Natural selection - last part of the chapter on Darwin in Mario Livio's book 'Genius Errors'

What is strange, the theory of evolution - the gradual change, hidden almost from the eye, caused a sudden revolution, one of the most dramatic in the history of science.

• to the first part of the episode
• For the second part of the chapter - revolution

One of the challenges presented by the concept of evolution concerns adaptation: the observation that species appear to be in perfect harmony with their environments, and the mutual adjustment of the properties of organisms - body parts or physiological procedures - to each other. This is a riddle that is beyond the ability of even the evolutionary-minded naturalists who preceded Darwin: if any species is so perfectly adapted to its environment, how can it change in an evolutionary process and still remain well-adapted? Darwin recognized the importance of this question, and made sure that his principle of natural selection would give it a satisfactory answer.

The basic idea behind natural selection is quite simple (after pointing it out...). As sometimes happens with discoveries whose time has come, the naturalist Alfred Russel Wallace - quite separately - formulated very similar ideas, around the same time. Nevertheless, Wallace knew full well who deserved the glory, and admitted it wholeheartedly. Thus he wrote in a letter he sent to Darwin on May 29, 1864:

"As for the theory of natural selection itself, I will always insist that it is yours and yours alone. You elaborated it in a detail that never occurred to me, years before I saw the first ray of light on this subject, and my article would not have convinced anyone in any way, and the humanities would not have given it any thought, except as a guess of the imagination, while your book revolutionized the study of nature. "

We will try to trace the thread of Darwin's thought. First, he noted, species tend to produce offspring in such large numbers that not all of them can survive. Second, the individuals within a given species are never exactly the same. If some of them are gifted with some advantage in terms of their ability to deal with the harms of their environment - and assuming that this advantage is hereditary and passed on to their offspring - then, over time, populations will gradually change towards the better adapted organisms. This is how Darwin himself put it, in Chapter 3 of the Origin of Species:

"[This war of existence] is what brings about the fact that even the easiest changes that come about because of anything, if they can add any advantage to their owners in their infinitely complicated relations with other organic creatures and [with] the conditions of their material life, help to preserve the sons of One species, and they are usually passed on to the offspring of their owners. The chances of the offspring's existence will also increase, since out of the multitude of individuals, which each species puts into periods, only a few will be able to survive. I called this principle of preserving every easy and useful change the name natural selection [natural selection]."

If we use modern genetic terminology (which Darwin knew nothing about), we can say that natural selection is simply the determination that those individuals whose genes are "better" (in terms of survival and reproduction) will be able to give birth to more offspring, and these offspring will also have good genes more (relatively). In other words, over the course of many generations, beneficial mutations will thrive and harmful mutations will be eliminated, and the result will be evolution toward better adaptation. For example, it is easy to see how greater speed would benefit both predators and prey. And so, on the open plains of the Serengeti in East Africa, natural selection produced some of the fastest animals on earth.

Several components fit together correctly to create the complete picture of natural selection. First, natural selection occurs in populations - in communities of individuals capable of interbreeding in given geographic locations - and not in individuals. Second, populations usually have such a high reproduction potential that their growth will be exponential if not controlled. For example, the female crystallized sunfish, Mola mola, produces up to three hundred million eggs at once. If even just one percent of these eggs are fertilized, and the fry survive and reach adulthood, the oceans will very quickly be filled with lumpy sunfish (and it is worth remembering that the average weight of an adult fish of this species exceeds a thousand kilograms). Happily, due to the competition for resources within the species, the struggles with predators and other obstacles posed by the environment, a pair of parents of each species produces on average only two offspring that survive and reproduce.
This description makes it clear that the word "selection" in Darwin's formulation of natural selection actually refers to the process of eliminating the "weak" (in terms of survival and reproduction) among the members of the population, and not to selection in the anthropomorphic sense of the word. Using the metaphor, the selection process can be described as filtering in a huge sieve. The largest grains (representing those that survive) remain in the chaff, while those that pass through the holes are eliminated. The environment is the factor that shakes the balance. For this reason, Wallace suggested to Darwin, in a letter he sent to him on July 2, 1866, to consider changing the name of the principle:

"Therefore I wish to offer you the possibility of completely distancing yourself from this source of misapprehension... and I think this can be done without difficulty, and most advantageously, by adopting the term coined by Spencer (and which he generally uses as a substitute for natural selection), namely, "survival the most suitable". This term is an open expression of the facts; "Natural selection" is a metaphorical expression, and to a certain extent an indirect and imprecise expression, because even if we punish nature, it does not actually choose special variations, but rather eliminates the least positive variations of all."

