DARWIN AND MENDEL

Myriad life forms have come and gone in a broad ecological panorama stretching across both space and time on planet Earth. Some were weak and trifling organisms, while others ruled the land, sea, and air. For hundreds of millions of years, a steady parade of new creatures has marched forth, many of whom led in turn. Yet only the latest of these dominant life forms—the men and women of today—know about all those that went before. Modern humans alone have been able to unearth and chronicle the absolutely amazing story of the now-extinct and bizarre life forms once prevalent on the 3rd planet out from the Sun.

What sense can be made of the copious array of past and present life on Earth? Does any logic link it all—anything to unify it into a coherent package of understanding? Classification is the first step in any attempt to discover underlying causes of the abundance and diversity of life on our planet. Some general trends are indeed apparent, though surprisingly biologists are still debating life’s basic categories.

All current life forms as well as fossilized remains of ancient life are often broadly classified as bacteria, plant, or animal. These classes, in turn, are further divided into different species, a subclassification generally used to denote not only structural similarity but also ability to mate and produce fertile offspring. But that was 20th-century biology, and this is now.

Recent discoveries of extremeophilic life as well as renewed awareness of the astounding biodiversity on Earth have caused biologists to revise the “universal tree (or bush) of life”—in other words, to rewrite a main and important part of all biology textbooks. A new consensus has emerged that molecular studies of extant life—especially the rates of change among genetic markers (mainly nucleotide-base sequences of RNA) shared by different groups of organisms—probably grant a more accurate picture of evolutionary relationships among all known life on Earth. Such molecular clocks stipulate that the three main branches, or domains, of life are now bacteria, eukarya, and archaea—the last of these containing mostly the newly found thermophilic (heat-loving) life forms residing in extreme environments.

Figure 6.14 shows a sketch of this new classification scheme, which is still subject to change as new findings accumulate. Plants and animals of the old classification scheme are now both grouped into eukarya, all of whose members are the eukaryotes noted earlier. And then there are the bacteria, whose microbial members probably comprise well more than half of Earth’s entire biomass. It is the past overlap of the bacteria and the third, archaean branch of prokaryotic life, way back in deep time, that might someday reveal the last common ancestor that sprang forth from life’s origin nearly 4 billion years ago.

FIGURE 6.14 FIGURE 6.14 — The modern "tree" of life looks more like a "bush," with its three main domains of life emanating out from a common center (yellow dot) at the origin of life. Magnitudes of the many lines imply extent of differences among various lineages.

Not all life forms, however, fit cleanly into neat categories. Life isn’t so simple and exceptions abound in Nature. The unicellular (~0.01-cm diameter) Euglena is a good example. Considered an animal by zoologists (animal biologists) since it can move rapidly like an animal, the Euglena is also claimed to be a plant by botanists (plant biologists) since it consumes energy much like a plant. Further research might resolve to which category it really belongs (it is however most definitely a eukaryote), as with fungi (including mushrooms, molds, and mildew) that are nearly always treated as plants yet don’t photosynthesize and have much in common structurally with animals. Diatoms, too—those simple, single-celled microbes that resemble microscopic, aqueous snowflakes—lie somewhere between plants and animals. Exceptions to rules are common in biology, because either the data are incomplete or the subject is complicated, and often both. The biological sciences have few hard-fast rules, quite unlike the physical sciences.

A few other very simple life forms also seem to be misfits, having little in common with either plants or animals. Bacteria, blue-green algae, and amoebas are especially good examples of small (~10-5 cm) unicellular life forms that generally lack structure within their cytoplasm; in fact, these prokaryotes have no well-defined biological nucleus and hark back to the very earliest forms of life on Earth. Some researchers argue that these ultrasimple life forms deserve a separate classification, regarding them as a "lower" form of life. By contrast, multicellular plants and animals (all of whose cells have nuclei and are hence eukaryotes) are said to comprise a "higher" form of life.

A thorough understanding of life goes beyond its cataloging; this isn’t mere stamp collecting. Real creatures don’t always match what’s expected of a species. Rather, individual species often show small, though noticeable, variations from their “ideal” categories—slight deviations from some standard specimen to which each individual organism may be compared. This is true of all species, whether they are now living, dead, or fossilized.

As with many types of matter in the Universe, change is key to their being, and mentally modeling that change is key to our understanding. Here too, as with other aspects of the cosmic-evolutionary scenario, the study of change contains insights needed to fathom how life has evolved throughout the course of time. We have entered the realm of biological evolution—the changes experienced by life forms, from generation to generation, throughout the history of life on Earth—perhaps the most intellectually powerful core of this, the sixth, BIOLOGICAL EPOCH.

