Complexity: A Guided Tour - Melanie Mitchell (2009)

Part I. Background and History

Chapter 5. Evolution

All great truths begin as blasphemies.

—George Bernard Shaw, Annajanska, The Bolshevik Empress

THE SECOND LAW OF THERMODYNAMICS states that the total entropy of an isolated system will always increase until it reaches its maximum value. Everyone knows this instinctively—it happens not only in our understanding of science, but also in our daily lives, and is ingrained in humans’ conceptions of history and in our art, literature, and religions. The Buddha tells us that “Subject to decay are all compounded things.” The Old Testament prophet Isaiah foretells that “The earth shall wax old like a garment.” Shakespeare asks,

O! how shall summer’s honey breath hold out,
Against the wrackful siege of battering days,
When rocks impregnable are not so stout,
Nor gates of steel so strong but Time decays?

It is a gloomy message, this inexorable march toward maximum entropy. But nature gives us a singular counterexample: Life. By anyone’s measure, living systems are complex—they exist somewhere in the middle ground between order and disorder. According to our intuitions, over the long history of life, living systems have become vastly more complex and intricate rather than more disordered and entropic.

We know that to decrease entropy, work must be done. Who or what is doing the work of creating and maintaining living systems and making them more complex? Some of the world’s religions propose that a deity is responsible, but in the mid-1800s, Charles Darwin proposed that instead, the history of life has resulted from the invisible hand of evolution via natural selection.

No idea in science has been more threatening to humans’ conceptions about themselves than Darwin’s theory of evolution; it arguably has been the most controversial idea in the history of science. But it is also one of the best ideas. The philosopher Daniel Dennett strongly affirms this:

If I were to give an award for the single best idea anyone has ever had, I’d give it to Darwin, ahead of Newton and Einstein and everyone else. In a single stroke, the idea of evolution by natural selection unifies the realm of life, meaning, and purpose with the realm of space and time, cause and effect, mechanism and physical law.

This chapter sketches the history and main ideas of Darwinian evolution and how it produces organization and adaptation. Concepts from evolutionary theory will come up again and again in the remainder of the book. In chapter 18, I describe how some of these concepts are being radically modified in light of the unexpected results coming out of the molecular revolution in biology and the results of complex systems ideas as applied to evolution.

Pre-Darwinian Notions of Evolution

The word evolution means “gradual change.” Biological evolution is the process of gradual (and sometimes rapid) change in biological forms over the history of life. Until the eighteenth century, the prevailing opinion was that biological forms do not change over time; rather, all organisms were created by a deity and have largely remained in their original form since their creation. Although some ancient Greek and Indian philosophers had proposed that humans arose via transmutation from other species, in the West the conception of divine creation began to be widely questioned only in the eighteenth century.

In the mid-1700s, 100 years before Darwin proposed his theory, a French zoologist named George Louis Leclerc de Buffon published a many-volume work entitled Historie Naturelle, in which he described the similarities between different species. Buffon suggested that the earth is much older than the Biblical 6,000 years and that all modern organisms evolved from a single ancestor, though he did not propose a mechanism for this evolution. Buffon’s work in biology and geology was a significant break from the prevailing creationist viewpoint. Not surprising, the Catholic Church in France burned copies of his books.

Charles Darwin’s grandfather, Erasmus Darwin, was another prominent eighteenth-century scientist who believed in the evolution of all species from a single ancient ancestor. He proposed mechanisms for evolution that were precursors to his grandson’s theory of natural selection. Erasmus Darwin expressed his ideas both in scientific writing and in poetry:

Organic life beneath the shoreless waves
Was born and nurs’d in ocean’s pearly caves;
First forms minute, unseen by spheric glass,
Move on the mud, or pierce the watery mass;
These, as successive generations bloom,
New powers acquire and larger limbs assume;
Whence countless groups of vegetation spring,
And breathing realms of fin and feet and wing.

If only modern-day scientists were so eloquent! However, like the Catholics in France, the Anglican Church didn’t much like these ideas.

