The Fundamentals of Evolution: Darwin and Modern Synthesis
- Darwin and the first evolutionary synthesis: Its grandeur, constraints, and difficulties
- Genetics and the "black day" of Darwinism
- Population genetics, Fisher's theorem, fitness landscapes, drift, and draft
- Positive and purifying (negative) selection: Classifying the forms of selection
- Modern Synthesis
- Recommended further reading
In this chapter and the next, I set out to provide a brief summary of the state of evolutionary biology before the advent of comparative genomics in 1995. Clearly, the task of distilling a century and a half of evolutionary thought and research into two brief, nearly nontechnical chapters is daunting, to put it mildly. Nevertheless, I believe that we can start by asking ourselves a straightforward question: What is the take-home message from all those decades of scholarship? We can garner a concise and sensible synopsis of the pregenomic evolutionary synthesis even while inevitably omitting most of the specifics.
I have attempted to combine history and logic in these first two chapters, but some degree of arbitrariness is unavoidable. In this chapter, I trace the conceptual development of evolutionary biology from Charles Darwin's On the Origin of Species to the consolidation of Modern Synthesis in the 1950s. Chapter 2 deals with the concepts and discoveries that affected the understanding of evolution between the completion of Modern Synthesis and the genomic revolution of the 1990s.
Darwin and the first evolutionary synthesis: Its grandeur, constraints, and difficulties
It is rather strange to contemplate the fact that we have just celebrated the 150th anniversary of the first publication of Darwin's On the Origin of Species (Darwin, 1859) and the 200th jubilee of Darwin himself. Considering the profound and indelible effect that Origin had on all of science, philosophy, and human thinking in general (far beyond the confines of biology), 150 years feels like a very short time.
What was so dramatic and important about the change in our worldview that Darwin prompted? Darwin did not discover evolution (as sometimes claimed overtly but much more often implied, especially in popular accounts and public debates). Many scholars before him, including luminaries of their day, believed that organisms changed over time in a nonrandom manner. Even apart from the great (somewhat legendary) Greek philosophers Empedokles, Parmenides, and Heraclites, and their Indian contemporaries who discussed eerily prescient ideas (even if, oddly for us, combined with mythology) on the processes of change in nature, Darwin had many predecessors in the eighteenth and early nineteenth centuries. In later editions of Origin, Darwin acknowledged their contributions with characteristic candor and generosity. Darwin's own grandfather, Erasmus, and the famous French botanist and zoologist Jean-Bapteste Lamarck (Lamarck, 1809) discussed evolution in lengthy tomes.1 Lamarck even had a coherent concept of the mechanisms that, in his view, perpetuated these changes. Moreover, Darwin's famed hero, teacher, and friend, the great geologist Sir Charles Lyell, wrote about the "struggle for existence" in which the more fecund will always win. And, of course, it is well known that Darwin's younger contemporary, Alfred Russel Wallace, simultaneously proposed essentially the same concept of evolution and its mechanisms.
However, the achievements of all these early evolutionists notwithstanding, it was Darwin who laid the foundation of modern biology and forever changed the scientific outlook of the world in Origin. What made Darwin's work unique and decisive? Looking back at his feat from our 150-year distance, three breakthrough generalizations seem to stand out:
- Darwin presented his vision of evolution within a completely naturalist and rationalist framework, without invoking any teleological forces or drives for perfection (or an outright creator) that theorists of his day commonly considered.
- Darwin proposed a specific, straightforward, and readily understandable mechanism of evolution that is interplay between heritable variation and natural selection, collectively described as the survival of the fittest.
- Darwin boldly extended the notion of evolution to the entire history of life, which he believed could be adequately represented as a grand tree (the famous single illustration of Origin), and even postulated that all existing life forms shared a single common ancestor.
Darwin's general, powerful concept stood in stark contrast to the evolutionary ideas of his predecessors, particularly Lamarck and Lyell, who contemplated mostly, if not exclusively, evolutionary change within species. Darwin's fourth great achievement was not purely scientific, but rather presentational. Largely because of a well-justified feeling of urgency caused by competition with Wallace, Darwin presented his concept in a brief and readable (even for prepared lay readers), although meticulous and carefully argued, volume. Thanks to these breakthroughs, Darwin succeeded in changing the face of science rather than just publishing another book. Immediately after Origin was published, most biologists and even the general educated public recognized it as a credible naturalist account of how the observed diversity of life could have come about, and this was a dynamic foundation to build upon.2
Considering Darwin's work in a higher plane of abstraction that is central to this book, it is worth emphasizing that Darwin seems to have been the first to establish the crucial interaction between chance and order (necessity) in evolution. Under Darwin's concept, variation is (nearly) completely random, whereas selection introduces order and creates complexity. In this respect, Darwin is diametrically opposed to Lamarck, whose worldview essentially banished chance. We return to this key conflict of worldviews throughout the book.