Darwin did adopt this phrase, which was coined in 1864 by the anthropologist Herbert Spencer, and used it as a synonym for natural selection, beginning with the fifth edition of the Origin of Species. But today's biologists almost never use this term, as it may give the false impression that only the strong or healthy survive. In practice, "survival of the fittest" told Darwin exactly what "natural selection" told him. That is, those organisms that have selectively and heritably preferred traits are the ones that are most successful in passing them on to their offspring. In this sense, although Darwin admitted that he drew inspiration from the ideas of radical philosophers such as the political economist Thomas Malthus - a kind of biological economy in a world of free competition - there are nevertheless important differences.

A third - and extremely important - point that must be emphasized in the discussion of natural selection is that, in practice, it consists of two stages one after the other: the first includes first and foremost randomness or chance, while the second is strictly non-random. In the first stage, a variation is created that can be inherited. In the language of modern biology, we understand this as genetic variation that originates from random mutations, gene mixing and all other processes related to sexual reproduction and the creation of a fertilized egg. In the second stage, selection, those individuals in the population whose ability to compete is the highest - whether in relation to members of their own species or in relation to members of other species, or in their ability to cope with the environment - have the best chances of surviving and reproducing. Contrary to these misconceptions about natural selection, chance plays a much smaller role in the second stage. Despite this, the selection process is still not completely deterministic - good genes would not have helped a species of dinosaur that was wiped out due to the impact of a giant meteorite, for example. On the one hand, then, evolution is nothing but a change in the frequencies of genes over time.

There are two main features that distinguish natural selection from the concept of "design" or "planning". First, natural selection has no long-term "strategic plan": it has no final destination. (In the language of the philosophers, it is not teleological.) In no way does it strive towards some ideal of perfection, but only works its action by eliminating the least adapted in each generation, and many times it changes direction or even brings about the extinction of entire dynasties. Second, since natural selection can only act on what already exists, there are therefore limits to what it can actually achieve. Natural selection begins by changing a species that has already reached a certain state in its evolution, and not by reshaping it out of nowhere. This is similar to the person who asks the tailor to make any repairs on an old dress, rather than the person who enters the Versace fashion house and asks to have a brand new dress designed for her. As such, natural selection is far from conceivable, as far as design is concerned. (Wouldn't it be nice if our field of vision encompassed all 360 degrees, or if we had four hands? And are having nerves in the teeth, or a prostate that surrounds the entire urethra, really such big ideas?) Therefore, even if certain characteristics would confer A fitness advantage, as long as there is no hereditary variation producing these characteristics, natural selection could in no way create them out of nothing. In fact, imperfection is the distinct fingerprint of natural selection.

Perhaps you have already stated that Darwin's theory of evolution, by its very nature, is not a theory that is easy to prove with direct evidence, because it usually operates on such long scales of time that observing the growing grass is compared to watching a movie in fast motion. Darwin himself wrote to the geologist Frederick Wolston Hatton on April 20, 1861, "I am quite tired of telling mankind that I do not pretend that I have evidence of the transformation of one species into another, but I am convinced that this view is essentially correct, because it is possible to group together and explain so many phenomena." Nevertheless, biologists, geologists, and paleontologists have accumulated an enormous wealth of circumstantial evidence for evolution, most of which is outside the scope of this book because it does not directly bear on Darwin's error. I will only state the following fact: the fossil record reveals a clear-as-sun evolution from simple to complex life. In more detail, over billions of years of geological time, the older the geological layer in which a fossil is found, the simpler the species of the fossil.