The most famous pre-Darwinian evolutionist is Jean-Baptiste Lamarck. A French aristocrat and botanist, Lamarck published a book in 1809, Philosophie Zoologique, in which he proposed his theory of evolution: new types of organisms are spontaneously generated from inanimate matter, and these species evolve via the “inheritance of acquired characteristics.” The idea was that organisms adapted to their environment during their lifetimes, and that these acquired adaptations were then passed directly to the organisms’ offspring. One example in Lamarck’s book was the acquisition of long legs by wading birds, such as storks. Such birds, he believed, originally had to stretch their legs in order to keep their bodies out of the water. This continual stretching made their legs longer, and the acquired trait of longer legs was passed on to the birds’ offspring, who stretched their legs even longer, passing this trait on to their own offspring, and so on. The result is the very long legs we now see on wading birds.

Lamarck gave many other such examples. He also asserted that evolution entails a “tendency to progression,” in which organisms evolve to be increasingly “advanced,” with humans at the pinnacle of this process. Thus, changes in organisms are predominately changes for the better, or at least, for the more complex.

Lamarck’s ideas were rejected by almost all of his contemporaries—not only by proponents of divine creation but also by people who believed in evolution. The evolutionists were not at all convinced by Lamarck’s examples of evolution via inheritance of acquired characteristics, and indeed, his empirical data were weak and were generally limited to his own speculations on how certain traits of organisms came about.


Jean-Baptiste Lamarck, 1744–1829 (Illustration from LES CONTEMPORAINS N 554: Lamarck, naturaliste {1744–1829}, by Louis Théret, Bonne Press, 1903. Photograph copyright © by Scientia Digital []. Reprinted by perimission.)

However, it seems that Charles Darwin himself was, at least at first, favorably impressed by Lamarck: “Lamarck…had few clear facts, but so bold and many such profound judgment that he foreseeing consequence was endowed with what may be called the prophetic spirit in science. The highest endowment of lofty genius.” Darwin also believed that, in addition to natural selection, the inheritance of acquired characteristics was one of the mechanisms of evolution (though this belief did not survive as part of what we now call “Darwinism”).

Neither Lamarck nor Darwin had a good theory of how such inheritance could take place. However, as the science of genetics became better understood in the years after Darwin, the inheritance of acquired characteristics seemed almost certain to be impossible. By the beginning of the twentieth century, Lamarck’s theories were no longer taken seriously in evolutionary biology, though several prominent psychologists still believed in them as an explanation of some aspects of the mind, such as instinct. For example, Sigmund Freud expressed the view that “if [the] instinctual life of animals permits of any explanation at all, it can only be this: that they carry over into their new existence the experience of their kind; that is to say, that they have preserved in their minds memories of what their ancestors experienced.” I don’t think these beliefs remained in psychology much beyond the time of Freud.

Origins of Darwin’s Theory

Charles Darwin should be an inspiration to youthful underachievers everywhere. As a child, he was a mediocre student in an overachieving family. (His usually loving father, a successful country doctor, in a moment of frustration complained bitterly to the teenaged Charles: “You care for nothing but shooting, dogs, and rat-catching, and you will be a disgrace to yourself and your family!”) Underachieving as he might have been then, he went on to be the most famous, and most important, biologist of all time.

In 1831, while trying to decide on his future career (country doctor or country parson seemed to be the choices), Darwin was offered a dual job as both “naturalist” and “captain’s dining companion” on a survey ship, the H.M.S. Beagle. The ship’s captain was a “gentleman,” and a bit lonely, so he wanted to dine with another gentleman rather than with the riff-raff of the ship’s crew. Darwin was his man.

Darwin spent almost five years on the Beagle (1831–1836), much of the time in South America, where, in addition to his dining duties, he collected plants, animals, and fossils and did a lot of reading, thinking, and writing. Fortunately he wrote many letters and kept extensive notebooks full of his observations, ideas, opinions, reactions to books, et cetera; his detailed recording of his thoughts went on for the rest of his life. If Darwin were alive today, he clearly would have been an obsessive blogger.


Charles Darwin, 1809–1882. Photograph taken in 1854, a few years before he published Origin of Species. (Reproduced with permission from John van Wyhe, ed., The Complete Work of Charles Darwin Online [].)