Indeed, with all due credit given to his geologist and early evolutionary biologist predecessors, Darwin was arguably the first scholar to prominently bring the possibility of evolutionary change (and, by implication, origin) of the entire universe into the realm of natural phenomena that are subject to rational study. Put another way, Darwin initiated the scientific study of the time arrow—that is, time-asymmetrical, irreversible processes. By doing so, he prepared the ground not only for all further development of biology, but also for the advent of modern physics. I believe that the great physicist Ludwig Boltzmann, the founder of statistical thermodynamics and the author of the modern concept of entropy, had good reason to call Darwin a "great physicist," paradoxical as this might seem, given that Darwin knew precious little about actual physics and mathematics. Contemporary philosopher Daniel Dennett may have had a point when he suggested that Darwin's idea of natural selection might be the single greatest idea ever proposed (Dennett, 1996).
Certainly, Darwin's concept of evolution at the time Origin was published and at least through the rest of the nineteenth century faced severe problems that greatly bothered Darwin and, at times, appeared insurmountable to many scientists. The first substantial difficulty was the low estimate of the age of Earth that prevailed in Darwin's day. Apart from any creation myth, the best estimates by nineteenth-century physicists (in particular, Lord Kelvin) were close to 100 million years, a time span that was deemed insufficient for the evolution of life via the Darwinian route of gradual accumulation of small changes. Clearly, that was a correct judgment—the 100 million years time range is far too short for the modern diversity of life to evolve, although no one in the nineteenth century had a quantitative estimate of the rate of Darwinian evolution. The problem was resolved 20 years after Darwin's death. In the beginning of the twentieth century, when radioactivity was discovered, scientists calculated that cooling of the Earth from its initial hot state would take billions of years, just about the time Darwin thought would be required for the evolution of life by natural selection.
The second, more formidable problem has to do with the mechanisms of heredity and the so-called Jenkin nightmare. Because the concept of discrete hereditary determinants did not exist in Darwin's time (outside the obscure articles of Mendel), it was unclear how an emerging beneficial variation could survive through generations and get fixed in evolving populations without being diluted and perishing. Darwin apparently did not think of this problem at the time he wrote Origin; an unusually incisive reader, an engineer named Jenkin, informed Darwin of this challenge to his theory. In retrospect, it is difficult to understand how Darwin (or Jenkin or Huxley) did not think of a Mendelian solution. Instead, Darwin came up with a more extravagant concept of heredity, the so-called pangenesis, which even he himself did not seem to take quite seriously. This problem was resolved by the (re)birth of genetics, although the initial implications for Darwinism3 were unexpected (see the next section).
The third problem that Darwin fully realized and brilliantly examined was the evolution of complex structures (organs, in Darwin's terms) that require assembly of multiple parts to perform their function. Such complex organs posed the classic puzzle of evolutionary biology that, in the twentieth century, has been evocatively branded 'irreducible complexity.'4 Indeed, it is not immediately clear how selection could enact the evolution of such organs under the assumption that individual parts or partial assemblies are useless. Darwin tackled this problem head-on in one of the most famous passages of Origin, the scenario of evolution of the eye. His proposed solution was logically impeccable, plausible, and ingenious: Darwin posited that complex organs do evolve through a series of intermediate stages, each of which performs a partial function related to the ultimate function of the evolving complex organ. Thus, the evolution of the eye, according to Darwin, starts with a simple light-sensing patch and proceeds through primitive eye-like structures of incrementally increasing utility to full-fledged, complex eyes of arthropods and vertebrates. It is worth noting that primitive light-sensing structures resembling those Darwin postulated on general grounds have been subsequently discovered, at least partially validating his scenario and showing that, in this case, the irreducibility of a complex organ is illusory. However, all the brilliance of Darwin's scheme notwithstanding, it should be taken for what it is: a partially supported speculative scenario for the evolution of one particular complex organ. Darwin's account shows one possible trajectory for the evolution of complexity but does not solve this major problem in general. Evolution of complexity at different levels is central to understanding biology, so we revisit it on multiple occasions throughout this book.
The fourth area of difficulty for Darwinism is, perhaps, the deepest. This major problem has to do with the title and purported main subject of Darwin's book, the origin of species and, more generally, large-scale evolutionary events that are now collectively denoted as macroevolution. In a rather striking departure from the title of the book, all indisputable examples of evolution that Darwin presented involve the emergence of new varieties within a species, not new species let alone higher taxa. This difficulty persisted long after Darwin's death and exists even now, although it was mitigated first by the progress of paleontology, then by developments in the theory of speciation supported by biogeographic data, and then, most convincingly, by comparative genomics (see Chapters 2 and 3). Much to his credit, and unlike detractors of evolution up to this day, Darwin firmly stood his ground in the face of all difficulties, thanks to his unflinching belief that, incomplete as his theory might be, there was no rational alternative. The only sign of Darwin's vulnerability was the inclusion of the implausible pangenesis model in later editions of Origin, as a stop-gap measure to stave off the Jenkin nightmare.