It is important to briefly mention some of the evidence supporting the idea of ​​natural selection, because the claim that life is able to develop and branch out without any goal to develop towards, was the most sensational aspect of Darwin's theory of evolution in the eyes of his contemporaries. I have already mentioned one clue that illustrates the reality of natural selection: the resistance to drugs that develop various pathogens. The bacterium Staphylococcus aureus, for example, is the most common cause of several types of infections, admitting to American hospitals no less than half a million patients a year. In the early 1961s, all known staphylococcus strains were susceptible to penicillin. But over the years, due to mutations that produced resistance through the power of natural selection, most strains of staphylococcus became resistant to penicillin. In this case, the entire process of evolution is dramatically compressed in time (partly due to the selection pressure exerted by man), because the bacterial generations are so short and their populations so huge. Since XNUMX, the strain of staphylococcus known as MRSA has developed resistance not only to penicillin but also to methicillin, amoxicillin and a host of other antibiotic drugs. It is hard to find a better illustration of natural selection in action.

Another fascinating (albeit controversial) example of natural selection is the evolution of the spotted skimmer Biston betularia. Before the industrial revolution, the bright colors of this falcon (of the species B. b. betularia morpha typica) provided it with adequate camouflage against the background of its habitat: lichens and trees. The industrial revolution in England brought with it terrible levels of pollution, which destroyed many lichens and blackened many trees with soot. And so, all of a sudden, the white-bodied falcon became conspicuous to the eyes of its many predators, and came almost to the brink of extinction. At the same time, the strain of the same flail whose body contains melanin, B. b. betularia morpha carbonaria, began to flourish around 1848, because its color now provided it with much better camouflage. Later, as if trying to prove the importance of a "green" policy, the flappers returned to the bodies and appeared, after better standards for the protection of the environment were adopted in England. Despite the criticism that some creationists have leveled at some of the studies of the spotted skimmer and the phenomenon described above ("industrial melanism"), even some of the critics agree that this is a clear case of natural selection, and claim only that this is not proof of evolution, since the net result is simply the transformation of one form of the skimmer to another, and not the emergence of a completely new species.

Another common objection to natural selection, on a more philosophical level, is that Darwin's definition is tautological - circular. In simple terms, the argument of the category is roughly this: natural selection means "survival of the fittest". But how do you define "most suitable"? by identifying those who survived better; Thus, the definition is tautological. This argument stems from a misunderstanding, and is completely false. Darwin did not use the word "fit" to indicate those who survive, but rather those who can be expected to survive, compared to the rest of their species, because they are better adapted to their environment. The interplay between a variable trait of an organism and that organism's environment is the essential point here. As organisms compete for limited resources, some survive and some do not. Moreover, in order for natural selection to work, the adaptive traits must be inherited, that is, passed from generation to generation in a genetic way.

Surprisingly, even Karl Popper, the celebrated philosopher of science, raised a suspicion of tautology against evolution by natural selection (albeit in a more sophisticated way). Essentially, Popper doubted the explanatory power of natural selection by relying on the following argument: if a certain species exists, it means that it has adapted to its environment (since if it had not adapted, it would have had to become extinct). In other words, Popper stated that adaptation is simply defined as a feature that guarantees existence, and nothing is ruled out. But since Popper published his argument some philosophers have shown him to be wrong. In practice, Darwin's theory rules out more scenarios than it accepts. For example, Darwin said that no species could appear without having a parent species. Similarly, in Darwin's theory, any variation that is not achieved in gradual stages is dismissed outright. In today's accepted terminology, "achievable" is a process subject to the laws of molecular biology and genetics. A key point here is the statistical nature of adaptation - in Darwin's theory there are no predictions about individual individuals, but only about probabilities. It is impossible to determine with certainty that two identical twins will give birth to the same number of offspring, or even that both will survive. By the way, Popper admitted his mistake years later, and declared: "I have changed my position on the ability to test and the logical status of natural selection; And I'm happy that I had the opportunity to repeat myself in public."

