by Livio, Mario
But, you may wonder, if all life on Earth originated from a single, common ancestor, how did the astonishing wealth of diversity arise? After all, this was the first hallmark of life that we have identified as one that requires an explanation. Darwin did not flinch, and took this challenge head-on—it was not an accident that the title of his book had the word “species” in it. Darwin’s solution to the diversity problem involved another original idea: that of branching, or speciation. Life starts from a common ancestor, just as a tree has a single trunk, Darwin reasoned. In the same way that the trunk develops branches, which then split into twigs, the “tree of life” evolves by many branching and ramification events, creating separate species at each splitting node. Many of these species become extinct, just like the dead and broken branches of a tree. However, since at each splitting the number of offspring species from a given ancestor doubles, the number of different species can increase dramatically. When does speciation actually occur? According to modern thinking, mainly when a group of members of a particular species becomes geographically separated. For instance, one group may wander to the rainy side of a mountain range, while the rest of the species stays on the dry slope. Over time, these rather different environments produce different evolutionary paths, eventually leading to two populations that can no longer interbreed—or in other words, different species. In rarer occasions, speciation could create new species that arise from interbreeding between two species. Such appears to have been the case of the Italian sparrow, which was shown in 2011 to be genetically intermediate between Spanish sparrows and house sparrows. Italian and Spanish sparrows behave like distinct species, but Italian and house sparrows do form hybrid zones, where the ranges of the two interbreeding species meet.
Amazingly, in 1945, author Vladimir Nabokov, of Lolita and Pale Fire fame, came up with a sweeping hypothesis for the evolution of a group of butterflies known as the Polyommatus blues. Nabokov, who had a lifelong interest in butterflies, speculated that the butterflies came to the New World from Asia in a series of waves lasting millions of years. To their surprise, a team of scientists using gene-sequencing technology confirmed Nabokov’s conjecture in 2011. They found that the New World species shared a common ancestor that lived about ten million years ago, but that many New World species were more closely related to Old World butterflies than to their neighbors.
Darwin was sufficiently aware of the importance of the concept of speciation to his theory to include a schematic diagram of his tree of life. (Figure 3 shows the original drawing from his 1837 notebook.) In fact, this is the only figure in the entire book. Fascinatingly, Darwin included the caveat “I think” at the top of the page!
In many cases, evolutionary biologists have been able to identify most of the intermediate steps involved in speciation: from pairs of species that have probably recently split from a single species, to pairs that are just about ready to be pushed into separation. At the more detailed level, a combination of molecular and fossil data has yielded, for instance, a relatively well-resolved and well-dated phylogenetic tree for all the families of living and very recently extinct mammals.
I cannot refrain at this point from digressing to note that from my own personal perspective, there is another aspect of the notions of a common ancestor and of speciation that makes Darwin’s theory truly special. About a decade ago, while working on the book The Accelerating Universe, I was trying to identify the ingredients that make a physical theory of the universe “beautiful” in the eyes of scientists. In the end, I concluded that two of the absolutely essential constituents were simplicity and something that is known as the Copernican principle. (In the case of physics, the third ingredient was symmetry.) By “simplicity,” I mean reductionism, in the sense that most physicists understand it: the ability to explain as many phenomena as possible with as few laws as possible. This has always been, and still is, the goal of modern physics. Physicists are not satisfied, for instance, with having one extremely successful theory (quantum mechanics) for the subatomic world, and one equally successful theory (general relativity) for the universe at large. They would like to have one unified “theory of everything” that would explain it all.
Figure 3
The Copernican principle derives its name from that of the Polish astronomer Nicolaus Copernicus, who in the sixteenth century removed the Earth from its privileged position at the center of the universe. Theories that obey the Copernican principle do not require humans to occupy any special place for these theories to work. Copernicus taught us that the Earth is not at the center of the solar system, and all the subsequent findings in astronomy have only strengthened our realization that, from a physics perspective, humans play no special role in the cosmos. We live on a tiny planet that revolves around an ordinary star, in a galaxy that contains hundreds of billions of similar stars. Our physical insignificance continues even further. Not only are there about two hundred billion galaxies in our observable universe, but even ordinary matter—the stuff that we and all the stars and gas in all the galaxies are made of—constitutes only a little over 4 percent of the universe’s energy budget. In other words, we are really nothing special. (In chapter 11 I will discuss some ideas suggesting that we should not take Copernican modesty too far.)