During and after the Beagle voyage, Darwin got a lot of ideas from his reading of scientific books and articles from various disciplines. He was convinced by Charles Lyell’s Principles of Geology (1830) that geological features (mountains, canyons, rock formations) arise from gradual processes of erosion, wind, myriad floods, volcanic eruptions, and earthquakes, rather than from catastrophic events such as the biblical Noah’s flood. Such a view of gradualism—that small causes, taken over long periods, can have very large effects—was anathema to religious fundamentalists of the day, but Lyell’s evidence was compelling to Darwin, especially as, on his voyage, he could see for himself the results of different kinds of geological processes.

Thomas Malthus’s Essay on the Principle of Population (1798) drew Darwin’s attention to the fact that population growth leads to competition for food and other resources. Malthus’s essay was about human population growth, but Darwin would adapt these ideas to explain the evolution of all living organisms via a continual “struggle for existence.”

Darwin also read Adam Smith’s free-market manifestoThe Wealth of Nations (1776). This book exposed him to Smith’s notion of the invisible hand in economics, whereby a collection of individuals acting in their own self-interest produces maximum benefit for the entire community.

From his own observations in South America and elsewhere, Darwin was acutely struck by the tremendous variation among living beings and by the apparent adaptation of different species to their environments. One of his most famous examples is the finches of the Galápagos Islands, 600 miles off the coast of Ecuador. Darwin observed that different species of these small birds, although otherwise quite similar to one another, have wide variations in beak size and shape. Darwin was eventually able to show that different species of finches had common ancestors who had evidently migrated to individual islands in the Galápagos chain. He also showed that the type of beak was adaptedto the individual species’ food sources, which differed among the islands. Darwin hypothesized that the geographical isolation imposed by the different islands, as well as the local environmental conditions, led to the evolution of these many different species from a small number of ancestors.

We can imagine Darwin with these ideas swirling in his head during his voyage and afterward, back in England, trying to make sense of the data he had collected. Gradual change over long periods can produce very large effects. Population growth combined with limited resources creates a struggle for existence. Collections of individuals acting in self-interested ways produce global benefit. Life seems to allow almost infinite variation, and a species’ particular traits seem designed for the very environment in which the species lives. Species branch out from common ancestors.

Over the years, it all came together in his mind as a coherent theory. Individual organisms have more offspring than can survive, given limited food resources. The offspring are not exact copies of the parents but have some small amount of random variation in their traits. The traits that allow some offspring to survive and reproduce will be passed on to further offspring, thus spreading in the population. Very gradually, through reproduction with random variation and individual struggles for existence, new species will be formed with traits ideally adapted to their environments. Darwin called this process evolution by natural selection.

For years after the development of his theories, Darwin shared his ideas with only a few people (Charles Lyell and some others). In part, his reticence was due to a desire for additional data to bolster his conclusions, but also contributing was a deep concern that his theories would bring unhappiness to religious people, in particular to his own wife, who was deeply religious. Having once considered becoming a country parson himself, he expressed discomfort with his main conclusion: “I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable.”

However, Darwin’s notebooks of the time also revealed his understanding of the philosophical implications of his work for the status of humans. He wrote, “Plato…says in Phaedo that our ‘necessary ideas’ arise from the preexistence of the soul, are not derivable from experience—read monkeys for preexistence.”

Competition is not only the centerpiece of evolution, but is also a great motivator in science itself. Darwin’s hesitation to publish his work quickly melted away when he discovered that he was about to be scooped. In 1858, Darwin received a manuscript from another English naturalist, Alfred Russell Wallace, entitled On the Tendency of Varieties to Depart Indefinitely from the Original Type. Darwin was alarmed to find that Wallace had independently come up with the same basic ideas of evolution by natural selection. Darwin expressed his dismay in a letter to Lyell: “[A]ll my originality, whatever it may amount to, will be smashed.” However, he generously offered to help Wallace publish his essay, but requested that his own work also be published at the same time, in spite of his worries about this request being “base and paltry.

Lyell agreed that, in order to solve the priority problem, Darwin and Wallace should publish their work together. This joint work was read to the Linnean Society in the summer of 1858. By the end of 1859, Darwin had published his 400-plus-page book On the Origin of Species.