Finally, to complete the picture, I should mention that even if natural selection is the main engine of evolution, there are other processes capable of bringing about evolutionary change. One of the examples (which Darwin could not have known about) is what contemporary evolutionary biologists call genetic drift: a change in the relative frequency of the appearance of a given form of a gene (in the professional language, an allele) in a population, due to chance or sampling errors. This result can be significant in small populations, as evidenced by the following example. When tossing a coin, the expectation is that a "tree" will appear in about 50 percent of the tosses. That is, if I toss a coin a million times, the number of tosses in which a "tree" will be obtained will be close to half a million. But if I toss the coin only four times, there is a non-negligible probability (of approximately 6.2 percent) that it will land on a "tree" all four times, and this would be a considerable deviation from the expectation. And here, we imagine a population of organisms on a very large island, where only one gene appears in one of two forms (alleles): X or Z. The frequency of the alleles in the population is equal; That is, the prevalence of both X and Z is about ½. But before the organisms could reproduce, a large tsunami swept the island and killed all but four of the organisms. The remaining four individuals can have one of sixteen combinations of alleles: XXXX, XXXZ, XXZX, XXXX, ZXXX, XXZZ, ZZXX, XZZX, ZXXZ, XXXZ, ZXZX, XZZZ, ZZZX, ZXZZ, ZZXZ, ZZZZ. It is easy to see that in ten of these sixteen combinations, the number of alleles X is not equal to the number of alleles Z. In other words, in the remaining population, there is a greater chance of genetic drift - a change in the relative frequency of the alleles - than of maintaining the original state of equal frequency.
Genetic drift can bring about a relatively rapid evolutionary change in the gene pool of a small population, without any dependence on natural selection. An example of genetic drift that is often talked about concerns the Homish community in eastern Pennsylvania. Among the Amish, polydactyly (a larger than normal number of fingers or toes) is many times more common than in the general population of the United States. This is one of the manifestations of the rare Ellis-Van Krefeld syndrome. Diseases of recessive genes, such as Ellis-Von Krefeld syndrome, need two copies of the gene that causes the disease in order for it to appear. That is, both parents have to be carriers of the recessive gene. The reason for the higher than usual frequency of these genes in the Amish community is marriage within the group, whose original population numbered only about two hundred immigrants from Germany. Thanks to the small size of this community, the researchers were able to trace Ellis-Ven Krefeld syndrome to one and only one couple, Samuel King and his wife, who arrived in the United States in 1744.

There are three points that need to be emphasized when it comes to genetic drift. First, the evolutionary changes resulting from genetic drift appear as the sole result of chance and sampling errors - they have nothing and nothing to do with selection pressures. Second, genetic drift cannot bring about adaptation, as it is the exclusive domain of natural selection. In fact, since genetic drift is completely random, it may result in the establishment of certain traits whose utility would otherwise be highly puzzling. And finally, although genetic drift clearly appears, to one degree or another, in every population (because all populations have a finite size), its results are most visible in small and isolated populations.
These were, on one foot, some of the central points of Darwin's theory of evolution by natural selection. Darwin revolutionized biological thinking in two important ways. Not only did he show that beliefs held for many generations could turn out to be false, he also showed that it is possible to arrive at scientific truth through the diligent collection of facts, combined with bold hypotheses about the theory that binds these facts together. As you probably already understood, his theory best explains why life on earth is so diverse, and why living organisms have the properties they do. Lydia Becker, the nineteenth-century English suffragist and botanist, wrote a beautiful description of Darwin's achievement:
How seemingly unimportant are the movements of insects crawling into flowers and out again in their search for the nectar they feed on! If we saw a man spending all his hours observing them and following their movements with curious eyes, because then it would be easy to forgive us if we said that he amuses his heart in a self-indulgent sit-down and watches strange things, indeed, but of no importance. But what a catch we would have had, if we had thought so! For these little winged messengers bring to mind the tidings of the philosopher of nature about mysteries not yet revealed to him; And just as Newton saw the law of gravity in the fall of an apple, so Darwin found, in the connection between flies and flowers, one of the most important facts to support his theory he proposed regarding the adaptation of certain forms in living creatures.

And really, Darwin was to the nineteenth century what Newton was to the seventeenth century, and Einstein was to the twentieth century. What is strange, the theory of evolution - the gradual change, almost hidden from view, caused a sudden revolution, one of the most dramatic in the history of science. As Ernst Meyer, the biologist and historian of science, said, it "caused a revolution in human thought, greater than any other scientific progress since the return of science and the birth of the Renaissance". The question is, then, what was Darwin's error?