery site, Neanderthal, or Neander Valley. The "missing link" between modern humans and their ancestors had been found; many more would follow.

Though critics often claim that there are few (or no) "missing links" in the fossil record, the 140-plus years since Darwin's book first appeared has yielded innumerable examples of links-no longer missing, of course-between many groups of organisms. Darwin's friend and supporter, the brilliant Thomas Henry Huxley, recognized the link between dinosaurs and birds as early as the 1860s. The term "Dinosaur" had barely been put forward by Sir Richard Owen at that time. In the 1970s, Yale's John Ostrom and his student Robert Bakker started advancing the idea that dinosaurs were warm blooded, like birds, and the subsequent decades have seen volumes of hard data support Huxley's original claim. Today, backed by hundreds of fossils and a wealth of other physiological and anatomical data, we classify birds as a group of dinosaurs, just as we put long-necked sauropods, horn-faced plant eaters, and two-legged tyrannosaurs into different groups on the dinosaur branch. The very last years of the 20th century offered Chinese fossils of feather-bearing dinosaurs, fossils that predate any known birds by at least 20 million years. It was once the major defining characteristic of birds that they were unique among animals in possessing feathers. Now we know that some dinosaurs had feathers long before there were any birds, and more are being discovered each year. Rare is the paleontologist who now denies that birds are dinosaurs.

In the same way that the details of the branch bearing the bird-dinosaur lineage have been slowly but thoroughly filled since Darwin's day, so too have other fossils and other biological advances filled in gaps in lineages of many other animal groups. Creationists, ever eager to mislead anyone who will listen to them, still claim that Darwin himself "admits" that the poor fossil record leaves too many links missing, and thus cannot be used as an important resource to support his theory. Darwin's lament over the "poor state" of the fossil record reflected paleontology in its infancy in 1860; it is no longer true in the 21st century! The fossil record is robust, diverse, and supports the theory of biological descent with modification. We have fossils of whales that lived on land, and snakes with legs. If anything, contemporary paleontologists are bedeviled by having too few funds and too few skilled workers to prepare all the materials that have been hauled into their museums. Now the poverty is no longer in the fossil record, but among those who must care for, expose, and interpret that record!

Finding the Key to the Material of Evolution.

More elusive than missing links was knowledge of what it was the sculptor (as in "evolution" or "natural selection") sculpted. The irony here is that the answer was also largely discovered within a few years of the publication of the Origin of Species, but it was so cryptic in nature that Darwin, who almost certainly read the paper about the discovery, did not understand its contents. In Brno (then in Austria-Hungary, now the Czech Republic) there lived an obscure monk named Gregor Mendel. His intent had been to get qualifications to be a high school teacher. Despite the support of his monastery, he thrice failed his exams, and failed his baccalaureate exam at Vienna, too. But if the conventional world spurned him, his abbot did not. He gave Mendel a greenhouse and a plot of land, and Mendel set out to see if he could discover nothing less than the laws of inheritance. Mendel's interest stemmed from his family's history as farmers. For so many peasants of the region, unlocking the secrets of inheritance could mean production of better crops and farm animals, and thereby making better incomes.

Mendel experimented with different plants, but eventually settled on the pea, because it provided him with seven traits that each occurred in only one of two forms. For example, peas were either green or yellow, flowers white or purple; there was no third choice. His experiment was to see how many of each trait appeared in each generation, while he controlled which plants would be parents. He would know the parents and their traits, each offspring, and all their offspring. By keeping meticulous records, Mendel was able to use his considerable mathematical skills to observe certain predictable patterns of probability in the way inheritance worked.

Due to his studies, Mendel established much of the foundation of modern genetics. He distinguished between the gene content of an organism and the way it actually looked (though the terms to describe these states, genotype and phenotype, respectively, were coined in 1909 by Wilhelm Johannsen), as well as a factor versus variant (gene and allele), dominant and recessive characters, and homozygous versus heterozygous genotypes (I shall return to these terms shortly). For his efforts, Mendel is considered the father of the science of genetics. He died in scientific obscurity, but would eventually be resurrected as a giant nearly on a par with Darwin.

Mendel was certainly not the first person to try to solve the question of inheritance: we already saw how the question plagued and ultimately eluded Charles Darwin, to name just one of many notable scientists. How, then, could a largely self-educated monk in a rural monastery accomplish something that the scientific establishment could not? The answer is a sobering lesson to anyone who would claim to be a scientist, because Mendel's success was based almost equally on sound experimental design, rigorous statistical analysis, and incredibly good luck.