It turns out that the priority issue was not fully solved. Unbeknown to Darwin, twenty-eight years before the publication of the Origin, a little-known Scot named Patrick Matthew had published an obscure book with an equally obscure title, On Naval Timber and Arboriculture, in whose appendix he proposed something very much like Darwin’s evolution by natural selection. In 1860, Matthew read about Darwin’s ideas in the periodical Gardiner’s Chronicle and wrote a letter to the publication citing his priority. Darwin, ever anxious to do the right thing, responded with his own letter: “I freely acknowledge that Mr. Matthew has anticipated by many years the explanation which I have offered of the origin of species, under the name of natural selection…I can do no more than offer my apologies to Mr. Matthew for my entire ignorance of his publication.”

So who actually is responsible for the idea of evolution by natural selection? Evidently, this is another example of an idea that was “in the air” at the time, an idea that someone would inevitably come up with. Darwin’s colleague Thomas Huxley realized this and chided himself: “How extremely stupid not to have thought of that!

Why does Darwin get all the credit? There are several reasons, including the fact that he was at that time a more famous and respected scientist than the others, but the most important reason is that Darwin’s book, unlike the works of Wallace and Matthew, contained a more coherent set of ideas and a tremendous amount of evidence supporting those ideas. Darwin was the person who turned natural selection from an interesting and plausible speculation into an extremely well-supported theory.

To summarize the major ideas of Darwin’s theory:

·        Evolution has occurred; that is, all species descend from a common ancestor. The history of life is a branching tree of species.

·        Natural selection occurs when the number of births is greater than existing resources can support so that individuals undergo competition for resources.

·        Traits of organisms are inherited with variation. The variation is in some sense random—that is, there is no force or bias leading to variations that increase fitness (though, as I mentioned previously, Darwin himself accepted Lamarck’s view that there are such forces). Variations that turn out to be adaptive in the current environment are likely to be selected, meaning that organisms with those variations are more likely to survive and thus pass on the new traits to their offspring, causing the number of organisms with those traits to increase over subsequent generations.

·        Evolutionary change is constant and gradual via the accumulation of small, favorable variations.

According to this view, the result of evolution by natural selection is the appearance of “design” but with no designer. The appearance of design comes from chance, natural selection, and long periods of time. Entropy decreases (living systems become more organized, seemingly more designed) as a result of the work done by natural selection. The energy for this work comes from the ability of individual organisms to metabolize energy from their environments (e.g., sunlight and food).

Mendel and the Mechanism of Heredity

A major issue not explained by Darwin’s theory was exactly how traits are passed on from parent to offspring, and how variation in those traits—upon which natural selection acts—comes about. The discovery that DNA is the carrier of hereditary information did not take place until the 1940s. Many theories of heredity were proposed in the 1800s, but none was widely accepted until the “rediscovery” in 1900 of the work of Gregor Mendel.

Mendel was an Austrian monk and physics teacher with a strong interest in nature. Having studied the theories of Lamarck on the inheritance of acquired traits, Mendel performed a sequence of experiments, over a period of eight years, on generations of related pea plants to see whether he could verify Lamarck’s claims. His results not only disconfirmed Lamarck’s speculations but also revealed some surprising facts about the nature of heredity.

Mendel looked at several different traits of pea plants: smoothness and color of seeds; shape of pea pod; color of pods and flowers; locations of flowers on the plants; and height of stems. Each of these traits (or “characters”) could have one of two distinct forms (e.g., the pod color could be green or yellow; the stem height could be tall or dwarf).

Mendel’s long years of experiments revealed several things that are still considered roughly valid in modern-day genetics. First, he found that the plants’ offspring did not take on any traits that were acquired by the parents during their lifetimes. Thus, Lamarckian inheritance did not take place.


Gregor Mendel, 1822–1884 (From the National Library of Medicine) [

Second, he found that heredity took place via discrete “factors” that are contributed by the parents, one factor being contributed by each parent for each trait (e.g., each parent contributes either a factor for tall stems or dwarf stems). These factors roughly correspond to what we would call genes. Thus, the medium of inheritance, whatever it was, seemed to be discrete, not continuous as was proposed by Darwin and others. (Note that pea plants reproduce via either self-pollination or cross-pollination with other pea plants.)