The Idea of Evolution The theory of biological evolution, independently conceived by the mid-19th-century British naturalists Charles Darwin and Alfred Wallace, can account for two outstanding features of the fossil record: First, living systems have generally become more complex with time. Second, variations among members of all species are more the rule than the exception. (Darwin gets most of the credit since his thinking predated that of Wallace and he provided many more data and examples in support of the theory. However, the very idea of evolution had been “in the air” at least 50 years before they made it famous in 1859.)

These two facts clash head-on with the age-old assumption that Nature is immutable. Like Copernicus and Galileo a few centuries before, and Heraclitus ~20 centuries before that, Darwin and his colleagues faced the same kind of opposition made popular by Aristotle, who refused to concede that species change. But given these undeniable facts of Nature, a static theory of life is simply untenable. Everything changes with time, life included. The only feasible explanation is a dynamic, evolutionary one.

The central tenet of biological evolution maintains that living things change, some for the better, others for the worse. Some species thrive, others go extinct, and yet others arise anew. Those that survive for lengthy periods of time are often drastically modified, sometimes becoming whole new species. Amidst all this change, organisms having similar structure share similar ancestry and are closely related. Those with very different structures have accumulated those differences over long durations and are therefore now only distantly related.

Natural Selection Biological evolution is not faith. It’s as much a fact as Earth orbiting the Sun. The fossil record no longer leaves room for reasonable doubt that evolution does happen. That “what” aspect of evolution is backed by data. The “how” aspect is less clear, which is why biological evolution is rightfully called a theory.

Although evolution itself is factual, the mechanisms that cause evolution remain theoretical. What we have here is a mental model, yet one solidly based on the scientific method: Observations were made of the fossilized remains of life; an idea was proposed to explain those facts; and subsequent data have served to strengthen and revise the intricacies of the theory during the past century and a half.

In particular, observations show that, although all species reproduce, few of them display huge increases in population. The total number of any one species remains fairly constant, there being no dramatic rise in offspring from generation to generation. Furthermore, the process of reproduction is almost never perfect; offspring in each generation are hardly ever exact copies of their parents. The implication is that not all offspring survive to reproduce. Life must struggle and compete in order to endure. (Humankind is an exception whose population is exponentially rising, but that’s because we are now affected more by cultural evolution, whereby not only do the fittest survive and reproduce but nearly everyone else does too.)

What is the primary agent of biological evolution? How does it work? The chief instigator is the environment, the physical conditions surrounding all living things. Temperature, density, foodstuffs, air composition and quality, in addition to natural barriers such as rivers, lakes, oceans, and mountains, are among a whole host of influential environmental factors. Other factors are more subtle, such as personality clashes (or individual love), neighborhood squabbles (or group harmony), among scores of exceedingly messy sociological pressures (some beneficial, some not). Further complications arise given that environmental conditions frequently change, albeit often slowly. Biological evolution asserts that life forms respond to their changing environments, inhibiting some traits while promoting others, thereby yielding an immense diversity of species throughout the course of time. Changes—in the environment and in life, and to repeat for emphasis—occur as a rule, not as an exception.

Natural selection, an expression coined by Darwin himself, is the mechanism that guides much of life’s evolution along time’s arrow. Recognizing that most members of a species exhibit some variation from their ideal standard, Darwin argued that organisms having a variation particularly suited to their environment would most likely survive. They’re quite naturally selected to live. By contrast, those organisms having unfavorable variations would most likely perish. They’re naturally selected to die. In short, only those life forms able to adapt to a changing environment tend to survive long enough to reproduce, thereby passing their favorable variations or traits on to their descendants.

A central feature of biological evolution is modification followed by adaptation—the positive response to a changing environment of an organism having some variation or trait that improves the organism’s chance for survival and reproduction. Pedagogically, though, it’s often more instructive to regard natural selection as a negative influence on organisms within a population. An even better word for selection would be elimination, for it is by means of non-random elimination that biological evolution really operates in Nature. Nature eliminates more than it selects and it does so deterministically in response to chancy events; there are many more "losers" than "winners" among life in the wild. That said, evolution rarely throws out a good scheme, but instead modifies and embellishes on whatever already exists.

In successive generations, advantageous traits become more pronounced in each individual; they accumulate. Not only that, but the numbers of individuals possessing favorable traits also increase. Favored individuals generally produce larger families, as they and their offspring have greater opportunities for survival. Their favored descendants multiply more rapidly than those of their less advantaged neighbors, and over many generations their progeny replace the heirs of individuals lacking the desirable trait.

Natural selection truly does smack of the well-known phrase “survival of the fittest.” It literally molds life forms. With the passage of sufficient time, the action of natural selection can greatly alter the shape, disposition, ability, character, and even the existence of individuals. Old species can disappear in response to changing conditions, while entirely new ones can arise anew. Yet because the element of chance is indeed a factor, the outcomes are not predictable. Evolutionary can display trends in general but is not a predictable science in detail.