Both reductionism and the Copernican principle are the true trademarks of Darwin’s theory of evolution. Darwin explained just about everything related to life on Earth (except its origin) with one unified vision. One can hardly be more reductionistic than that. At the same time, his theory was Copernican to the core. Humans evolved just like every other organism. In the tree analogy, all of the youngest buds are separated from the main trunk by a similar number of branching nodes, the only difference being that they point in different directions. Equivalently, in Darwin’s evolutionary scheme, all the present-day living organisms, including humans, are the products of similar paths of evolution. Humans definitely do not occupy any exceptional or unique place in this scheme—they are not the lords of creation—but an adaptation and development of their ancestors on Earth. This was the end of “absolute anthropocentrism.” All the terrestrial creatures are part of the same big family. In the words of the influential evolutionary biologist Stephen Jay Gould, “Darwinian evolution is a bush, not a ladder.” To a large extent, what has fueled the opposition to Darwin for more than 150 years is precisely this fear that the theory of evolution displaces humans from the pedestal on which they have put themselves. Darwin has initiated a rethinking of the nature of the world and of humans. Note that in a picture in which only the “fittest” survive (as we shall soon discuss in the context of natural selection), one could argue that insects have clearly outclassed humans, since there are so many more of them. Indeed, the British geneticist J. B. S. Haldane is cited (possibly apocryphally) as having replied to theologians who inquired whether there was anything that could be concluded about the Creator from the study of creation, with the observation that God “has an inordinate fondness for beetles.” Today we know that even in terms of genome size—the entirety of the hereditary information—humans fall far short of, believe it or not, a fresh water ameboid named Polychaos dubium. With 670 billion base pairs of DNA reported, the genome of this microorganism may be more than two hundred times larger than the human genome!
Darwin’s theory, therefore, amply satisfies the two applicable criteria (which admittedly are somewhat subjective) for a truly beautiful theory. No wonder, then, that The Origin has elicited perhaps the most dramatic shift of thought ever brought about by a scientific treatise.
Returning now to the theory itself, Darwin was not content with merely making statements about evolutionary changes and the production of diversity. He regarded it as his main task to explain how these processes have occurred. To achieve this goal, he had to come up with a convincing alternative to creationism for the apparent design in nature. His idea—natural selection—has been esteemed by Tufts University philosopher Daniel C. Dennett as no less than “the single b
est idea anyone has ever had.”
Natural Selection
One of the challenges that the concept of evolution posed concerned adaptation: the observation that species appeared to be perfectly harmonized with their environments, and the mutual adaptedness of the traits of organisms—body parts and physiological processes—to one another. This created a puzzle that confounded even the evolutionary minded among the naturalists that preceded Darwin: If species are so well adapted, how could they evolve and still remain well adapted? Darwin was fully aware of this conundrum, and he made sure that his principle of natural selection provided a satisfactory solution.
The basic idea underlying natural selection is quite simple (once it is pointed out!). As it sometimes happens with discoveries whose time has come, the naturalist Alfred Russel Wallace independently formulated very similar ideas at about the same time. Wallace was nevertheless very clear on who he thought deserved most of the credit. In a letter to Darwin on May 29, 1864, he wrote:
As to the theory of Natural Selection itself, I shall always maintain it to be actually yours and yours only. You had worked it out in details I had never thought of, years before I had a ray of light on the subject, and my paper would never have convinced anybody or been noticed as more than an ingenious speculation, whereas your book has revolutionized the study of Natural History.
Let us attempt to follow Darwin’s train of thought: First, he noted, species tend to produce more offspring than can possibly survive. Second, the individuals within a given species are never all precisely identical. If some of them possess any kind of advantage in terms of their ability to cope with the adversity of the environment—and assuming that this advantage is heritable, and passed on to their descendants—then over time, the population will gradually shift toward organisms that are better adapted. Here is how Darwin himself put it, in chapter 3 of The Origin:
Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection.
Using the modern gene terminology (of which Darwin knew absolutely nothing), we would say that natural selection is simply the statement that those individuals whose genes are “better” (in terms of survival and reproduction) would be able to produce more offspring, and that those offspring will also have better genes (relatively speaking). In other words, over the course of many generations, beneficial mutations will prevail, with harmful ones eliminated, resulting in evolution toward better adaptation. For instance, it is easy to see how being faster could benefit both predator and prey. So in East Africa’s open plains of the Serengeti, natural selection has produced some of the fastest animals on Earth.
There are several elements that combine effectively to create the complete picture of natural selection. First, natural selection takes place in populations—communities of interbreeding individuals at given geographical locations—not in individuals. Second, populations typically have such high reproduction potential that if unchecked they would increase exponentially. For example, the female of the ocean sunfish, Mola mola, produces as many as three hundred million eggs at a time. If even just 1 percent of those eggs are fertilized and survive to adulthood, we soon would have oceans filled with Mola molas (and the average weight of an adult ocean sunfish exceeds two thousand pounds). Fortunately, due to competition for resources within the species, struggles with predators, and the environment’s other adversities, from a set of parents belonging to any species, an average of only two offspring survive and reproduce.