For each trait he was studying, Mendel found that each plant has a pair of genes responsible for the trait. (For simplicity, I am using more modern terminology; the term “gene” was not used in Mendel’s time.) Each gene of the pair encodes a “value” for the trait—for example, tall vs. dwarf. This value is called an allele. For stem height there are three possibilities for the allele pairs encoded by these two genes: both alleles the same (tall/tall or dwarf/dwarf) or different (tall/dwarf, which is equivalent to dwarf/tall).

Moreover, Mendel found that, for each trait, one allele is dominant (e.g., tall is dominant for stem height) and the other recessive (e.g., dwarf is recessive for stem height). A tall/tall individual will always be tall. A tall/dwarfindividual will also be tall since tall is dominant; only one copy of the dominant allele is needed. Only a dwarf/dwarf individual—with two copies of the recessive allele—will be dwarf.

As an example, suppose you have two tall/dwarf individuals that cross-pollinate. Both the parents are tall, but there is a 25% chance that both will pass on their dwarf gene to the child, making it dwarf/dwarf.

Mendel used such reasoning and the laws of probability to predict, very successfully, how many plants in a given generation will display the dominant or recessive version of a given trait, respectively. Mendel’s experiments contradicted the widely believed notion of “blending inheritance”—that the offspring’s traits typically will be an average of the parents’ traits.

Mendel’s work was the first to explain and quantitatively predict the results of inheritance, even though Mendel did not know what substance his “factors” were made out of, or how they recombined as a result of mating. Unfortunately, his 1865 paper, “Experiments in Plant Hybridization,” was published in a rather obscure journal and was not appreciated as being of great importance until 1900, after which several scientists had obtained similar results in experiments.

The Modern Synthesis

You would think that the dissemination of Mendel’s results would be a big boost for Darwinism, since it provided Darwin’s theory with an experimentally tested mechanism of inheritance. But for decades, Mendel’s ideas were considered to be opposed to Darwin’s. Darwin’s theory asserted that evolution, and therefore variation, is continuous (i.e., organisms can differ from one another in arbitrarily minute ways) and Mendel’s theory proposed that variation is discrete (a pea plant is either tall or dwarf, but nothing in between). Many early adherents to Mendel’s theories believed in mutation theory—a proposal that variation in organisms is due to mutations in offspring, possibly very large, which themselves drive evolution, with natural selection only a secondary mechanism for preserving (or deleting) such mutations in a population. Darwin and his early followers were completely against this idea; the cornerstones of Darwin’s theory were that individual variations must be very small, natural selection on these tiny variations is what drives evolution, and evolution is gradual. “Natura non facit saltum” (Nature does not make leaps) was Darwin’s famous dismissal of mutation theory.

After many bitter arguments between the early Darwinists and Mendelians, this false opposition was cleared up by the 1920s when it was discovered that, unlike the traits of Mendel’s pea plants, most traits in organisms are determined by many genes, each with several different alleles. The huge number of possible combinations of these many different alleles can result in seemingly continuous variation in an organism. Discrete variation in the genes of an organism can result in continuous-seeming variation in the organ-ism’s phenotype—the physical traits (e.g., height, skin color, etc.) resulting from these genes. Darwinism and Mendelism were finally recognized as being complementary, not opposed.

One reason the early Darwinists and Mendelians disagreed so strongly is that, although both sides had experimental evidence supporting their position, neither side had the appropriate conceptual framework (i.e., multiple genes controlling traits) or mathematics to understand how their respective theories fit together. A whole new set of mathematical tools had to be developed to analyze the results of Mendelian inheritance with many interacting genes operating under natural selection in a mating population. The necessary tools were developed in the 1920s and 1930s, largely as a result of the work of the mathematical biologist Ronald Fisher.

Fisher, along with Francis Galton, was a founder of the field of modern statistics. He was originally spurred by real-world problems in agriculture and animal breeding. Fisher’s work, along with that of J.B.S. Haldane and Sewall Wright, showed that Darwin’s theories were indeed compatible with Mendel’s. Moreover, the combined work of Fisher, Haldane, and Wright provided a mathematical framework—population genetics—for understanding the dynamics of alleles in an evolving population undergoing Mendelian inheritance and natural selection. This unification of Darwinism and Mendelism, along with the framework of population genetics, was later called “the Modern Synthesis.”