Note once more the twin roles played by chance and necessity, as elsewhere in cosmic evolution. The mechanism of natural selection isn’t some proactive force, like those guiding strictly physical change. Instead, natural selection in biology acts as a sifting process—an “editor” of sorts—permitting some species to thrive quite naturally all the while others become extinct. Chance events admittedly often trigger evolutionary change, but natural selection has a decidedly deterministic part that directs that change—though mostly by a process of elimination. Contrary to popular opinion, Darwin never said that the order so prevalent in our biosphere arises from chance alone. Yet even the limited role of chance in modern neo-Darwinism, when coupled with the deterministic part of natural selection, is capable of generating highly improbable results. Chance and necessity, mutation and selection: It is the synthesis of randomness and determinism that ultimately gives rise to the spectacular novelty and creativity seen among the wonderfully diverse and talented life forms adorning planet Earth today.

Examples of Evolution Natural selection cannot be easily observed at work. Passages of time usually far longer than a human lifespan are needed to witness large-scale evolutionary change in any population of a species. Some experimental success has been achieved in laboratory settings that mimic those of Nature. Like the origin-of-life experiments of the earlier CHEMICAL EPOCH, these simulations study the adaptation of life to a changing environment. In all cases, the results support the theory of biological evolution via natural selection.

Here’s one such experiment, conducted under carefully controlled conditions: Two groups of field mice, one group with dark fur and the other with light, were let loose in a small barn with an owl. The straw and ground cover were chosen to match closely the dark coloring of one set of mice. This camouflage then gave the darker-colored mice an environmental advantage to hide; the lighter-colored mice were clearly at a disadvantage. At the end of this well-documented experiment, the owl had captured many more lighter-colored mice. When the ground cover was lightened—corresponding to an environmental change granting the lighter-colored mice a greater opportunity to survive—the results were reversed; the owl readily captured the darker-colored mice. This is an example of how small variations in one species can grant a competitive advantage. As might be expected in the real world outside the laboratory, the natural habitat of the light mice is cornfields, of the dark mice, forests. In each case, the mice best adapted to their environment were naturally selected to live and thus to reproduce their kind.

Recently, the tools of molecular biology have allowed researchers to track minute changes among life in test tubes. With computers that can store lots of numbers and analyze them quickly, evolution has become visible as whole populations move from generation to generation. For example, in one such laboratory experiment that ran for many years, a sparse sugar broth was laced with a quick-replicating bacteria and each day siphoned off into a fresh flask of food. Over the course of several thousand generations of the bug—known as Escherichia coli—biologists were able to study evolutionary dynamics in action. The result of the experiment—with food availability acting as the selector—suggests that rare, beneficial changes gradually led to increased cell size, much as expected if natural selection is truly at work.

Examples of natural selection have also been observed in Nature’s outdoor setting whenever environmental factors change exceptionally fast. Consider a classic study done several times over the course of the past century. Normally, in rural areas, the bark of many trees in the English countryside is abundant with light-colored lichens growing freely on their dark trunks, enabling the famous “peppered moths” to blend nicely with their environment. In their struggle to survive, the moths prospered while resting on the trees against whose bark they were nearly invisible. By contrast, their darker-colored moth relatives lacked this competitive advantage because they stood out clearly against the lichen-rich bark, allowing birds to snare an easy meal. As shown in Figure 6.15, ~100 years ago at the height of the Industrial Revolution, however, many trees near manufacturing cities had become heavily soiled since lichens are highly susceptible to airborne pollutants. This environmental change—rapid by Nature’s standards—had killed the lichens and thus removed the benefit previously enjoyed by the lighter-colored peppered moths. The result was that few pale moths prevailed, at least near industrialized areas; instead, the darker moths possessed the advantage of camouflage, enabling them to avoid the birds, mate in peace, and freely reproduce. Today, the situation has once again reversed: With the reduction of industrial pollution in recent years, the number of peppered moths has rebounded along with the growth of lichens, each inversely correlated with the lower levels of soot and sulfur dioxide in the cleaner air. This is an example of how simple variations—in this case color, as with the mice in the barn—serve to guide natural selection within a changing environment. For some moths, in fact, a small change became an issue of life or death.