This description makes it clear that the word “selection” in Darwin’s formulation of natural selection really refers more to a process of elimination of the “weaker” (in terms of survival and reproduction) members of a population, rather than to a selection by an anthropomorphic nature. Metaphorically, you could think of the process of selection as one of sifting through a giant sieve. The larger particles (corresponding to those that survive) remain in the sieve, while the ones that pass through are eliminated. The environment is the agent that does the shaking of the sieve. Consequently, in a letter that Wallace wrote to Darwin on July 2, 1866, he actually suggested that Darwin should consider changing the name of the principle:
I wish, therefore, to suggest to you the possibility of entirely avoiding this source of misconception . . . and I think it may be done without difficulty and very effectually by adopting Spencer’s term (which he generally uses in preference to Natural Selection), viz. “Survival of the Fittest.” This term is the plain expression of the facts; “Natural Selection” is a metaphorical expression of it, and to a certain degree indirect and incorrect, since, even personifying Nature, she does not so much select special variations as exterminate the most unfavourable ones.
Darwin adopted this expression, coined in 1864 by the polymath Herbert Spencer, as a synonym for natural selection in his fifth edition of The Origin. However, present-day biologists rarely use this term, since it may give the wrong impression that it means that only the strong or healthy survive. In fact, “survival of the fittest” meant to Darwin precisely the same as “natural selection.” That is, those organisms with selectively favored and heritable characteristics are the ones who most successfully pass those to their offspring. In this sense, even though Darwin admitted to having been inspired by ideas of philosophical radicals such as the political economist Thomas Malthus—some sort of biological economics in a world of free competition—important differences exist.
A third and extremely important point to note about natural selection is that it really consists of two sequential steps, the first of which involves primarily randomness or chance, while the second one is definitely nonrandom. In the first step, a heritable variation is produced. In modern biological language, we understand this to be a genetic variation introduced by random mutations, gene reshuffling, and all the processes associated with sexual reproduction and the creation of a fertilized egg. In the second step, selection, those individuals in the population that are best suited to compete, be it with members within their own species, with members of other species, or in terms of their ability to cope with the environment, are more likely to survive and reproduce. Contrary to some misconceptions about natural selection, chance plays a much smaller role in the second step. Nevertheless, the process of selection is still not entirely deterministic—good genes are not going to help a species of dinosaurs wiped out by the impact of a giant meteorite, for instance. In a nutshell, therefore, evolution is really a change over time in the frequency of genes.
There are two main features that distinguish natural selection from the concept of “design.” First, natural selection does not have any long-term “strategic plan” or ultimate goal. (It is not teleological.) Rather than striving toward some ideal of perfection, it simply tinkers by elimination of the less adapted with generation after generation, often changing direction or even resulting in the extinction of entire lineages. This is not what one would expect from a master designer. Second, because natural selection is constrained to work with what already exists, there is only so much that it can actually achieve. Natural selection starts by modifying species that have already evolved to a certain state, rather than by redesigning them from scratch. This is similar to asking a tailor to do some alterations to an old dress instead of asking the Versace fashion house to design a new one. Consequently, natural selection leaves quite a bit to be desired in terms of design. (Wouldn’t a visual field covering all 360 degrees or having four hands be nice? And were having nerves in the teeth or a prostate gland that
totally surrounds the urethra really such great ideas?) So even if certain characteristics confer a fitness advantage, as long as there is no heritable variation that achieves this result, natural selection could never produce such characteristics. Imperfections are, in fact, natural selection’s unmistakable fingerprint.
You have probably noticed that Darwin’s theory of evolution is, by its very nature, not easily provable by direct evidence, since it typically operates on such long timescales that watching grass grow feels like a fast-paced action movie by comparison. Darwin himself wrote to the geologist Frederick Wollaston Hutton on April 20, 1861, “I am actually weary of telling people that I do not pretend to adduce evidence of one species turning into another, but I believe that this view is in the main correct, because so many phenomena can thus be grouped and explained.” Nevertheless, biologists, geologists, and paleontologists have amassed a huge body of circumstantial evidence for evolution, most of which is beyond the scope of this book, since it is not related directly to Darwin’s blunder. Let me only note the following fact: The fossil record reveals an unmistakable evolution from simple to complex life. Specifically, over the billions of years of geological time, the more ancient the geological layer in which a fossil is uncovered, the simpler the species.
It is important to mention briefly a few of the pieces of evidence supporting the idea of natural selection, since it was the notion that life could evolve and diversify without there being a goal to evolve toward that was the most deeply unsettling aspect of the theory to Darwin’s contemporaries. I have already mentioned one clue demonstrating the reality of natural selection: the resistance to drugs developed by various pathogens. The bacterium known as Staphylococcus aureus, for instance, is the most common cause for the types of infections known as staph infections, which affect no fewer than a half million patients in American hospitals each year. In the early 1940s, all the known strains of staph were susceptible to penicillin. Over the years, however, due to mutations producing resistance and through natural selection, most staph strains have become resistant to penicillin. In this case, the entire process of evolution has been compressed in time dramatically (due partly to the selective pressure exerted by humans), since the generations of bacteria are so short lived and the population is so enormous. Since 1961, a particular staph strain known as MRSA (an acronym for methicillin-resistant Staphylococcus aureus) has developed resistance not just to penicillin but also to methicillin, amoxicillin, oxacillin, and a whole host of other antibiotics. There is hardly a better manifestation of natural selection in action.