First, Mendel used pea plants that displayed seven very clearly distinct characters in discrete either/or states. That is, a pea plant's flower was either white or purple, never a third or intermediate color. The peas were either green or yellow, and either smooth or wrinkled. Not only were the states discreet but it only took one gene per state to affect a particular trait's appearance. (In contrast, many traits involve two or more genes working together-human eye and skin color vary depending upon how many of several genes produce or do not produce the pigment melanin.) This simplicity of experimental design provided an easy to analyze model. In addition, peas have only seven chromosomes, and each trait Mendel observed was on a different one of those chromosomes. This reduced the chance of pea genes crossing over, which is normally a common source of genetic variation. Peas are also sexually reproducing and not subject to self-producing offspring with an increased number of chromosomes (polyploidy), another very common genetic trait in plants. In sum, Mendel found a nearly perfect experimental species, though at the time he could not possibly have known why his peas were a good model for study or how their inheritance worked.

Second, and possibly most important, Mendel evaluated each trait individually. Unlike his predecessors who would have tried to figure out how many white-flowered, long-stemmed, wrinkled-green peas a pair of plants might produce, Mendel kept separate records for how many green peas each plant produced, how many purple flowers would be produced, and so on individually. Today we say that he performed monohybrid crosses. Although this may sound logical a century and a half later, it was actually a radical procedure in 1860. Had Mendel tried to track even two traits together, it is very doubtful he would have become anything other than an obscure monk.

The third thing Mendel did was also inspired, to the point that it is probably the single most significant reason why his work went unappreciated for about 40 years. The monk was the first biologist to mate biology to statistics, explaining his results not as progeny and anatomy so much as numbers, formulae and ratios. The biologists of his day were simply too awed by such mathematics to plow through the comparatively heavy reading of the short but data-loaded papers Mendel published. While math may still intimidate many biologists, its use in biological science is almost universal, and few quality studies are published in evolutionary analysis, ecology or genetics without extensive inclusion of statistics. (To the biology undergrads reading this as you wrestle with probability and calculus courses, now you know whom to blame!)

Fourth, Mendel bred his peas into multiple generations. Had he done a single crossing, he could not have seen the statistical pattern affecting each trait. Let me explain: suppose Mendel crossed a purebred plant with green peas ("purebred" meaning no alleles for yellow were present) with a purebred plant for yellow peas. Because yellow is the dominant allele, all the offspring would have yellow peas, even though they all carried one allele for green and one for yellow (called hybrids). Had Mendel stopped there, he (like others before him) would have had no idea where the green "factors" had gone. It was only after the monk crossed the hybrids and looked at their progeny that green peas again appeared. Counting all the green and yellow peas of this generation, called the F2, lead to the observation that the colors occurred in a 3 yellow to 1 green ratio. In fact, there was a close 3-to-1 ratio for each of the seven traits Mendel studied, and a man who had been well trained in physics and mathematics easily noticed such a statistical signature. It did not take long for Mendel to perform agricultural parlor magic by accurately predicting the outcomes of crosses between any peas in his garden.

I mentioned another factor that allowed Mendel to succeed where so many others failed, a factor without which many of the really great scientific discoveries would remain mysteries. It is also a factor that many of my colleagues are loathe to acknowledge, for the academy puts a heavy load on scientists to be brilliant merely because they are scientists. That factor is dumb luck. Mendel was fortunate in choosing pea plants because they are among the small percentage of plants that express their traits as discrete characters. How lucky was he on this count? Botanists tell me that the likelihood of randomly picking a plant that would behave genetically the way the peas do is about 270 to one, pretty slim odds. More than nine out of ten plants would yield multiple character states, such as white, purple, and grayish flowers, or green, yellow, and brownish seed pods. Look at human eye color: we have much more variety than brown or blue eyes, including shades of both along with green, gray, and hazel (and, often, bloodshot). A trait such as human eye color is called a continuously variable trait, meaning we have a broad range of possible outcomes. Another example is height. Not so in peas. Not only did pea plants have discrete characters, those characters were discrete for every one of the seven traits Mendel observed. Later in his career, he tried to obtain similar results using a different type of plant, but figuring out the pattern of its inheritance eluded the monk (This tendency of other plants to behave "differently" does not mean that Mendel's rules of genetics are sometimes wrong, but (as shown by many later studies) that the genes often "gang up" to make reading their effects more difficult than in simple peas. These confounding traits now are known as "codominant," "incompletely dominant," "multiple gene effect," and other conditions). Someone said that fortune favors the prepared mind; sometimes, one must also be prepared to be lucky!

Despite the popular impression of how scientists make great discoveries, shout "Eureka!" and have a grateful world accept their findings, many (if not most) new ideas take years or decades to be accepted. Most of the scientific community quickly embraced Darwin's thesis and evidence that evolution had occurred, but most, including Darwin's staunchest supporter, Thomas Henry Huxley (1825-1895), rejected natural selection as the primary mover. Ignorant of Mendel's work, Darwin's subsequent revisions of On the Origin of Species offered progressively more convoluted incarnations of Lamarck's ideas of acquired characters. Mendel's work remained on dusty shelves, unread for another four decades. For Darwinian evolution, publication followed "Eureka!" by some twenty years; it would be nearly another century before it would be melded with Mendelian genetics and emerge as an almost universally held theory and mechanism to explain precisely how species originate. By the early twentieth century, botanists had started realizing the value of coupling their observations with statistical analyses. The stage was thus set for Mendel's work to be "discovered" and posthumously hailed with "Eureka!"