FIGURE 6.15 FIGURE 6.15 — A small variation among some members of a single species can sometimes be advantageous in the struggle for survival. Here, on the dirty bark of a tree, a darker-colored moth at the bottom blends with its environment better than does a lighter-colored moth at the top. Hungry birds clearly pick off such light moths, allowing the darker ones to prosper and reproduce. (British Museum)

Literally hundreds of similar cases of natural selection have been reported in the medical and agricultural literature. The common housefly presents yet another natural example of how some members of a single species can adapt to a changing environment, granting them a better opportunity to be naturally selected for survival—or, better and boldly stated, to avoid being eliminated. Originally, the pesticide DDT was successful in killing houseflies. During the first several years of use, DDT killed almost all the flies; few of them could adapt to this sudden environmental change caused by the chemical DDT in the air. A small minority of flies, however, managed to survive because they possessed a chance variation or trait that made them resistant to this chemical. These oddball survivors reproduced freely and thus passed the advantageous trait on to their descendants. Within a decade, the offspring of the survivors outnumbered the original majority type of fly. Accordingly, DDT has grown less effective over the years. Now (~50 years later) most houseflies have inherited a resistance to DDT and the pesticide is useless against these flies. The chemical DDT didn’t give this resistance to the flies; rather, it provided an environmental change enabling natural selection to go to work. To survive as a species, the housefly had to adapt to the changing environment. Those that managed, survived. Those that were unable are long gone.

These last two cases are examples of evolutionary response to environmental change induced by humans—a whole new aspect of evolution whereby technologically equipped beings play the role of Nature, a topic best left for the next CULTURAL EPOCH.

Types of Selection Variations among members of a particular species can be illustrated in graphical form. Figure 6.16 is a plot of the distribution of a particular trait among members of a given population of a species. The curve is bell-shaped, implying that most members have some average properties, while fewer members have extreme properties to either side of the average. Known as the gene pool, this type of spread in variations can represent any trait of a species—hair color, eye color, size, shape, appearance, whatever. In the case of the species of mice or moths discussed above, the variation is their color. As shown by the graph, most moths are sort of tannish, although some are white and brown.

FIGURE 6.16 FIGURE 6.16 — All members of any species show some variation in their properties (or traits). These properties can be height, weight, color, leg length, eyesight, or whatever. In the example chosen here, we have plotted the number of mice having different colors.

Now imagine an environmental change. For the mice, consider a ground cover that becomes progressively darker. And for the moths, consider birch trees that gradually became dirtier. After natural selection has had a chance to take its course, the distribution of mice and moths will have shifted. Figure 6.17(a) schematically shows how the darker-colored mice or moths are favored for survival. Those darker mice or moths will tend to increase in number; the others will tend to die off. Hence in this case, one of the extreme properties—dark color—is favored. Over a period of time, the gene pool representing the color of all members of the mice and moth species will have shifted, as shown in Figure 6.17(b). This is known as “directional selection.”

FIGURE 6.17 FIGURE 6.17— Directional selection enhances one extreme trait of a species(a), eventually producing a shift in the distribution of that trait within the species (b).

A second kind of species change is called “stabilizing selection.” Figure 6.18(a) illustrates this case, whereby environmental change causes both extremes to do poorly, while the average traits are enhanced. The result, after a period of natural selection, is the more peaked gene pool shown in Figure 6.18(b).

FIGURE 6.18 FIGURE 6.18— Stabilizing selection enhances the average traits of members of a single species (a), eventually producing a more peaked distribution (b).

Over lengthy durations, chance variations in living things can accumulate. Hair and eye color, size and shape, among a host of other attributes all change as Nature naturally selects for survival those life forms best adapted to the environment at any given time—or again to stress the real action in the wild, eliminates those life forms that are poorly adapted. Eventually, some life forms come to differ noticeably from members of their original species. In this way, the environment helps new species to evolve from old ones.

For example, some members of a single species might become isolated by some physical change in the environment, such as a new river that reroutes (due to flooding or plate tectonics, for instance) through an area inhabited by a population of butterflies. As illustrated in Figure 6.19, should the river act as a physical barrier too wide to be crossed, butterflies on one side would be unable to mate with those on the other side. The two populations of butterflies, then completely separated, will begin to differ as minute changes accumulate in each of them over long periods of time. In some cases, one group of butterflies—often called a “founder population”—will eventually differ greatly, even in outward appearance. Should the geographical barrier later be removed—if the river dries up, for instance—then the two populations would be able to intermingle once again. Provided they were separated long enough to allow real changes in form, they will be unable to interbreed. Two new species of butterflies will then exist where previously there was only one. Furthermore, each new species will stake out its own claim or fill a separate niche, thereby coexisting within the new environment.

FIGURE 6.19 FIGURE 6.19 — A species of butterflies that normally interbreed (a) can be disrupted by some physical barrier such as the appearance of a mountain or river (b). When the barrier disappears later (c), the populations of butterflies on the two sides might have changed so much that they can no longer interbreed.