Fisher, Wright, and Haldane are known as the three founders of the Modern Synthesis. There were many strong disagreements among the three, particularly a bitter fight between Fisher and Wright over the relative roles of natural selection and “random genetic drift.” In the latter process, certain alleles become dominant in a population merely as a chance event. For instance, suppose that in a population of pea plants, neither the dwarf nor tall alleles really affect the fitness of the plants as a whole. Also suppose that at some point the dwarf allele, just by chance, appears in a higher fraction of plants than the tall allele. Then, if each dwarf and tall plant has about the same number of offspring plants, the dwarf allele will likely be even more frequent in the next generation, simply because there were more parent plants with the dwarf allele. In general, if there is no selective advantage of either trait, one or the other trait will eventually be found in 100% of the individuals in the population. Drift is a stronger force in small rather than large populations, because in large populations, the small fluctuations that eventually result in drift tend to cancel one another out.

Wright believed that random genetic drift played a significant role in evolutionary change and the origin of new species, whereas in Fisher’s view, drift played only an insignificant role at best.

These are both reasonable and interesting speculations. One would think that Fisher and Wright would have had lots of heated but friendly discussions about it over beer (that is, when the Briton, Fisher, and the American, Wright, were on the same continent). However, what started as a very productive and stimulating interchange between them ended up with Fisher and Wright each publishing papers that offended the other, to the point that communication between them basically ended by 1934. The debate over the respective roles of natural selection versus drift was almost as bitter as the earlier one between the Mendelians and the Darwinists—ironic, since it was largely the work of Fisher and Wright that showed that these two sides actually need not have disagreed.

The Modern Synthesis was further developed in the 1930s and 1940s and was solidified into a set of principles of evolution that were almost universally accepted by biologists for the following fifty years:

·        Natural selection is the major mechanism of evolutionary change and adaptation.

·        Evolution is a gradual process, occurring via natural selection on very small random variations in individuals. Variation of this sort is highly abundant in populations and is not biased in any direction (e.g., it does not intrinsically lead to “improvement,” as believed by Lamarck). The source of individual variation is random genetic mutations and recombinations.

·        Macroscale phenomena, such as the origin of new species, can be explained by the microscopic process of gene variation and natural selection.

The original architects of the Modern Synthesis believed they had solved the major problems of explaining evolution, even though they still did not know the molecular basis of genes or by what mechanism variation arises. As the evolutionist Ian Tattersall relates, “Nobody could ever again look at the evolutionary process without very consciously standing on the edifice of the Synthesis. And this edifice was not only one of magnificent elegance and persuasiveness; it had also brought together practitioners of all the major branches of organismic biology, ending decades of infighting, mutual incomprehension, and wasted energies.”

Challenges to the Modern Synthesis

Serious challenges to the validity of the Modern Synthesis began brewing in the 1960s and 1970s. Perhaps the most prominent of the challengers were paleontologists Stephen Jay Gould and Niles Eldredge, who pointed out some discrepancies between what the Modern Synthesis predicted and what the actual fossil record showed. Gould went on to be simultaneously the best-known proponent and expositor of Darwinian evolution (through his many books and articles for nonscientists) and the most vociferous critic of the tenets of the Synthesis.

One major discrepancy is the prediction of the Modern Synthesis for gradual change in the morphology of organisms as compared with what Gould, Eldredge, and others claimed was the actual pattern in the fossil record: long periods of no change in the morphology of organisms (and no new species emerging) punctuated by (relatively) short periods of large change in morphology, resulting in the emergence of new species. This pattern became labeled punctuated equilibria. Others defended the Modern Synthesis, asserting that the fossil record was too incomplete for scientists to make such an inference. (Some detractors of punctuated equilibria nicknamed the theory “evolution by jerks.” Gould countered that the proponents of gradualism supported “evolution by creeps.”) Punctuated equilibria have also been widely observed in laboratory experiments that test evolution and in simplified computer simulations of evolution.


Stephen Jay Gould, 1941–2002. (Jon Chase/Harvard News Office, © 1997 President and Fellows of Harvard College, reproduced by permission.)


Niles Eldredge (Courtesy of Niles Eldredge.)