Combining Darwin and Mendel.

It would fall upon another organism completely to demonstrate the validity of the genetics demonstrated with peas. Beginning in 1909, Columbia University would become the focal point of gene research. Lead by Thomas Hunt Morgan, the team would turn from studying plants to looking at an animal. They wanted a creature that would be small (for the University gave them very little work space), reproduce very rapidly, and produce lots of offspring. They settled on a tiny insect, the fruit fly known as Drosophila melanogaster (the name translates from Greek δροσου + φιλος and Latin melanus + gaster, to "black-bellied fruit lover"), a species that has been the bedrock organism for genetics research ever since.

It is particularly noteworthy that Morgan was the guiding force on this team, for he was a true skeptic when it came to what was, at the time, believed about genetics. He did not believe in genes, did not believe in mutations, and did not believe that sex was genetically determined. It would take mathematically precise evidence to make this man believe anything, so it is particularly advantageous to genetics to have had such a strong critic as the main verifier of its scientific principles. To his great credit, Morgan's work fully vindicated that all the early beliefs were correct, and he changed his worldviews accordingly (Eddy and Johannsen, 19XX).

The big break was also unusual because scientific advances usually come from a sequence of related but separate experiments or discoveries. For Morgan, though, all the big discoveries would come from a single fruit fly. He was a white-eyed fly who was in poor physical condition and barely lived long enough to produce a single mating. The full story of the fly and its ramifications has been given elsewhere, but I shall present a summary of the importance of that fly to all subsequent genetic studies.

From studying the offspring and grand-offspring of that male, Morgan and his graduate students were able to conclusively demonstrate that 1) chromosomes are real and they carry genetic information (this from studying the male's Y chromosome), 2) mutations are real and do introduce the genetic novelties that Darwin's theory expected (for the white-eye developed de novo, without any white-eyed ancestor), 3) sex is determined by chromosomes (and hence our system of XX = female and XY = male), and 4) genes can link (meaning that one condition and another must go together: i.e., white eyes only occur in male flies). Later work, based on these foundations, allowed Morgan's team to discover recombination (when genes shuffle to produce combinations of traits not found in either parent) and develop gene mapping. The legacy of Morgan's "Fly Room" at Columbia came full circle in March 2000, when the full genome of the fruit fly was sequenced and published in the journal Science.

Many variations of these foundations have subsequently been discovered. For example, we know that the XX/XY sex determination system is not universal. Some insects have an XX/XO/XY system (where XO are infertile females called workers, XX are egg-laying queens, and XY fertile male drones), while birds and many reptiles have a ZZ/ZW system (where ZZ is male and ZW female). Nevertheless, these systems still follow the principles laid down by Gregor Mendel.

The important contribution of Morgan and associates to an evolutionary scenario is that in confirming both the reality of genes and the ability of genes to change (either by shuffling or mutating), Darwin's theory now had something upon which natural selection could act-Mendel's genes were demonstrated to be real. The "stuff" that Darwin's Natural Selection sculpted had been found and verified. To many biologists, organisms would now be perceived as the mechanism by which genes produce more genes. There is considerable dialogue about the relative importance of gene/organism/selection in regards to moving evolution (The main arguments can be broken down into a) organisms are a gene's way of making new genes and b) genes are an organism's way of making new organisms. It has also been suggested by some biologists that resolution of such a question is really in the realm of philosophy, not science, since the ultimate answer is, in all probability, not knowable), but no doubt exists that these three factors are the stuff of evolution. We can now address, at long last, precisely what is happening when organisms evolve.

Dr. Sprackland is an evolutionary zoologist who has taught college biology and anatomy and physiology. His latest book, Giant Lizards, 2nd Edition, is set for release in October 2008.

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Comments 3 comments

tonymac04 profile image

tonymac04 6 years ago from South Africa

Another brilliant Hub. Will go on to reading part III!

Thanks for the great lessons.

Love and peace

Tony


Baileybear 5 years ago

Superb account of scientific history. I am considering writing a hub about some aspects of Darwin, the person, and this would be a great one to link to for historical aspects (which I don't really want to go into).


Torrent 5 years ago

There's no doubt that some animals have "evolved" over time, but if you're suggesting that human beings came from pond slime, you've got to be kidding. Frankly, I find it hard to believe that any intelligent human would believe in "from the goo to the zoo to you." Come on.

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