Environmental disruptions of this sort often guide the transformation of a single species into two or more species. Known as “disruptive selection” or speciation, this is the mechanism behind the diversification of all life. It might take centuries, or even millions of years. The rate of evolution depends on a whole array of factors including the amount of initial environmental change and the extent of resulting adaptation.

Figure 6.20 is a graphical representation of how speciation works. In this case, the physical barrier of a new river, among many other possible environmental changes, can sometimes cause the average traits to do poorly while enhancing the extreme traits. One extreme trait may enable butterflies on one side of the river to increase their survivability; another extreme trait may be useful for butterflies on the other side. The result, shown in Figure 6.20(b), is a complete disruption in the gene pool which effectively creates two new species.

FIGURE 6.20 FIGURE 6.20 — Disruptive selection enhances the extreme traits (a), eventually producing two completely new populations that can no longer interbreed (b). Two species now exist whereas previously there was only one.

An actual case study entails recently upthrust mountains and eroded ravines in the Grand Canyon—dramatic environmental change over geological timescales. Two distinctly different populations of squirrels live on the north and south rims of the canyon. The Kaibib squirrels of the north rim have black bellies and white tails, whereas the Abert squirrels of the south rim have white underparts and gray tails. Both feed on pine-tree bark growing only on the kilometer-high plateaus. The two populations are now separated, and presumably have been for thousands of years, by the intensely hot and dry conditions in the canyon. But they have so many similarities that it seems safe to assume that their ancestors were once members of the same species.

Other examples abound of many slightly different species, coexistent yet isolated and apparently sharing a common ancestral heritage. Scientists find more of them every day, including members of species that aren’t even separated by a physical barrier, but that for one reason or another don’t interbreed. The factors are so numerous and the timescales so long that it’s often impossible to reconstruct the myriad changes that led to the current state of biological affairs.

One final clarification about the distributions of traits: When biologists refer to populations of species, they are really thinking in terms of gene pools—the whole spread of variations within a given population of a species. It’s the genes, or DNA fragments, of different populations that are isolated from one another and that gradually develop variations. Therefore, we need a better understanding of changes among the microscopic genes if we are to fully appreciate modern biological evolution. This we shall take up after the next sub-section below, but before leaving our discussion of Darwinism, let’s consider one other, currently popular yet controversial, alternative to classical Darwinian evolution.

Punctuated Equilibrium The fossil record of the history of life on Earth clearly documents many episodes of mass extinction—times when at least half of all life perished, after which life’s diversity lay dormant for up to several million years. Figure 6.21 graphs the ups and downs of species extinction, noting five prominent periods of death and destruction since Cambrian times.

FIGURE 6.21 FIGURE 6.21 — Five major mass extinctions have been gleaned from the fossil record over the past 0.5 billion years. The most most recent one, "End-Cretaceous," occurred ~65 million years ago when the dinosaurs and many other life forms perished; the "End-Permian" ~250 million years ago nearly destroyed all of life on Earth entirely.

In addition to the “great dying” ~65 million years ago that ended the reign of the dinosaurs (as well as ~2/3 of all other living things), several other dark times devastated biodiversity. To cite two of the worst episodes, ~440 million years ago vast numbers of animals then living in the sea quickly vanished, and ~250 million years ago as much as 90% of all marine species and 70% of all land species suddenly became extinct. The last of the trilobites disappeared during this quarter-billion-year-old event, as did all of Earth’s ancient corals, most of its amphibian families, and nearly all of its reptiles. Life itself was nearly extinguished on our planet; it's amazing that it managed to survive at all. As for the dinosaurs later in time, a cosmic killer might have been the trigger, namely, an asteroid that impacted southern Pangaea and likely caused huge volcanic eruptions and massive lava outflows, supplemented by dramatic fluctuations in climate and sea level. Ironically, this potential impact might have opened the door to the later emergence of the dinosaurs, though not all experts look to the sky for causes of dramatic changes on Earth.

Some paleontologists have proposed that the most notable mass extinctions have been periodic, occurring roughly every 30 million years. One possible reason for this (hotly debated) periodicity in the fossil record conjures up another astrobiological connection: The remote Oort Cloud of comets, ~50,000 A.U. away, is disturbed each time the Solar System oscillates above and below the galactic plane of our Milky Way, potentially causing numerous comets to be ejected from their regular orbits and toward the Sun. Some of these comets would rain down on Earth, disrupting the climate, reversing the poles, or otherwise upsetting our planet’s environment, thus causing mass extinction of life on Earth. Another, astounding proposal is that our Sun might have a companion star which, in a highly elliptical orbit, would periodically pass through the Oort Cloud every ~30 million years, thereby creating a gravitational uproar that sends comets sunward. Observational efforts using infrared equipment have searched for this dim, dwarf, and decidedly hypothetical star—already named Nemesis, after the Greek goddess who relentlessly persecutes the excessively rich, proud, and powerful—but thus far to no avail. It probably doesn’t exist.

