Authors: Mario Livio
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.
Another fascinating (although controversial) example of natural selection is
the evolution of the peppered moth. Prior to the industrial revolution, the light colors of this moth (known among biologists as
Biston betularia betularia morpha typica
) provided ample camouflage against the background of its habitat: lichens and trees. The industrial revolution in England brought with it immense levels of pollution that destroyed many lichens and blackened many trees with soot. Consequently, the white-bodied moths were exposed suddenly to massive predation, which led to their near extinction. At the same time, the melanic, dark-colored variety of the moth (
carbonaria
) started to flourish around 1848, because of its much improved camouflage characteristics. As if to demonstrate the importance of “green” practices, the white-bodied moths started reappearing again once better environmental standards had been adopted. While some studies of the peppered moth and the phenomenon described above (“industrial melanism”) have been criticized by a number of creationists, even some of the critics agree that this is a clear case of natural selection, and they argue only that this does not provide proof of evolution, since the net result is merely of one type of moth morphing into another rather than into an entirely new species altogether.
Another common, more philosophical, objection to natural selection is that Darwin’s definition of it is circular, or
tautological.
Put in simple terms, the adverse judgment goes something like this: Natural selection means “survival of the fittest.” But how do you define the “fittest”? They are identified as those that survive best; hence, the definition is a tautology. This argument stems from a misunderstanding, and it is absolutely false. Darwin did not use “fitness” to refer to those who survive but to those who, when compared with other members of the species, could be
expected
to survive
because they were better adapted to the environment.
The
interaction between a variable feature of an organism and the environment of that organism is crucial here. Since the organisms compete for limited resources, some survive and some don’t. Furthermore, for natural selection to operate, the adaptive characteristics need to be
heritable,
that is, capable of being genetically passed on.
Surprisingly, even
the famous philosopher of science Karl Popper raised a suspicion of tautology against evolution by natural selection (albeit a more subtle one). Popper basically questioned natural selection’s explanatory power based on the following argument: If certain species exist, this means that they were adapted to their environment (since those that were not adapted became extinct). In other words, Popper asserted, adaptation is simply
defined
as the quality that guarantees existence, and nothing is ruled out. However, since Popper published this argument, a number of philosophers have shown it to be erroneous. In reality, Darwin’s theory of evolution rules out more scenarios than it leaves in. According to Darwin, for instance, no new species can emerge without having an ancestral species. Similarly, in Darwin’s theory, any variations that are not achievable in gradual steps are ruled out. In modern terminology, “achievable” would refer to processes governed by the laws of molecular biology and genetics. A key point here is the statistical nature of adaptation—no predictions can be made about individuals, just about probabilities. Two identical twins are not guaranteed to produce the same number of offspring, or even to both survive. Popper, by the way,
did recognize his error in later years, declaring, “I have changed my mind about the testability and the logical status of natural selection; and I am glad to have an opportunity to make a recantation.”
Finally, for completeness, I should mention that although natural selection is the main driver of evolution, other processes can bring about evolutionary changes. One example (which Darwin could not have known about) is provided by what has been
termed by modern evolutionary biologists
genetic drift:
a change in the relative frequency in which a variant of a gene (an
allele
) appears in a population due to chance or sampling errors. This effect can be significant in
small populations, as the following examples demonstrate. When you flip a coin, the expectation is that heads will turn up about 50 percent of the time. This means that if you flip a coin a million times, the number of times you’ll get heads will be close to a half million. If you toss a coin just four times, however, there is a nonnegligible probability (of about 6.2 percent) that it will land heads each time, thus deviating substantially from the expectation. Now imagine a very large island population of organisms in which just one gene appears in two variants (alleles): X or Z. The alleles have an equal frequency in the population; that is, the frequency of X and Z is
1
/
2
for each. Before these organisms have a chance to reproduce, however, a huge tsunami wave washes the island, killing all but four of the organisms. The surviving four organisms could have any of the following sixteen combinations of alleles: XXXX, XXXZ, XXZX, XZXX, ZXXX, XXZZ, ZZXX, XZZX, ZXXZ, XZXZ, ZXZX, XZZZ, ZZZX, ZXZZ, ZZXZ, ZZZZ. You will notice that in ten out of these sixteen combinations, the number of X alleles is
not
equal to the number of Z alleles. In other words, in the surviving population, there is a higher chance for a genetic drift—a change in the relative allele frequency—than for keeping the initial state of equal frequencies.