Thus Gould and his collaborators asserted that the “gradualism” pillar of the Modern Synthesis is wrong. They also believed that the other two pillars—the primacy of natural selection and small gene variations to explain the history of life—were not backed up by evidence.

Although Gould agreed that natural selection is an important mechanism of evolutionary change, he asserted that the roles of historical contingency and biological constraints are at least as important as that of natural selection.

Historical contingency refers to all the random accidents, large and small, that have contributed to the shaping of organisms. One example is the impact of large meteors wiping out habitats and causing extinction of groups of species, thus allowing other species to emerge. Other examples are the unknown quirks of fate that gave carnivorous mammals an advantage over the carnivorous birds that once rivaled them in numbers but which are now extinct.

Gould’s metaphor for the role of contingency is an imaginary “tape of life”—a kind of time-lapse movie covering all of evolution since the beginning of life on Earth. Gould asks, What would happen if the tape were restarted with slightly different initial conditions? Would we see anything like the array of organisms that evolved during the first playing of the tape? The answer of the Modern Synthesis would presumably be “yes”—natural selection would again shape organisms to be optimally adapted to the environment, so they would look much the same as what we have now. Gould’s answer is that the role played by historical contingency would make the replayed tape much different.

Biological constraints refer to the limitations on what natural selection can create. Clearly natural selection can’t defy the laws of physics—it can’t create a flying creature that does not obey the laws of gravity or a perpetual-motion animal that needs no food. Gould and many others have argued that there are biological constraints as well as physical constraints that limit what kind of organisms can evolve.

This view naturally leads to the conclusion that not all traits of organisms are explainable as “adaptations.” Clearly traits such as hunger and sex drive lead us to survival and reproduction. But some traits may have arisen by accident, or as side effects of adaptive traits or developmental constraints. Gould has been quite critical of evolutionists he calls “strict adaptationists”—those who insist that natural selection is the only possible explanation for complex organization in biology.

Furthermore, Gould and his colleagues attacked the third pillar of the Synthesis by proposing that some of the large-scale phenomena of evolution cannot be explained in terms of the microscopic process of gene variation and natural selection, but instead require natural selection to work on levels higher than genes and individuals—perhaps entire species.

Some evidence for Gould’s doubts about the Modern Synthesis came from work in molecular evolution. In the 1960s, Motoo Kimura proposed a theory of “neutral evolution,” based on observations of protein evolution, that challenged the central role of natural selection in evolutionary change. In the 1970s, chemists Manfred Eigen and Peter Schuster observed behavior analogous to punctuated equilibria in evolution of viruses made up of RNA, and developed an explanatory theory in which the unit of evolution was not an individual virus, but a collective of viruses—a quasi-species—that consisted of mutated copies of an original virus.

These, and other challenges to the Modern Synthesis were by no means accepted by all evolutionists, and, as in the early days of Darwinism, debates among rival views often became rancorous. In 1980, Gould wrote that “[T]hesynthetic theory…is effectively dead, despite its persistence as textbook orthodoxy.” Going even further, Niles Eldredge and Ian Tattersall contended that the view of evolution due to the Modern Synthesis “is one of the greatest myths of twentieth-century biology.” On the other side, the eminent evolutionary biologists Ernst Mayr and Richard Dawkins strongly defended the tenets of the Synthesis. Mayr wrote, “I am of the opinion that nothing is seriously wrong with the achievements of the evolutionary synthesis and that it does not need to be replaced.” Dawkins wrote, “The theory of evolution by cumulative natural selection is the only theory we know of that is in principle capable of explaining the existence of organized complexity.” Many people still hold to this view, but, as I describe in chapter 18, the idea that gradual change via natural selection is the major, if not the only force in shaping life is coming under increasing skepticism as new technologies have allowed the field of genetics to explode with unexpected discoveries, profoundly changing how people think about evolution.

It must be said that although Gould, Eldredge, and others have challenged the tenets of the Modern Synthesis, they, like virtually all biologists, still strongly embrace the basic ideas of Darwinism: that evolution has occurred over the last four billion years of life and continues to occur; that all modern species have originated from a single ancestor; that natural selection has played an important role in evolution; and that there is no “intelligent” force directing evolution or the design of organisms.