The last known sizeable asteroid to strike North America occurred ~35 million years ago at the base of the present-day Chesapeake Bay. A submerged and mostly buried crater nearly 100 km across (found by energy companies while prospecting for oil) implies that an impacting rock ~3 km in diameter flung out a hail of white-hot debris and a surging tsunami that must have turned the eastern seaboard into a wasteland. The fossil record shows a moderate extinction of numerous sea creatures about a million years later. Though not as large as the asteroid that likely felled the dinosaurs, the resulting damage must have been awesome nonetheless—and if it happened today would likely wipe out most East Coast cities, threatening millions of people.

The most recent significant hit was in 1908, when an asteroid (or comet) exploded over Tunguska, in Siberia. That object was probably only ~100 m across (or the size of an apartment building), yet was still able to flatten trees over thousands of square kilometers with the force of a multi-megaton nuclear blast. And in early 2002, an even larger asteroid whizzed past Earth at about double the distance to the Moon—a relatively close call. Stunned astronomers didn’t see it coming until it was about a week away.

Regardless of whether comets and asteroids have regularly belted Earth sufficiently to cause mass extinctions, biologists now realize that the rate and tempo of evolution have apparently not been steady throughout the history of life on Earth. By contrast, Darwin himself argued that natural selection operates gradually, with slow, steady, uniform changes occurring through time. He envisioned the development of species much as he saw environmental change on Earth—as a process of smooth, gradual change proceeding at a uniform rate. But paleontologists are now gathering increasing evidence that biological evolution may well have operated more erratically, with occasional rapid, even catastrophic changes occurring every so often.

The fossil record is spotty and not always seemingly in accord with the gradualism posited by Darwin. Few fossils show clean, continuous transitions from one form to another with myriad small, intermediate steps linking one species to another. Of course, one possible explanation of these so-called gaps in the fossil record is that that record is thus far incomplete—which it surely is to some extent. But perhaps Nature is also not so continuous; perhaps it changes abruptly owing to irregular upheavals. Today’s fossil record implies that many species have remained more or less the same for long stretches of time often measured in millions of generations, after which they rather suddenly underwent bursts of evolution in perhaps less than a thousand generations—too short to have left clear, transitional forms in the rock layers.

To more closely match the geological record, a contrasting idea, called punctuated equilibrium, has attracted some adherents during the past few decades. According to this theory, life stays pretty much unchanged until something drastic happens and then changes fast—sometimes for the better and species diversification, at other times for the worse and species extinction. Life’s “equilibrium,” or “stasis,” is said to be upset, or “punctured,” by rapid environmental change. And when speciation rates—the pace at which new organisms emerge—cannot keep up with extinction rates—the pace at which they vanish—the net result is often long-lasting depletion of biodiversity.

The theory of punctuated equilibrium is a slight variation on classical Darwinism. It’s not at all a violation of the basic ways and means of biological evolution; natural selection remains the principal way that life changes from species to species. Punctuated equilibrium merely claims that the rate of evolutionary change isn’t so gradual. Instead, the “motor of evolution” occasionally accelerates during periods of dramatic environmental change, such as asteroid impacts, magnetic reversals, volcanic eruptions, and the like. We might say that evolution is imperceptibly gradual most of the time and shockingly sudden some of the time.

Not all scientists have yet accepted the notion of punctuated equilibrium. Perhaps the rate of evolution is largely a matter of perspective: Those who examine the fossil record in fine detail, across short durations, will occasionally see evidence for periods of rapid speciation; yet those who step back and take a broader view, over very long periods of time, will see more gradual change. This debate is still underway—a perfectly reasonable parley about the tempo and not the mode of change—and is not likely to be resolved until the fossil record is more complete or the genetic record better understood.

Genetics What is it that alters living systems to make members of a single species sometimes unable to interbreed? Basically, the minuscule gene is the culprit, for it’s the genetic code that dictates if and how life forms reproduce—a subject that was introduced briefly in the previous CHEMICAL EPOCH. Pioneered more than a century ago by the Austrian monk Gregor Mendel, the subject of genetics has become a good deal more elaborate than he could have ever imagined. Darwin himself would probably be surprised at the microscopic roots of biological evolution as we know it today—a modern synthesis of Darwinian and Mendelian ideas, often referred to as neo-Darwinism.

Still, what causes the genetic alterations? What factors contribute to the similarities and differences in organisms? In short, what is the origin of all the myriad variations seen throughout the living world?

Hereditary error is a major factor promoting the evolution of living systems; it’s in fact a prerequisite for evolutionary change. Note the phrase hereditary error, not heredity itself, which is an agent of continuity, not change. Heredity, by definition, is the transmission of genetic traits from parents to offspring, thus ensuring the preservation of certain characteristics among future generations of species. Otherwise, both the structure and function of each and every organism entering the world would have to be created from first principles. Normally, chemically coded instructions of the DNA molecules enable cells to duplicate themselves flawlessly millions of times. But occasionally, mistakes do occur at the microscopic level. Not even genes are immutable. Everything changes.

For reasons only partly understood, a DNA molecule can sometimes drop one of its nucleotide bases during replication. Or it may pick up an extra one. Further, a single base can suddenly change into another type of base. Even such slight errors in the DNA molecule’s copying mechanism mean that the genetic message carried in the DNA molecule for that particular cell is changed. The change doesn’t have to be large; even a change in a single nucleotide base among millions strung along the DNA molecule can produce a distinct difference in the genetic code. This, in turn, causes a slightly modified protein to be synthesized in the cell. Furthermore, the error is perpetuated, spreading to all subsequent generations of cells containing that DNA.

Microscopic changes in the genetic message—mutations—can affect offspring in various ways. Sometimes the effect is small and newly born organisms seem hardly any different. At other times, mutations can alter a more important part of a DNA molecule, inducing marked change in the makeup of an organism. And, at still other times, a single mutation can rupture a DNA molecule severely enough to cause the death of individual cells, and even whole organisms. An example of the last of these is cancer.

Mutations are responsible for differences in hair and eye color, body height and finger length, skin texture, internal organs, individual talents, and numerous other traits among a population of life forms of any given species. Virtually any aspect of the life of an organism can be modified by genetic mutations. Such mutations provide a never-ending variety of new kinds of DNA molecules.

Not all mutations are detrimental. Most of them do indeed create traits inferior to the previous generation’s, especially in many of today’s highly evolved and exquisitely adapted organisms. But some mutations are favorable and serve to better the life of an individual. These can then be passed on to succeeding generations, making life more bearable for members of that species. Nature provides a fine balance between error and accuracy in replication: Too many mutations and an organism can’t function; too few and it loses adaptability. Beneficial mutations aid the motor of evolution, steering life forms toward increasing opportunities to adapt further to ever-changing environments.

What causes genes to mutate? Why do some DNA molecules occasionally replicate differently, though they may have copied themselves exactly for millions or even billions of previous cell divisions? Whatever the reasons the outcome is unpredictable, for apparently chance, indiscriminate events are at work, at least in part. Biologists have performed laboratory experiments with cells and have managed to increase the number of mutations by artificial means, thereby helping to unravel some of the causes of genetic change. The results so far show that the easiest way to enhance gene mutations is to treat reproductive cells with external agents.

Three of the most important mutation-inducing agents revealed in the last few decades are temperature, chemicals, and radiation. When cells are heated or treated with industrially generated nerve gas or chemical drugs, mutations are clearly enhanced. In addition, ultraviolet and x-ray radiation seem to be an especially striking source of genetic mutations. Radiation, in one form or another, has been present on Earth since the beginning of geologic time. Radioactive elements embedded in rocks, cosmic rays bombarding Earth from outer space, and even small amounts of solar ultraviolet energy reaching the ground all serve to prove that life originated and evolved in a radiation-filled environment.

Generally, there’s nothing wrong with immersion in radiation. We and other life forms probably wouldn’t be here without it. In the absence of radiation, life itself might not have progressed beyond the primitive, unconscious, unicellular organisms drifting in the oceanic slime. Of some valid concern now, however, is the fact that human inventions such as atomic bombs, nuclear reactors, and some medical devices also release radiation. Intense doses of radiation, let alone nerve-gas agents, can kill directly, though more subtle doses cause changes in the reproductive cycle that are then passed on to future generations. It’s unclear that these human-induced mutations are in all cases harmful, but, in the absence of evidence to the contrary, a healthy degree of skepticism is surely warranted.

We must not risk damaging the refined work of several billion years of organic evolution, for the long sequence of changes that evolution engenders cannot likely easily be repeated.

Selection, Broadly Considered Natural selection is central to all of biological evolution. Darwin’s idea was novel and innovative— surely one of the most significant advances in all of post-Renaissance science. Yet aspects of selection are seen throughout much of Nature—during physical evolution in simpler systems and during cultural evolution in some of the most complex systems. Not that genes are involved in physical or cultural change; not that inheritance and reproduction are prominent for any but biological evolution; not that natural, or Darwinian, selection works among inanimate objects. Yet, the process of selection, generally construed, is present all through cosmic evolution, operating in realms well beyond biology. To be sure, selection functions more robustly for living systems than for those non-living—and probably even more actively, or at least faster, for cultural systems. But selection, in the main, is a common feature at work throughout Nature writ large.

Both to clarify and to stress a point made earlier, the term “selection” is actually a bit of a misnomer, for there’s no known agent in Nature that deliberately selects. Selection itself is not an active force or promoter of evolution as much as a passive pruning device to weed out the unfit. As such, selected objects are simply those that remain after all the poorly adapted or less fortunate ones have been removed from a population. The better term again might be “non-random elimination,” since what is really meant here is the aggregate of adverse circumstances responsible for the deletion of some members of a group. Accordingly, selection can be broadly taken to mean preferential interaction of any system—living or nonliving—with its surrounding environment.

Selection works alongside the flow of energy into and out of all open systems, not just life forms, often providing a formative step in the production of order. In short, all ordered systems are selected by their ability to command energy resources—not too much energy as to be destructive and not so little as to be ineffective. Sometimes, when the energy flow exceeds a critical threshold thereby driving a system well beyond equilibrium, selection aids in generating newly ordered forms much as described toward the end of the previous CHEMICAL EPOCH.

Nature displays numerous examples of the process of selection operating among inanimate systems, but always in ways simpler than among living systems, and always, it seems, in the presence of energy. We noted earlier how prebiological molecules bathed in energy were “selected” in a soupy sea to become the building blocks of life. Certain bonding of amino acids were advantaged while others were excluded, implying that the chemical evolutionary steps toward life yielded new states more thermodynamically stable than their precursor molecules, all the while entropy increased in their watery surroundings. Selection—call it chemical selection—was clearly working to help tame chance, albeit not the more subtle yet powerful Darwinian, biological selection involving species modification, inheritance, and adaptation.

Crystal growth also demonstrates aspects of selection—call it physical selection—helping to order nonliving substances in ways much simpler than biological selection. To grow an ice crystal, water molecules must collide so that they stick and are not rejected. The initial molecular collisions are entirely random, but once they occur the migrating molecules are then guided by well-known electromagnetic forces into favorable positions on the surface. If the incoming molecule lands at a surface position physically conducive to the growth of ice crystal structure, it’s “selected” to stay and contribute to the crystal; otherwise it’s expelled. Its arrival is random, but the result is not.

An atmospheric storm is another example of a physical system undergoing selection while complexifying. Kilometer-sized vortices come and go at random in Earth’s atmosphere, mostly the result of swirling updrafts and turbulent eddies caused by winds hitting high-relief surface areas, such as mountains or islands. Patterns of cumulous clouds develop as rising currents form small competing cumuli that draw on the solar energy stored as latent heat in water vapor molecules. Those cumuli able to attract more air flow and thus more energy are “selected,” leaving the others to dissipate. By the end of a typical hot, sunny summer day, selection has fostered the growth of a few large-scale thunderstorms—and sometimes, with adequate moisture and energy, occasional eddies mature into full-scale hurricanes hundreds of kilometers across.

Stars, too, are subject to selection. Our Sun, for example, in ~5 billion years will become a red-giant star, increasing its gradient of temperature and chemical composition from core to surface. But, as noted in the earlier STELLAR EPOCH, the Sun will never fuse carbon into heavier elements, never become more complex than an old red giant, and never detonate as a supernova. In short, the Sun will not be selected to evolve much further, since its energy flow will likely never reach those critical values needed for the natural emergence of greater complexity. Although our Sun is not alive by any stretch of the imagination, it will have been non-randomly eliminated from further stellar evolution.

Many other examples of non-biological selection pervade the physical world, affecting both matter and radiation, including, for example, galaxies that are “selected” to form in the earlier Universe by means not too different from that described above for hurricanes, and certain modes of radiation able to handle energy coherently that are “selected” for laser propagation in the laboratory. Likewise, as will be noted in the next CULTURAL EPOCH, selection—call it cultural selection—was just as surely at work among our ancestors. The ability to start a fire, for example, would have been a major selective asset for those who possessed it, as do, in more recent times, dealer competition and customer demand when combining to create selective pressure for better automobiles in the social marketplace.

Selection does operate among inanimate, non-biological systems, even if not as robustly as for animate, biological systems. Physical and chemical selection obeys well-understood, if statistical, laws of physics, whereas biological (Darwinian) selection is, appropriately, richer and more multifaceted, drawing on genetic exchange and vast information storage. Even so, all these selective mechanisms, including accelerated cultural selection to come in the next epoch, help to build order and complexity in basically the same way: They all mix a random initiator with a deterministic response in the presence of energy, a theme integral to the onset of structure throughout all of Nature. Provided we think broadly enough, there is indeed unity among the natural sciences.


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