THE IDEA that several million different species of living things needed nothing more than the ordinary processes of life to explain their origin had been growing in the minds of many scientists of the early nineteenth century, but few were willing to come out into the open about it. Darwin's contribution was an idea capable of persuading people how such an unlikely result could flow from such ordinary causes. He succeeded not only with the elaboration of the concept of natural selection but also with its popularization, and his skill in putting these two together is the main reason for the continued popularity of his theory.
In spite of this, a hundred years of research have not produced any large-scale substantiation of the theory of "evolution by natural selection." To be sure, there is the geological evidence of major differences in fossil species be tween one stratum and the next just as there is the undoubted contemporary evidence to support Darwin's concept about how living populations change. What is lacking is any hint that one is the cause of the other. It still remains to be shown that real evolutionary novelty, the production of genuinely different kinds of organisms, ever did or ever could be the result of the microevolutionary changes observed within natural populations.
In this article we will review the evidence about variation and natural selection and consider its relevance to the claims made about the evolution of life on this planet.
Only the Fittest Survive
Darwin's line of reasoning is some times misunderstood by critics who are not always fair to him. He developed his argument as follows:
1. Many more egg cells are fertilized than ever develop, and many more young are produced than ever reach maturity.
2. The differences among offspring, significantly affect their ability to survive, so it is usually the least fit that are caught by predators or that die of starvation.
3. Many of the differences affecting survival are heritable, and favorable changes among the surviving members of a population are perpetuated. The less favorable characteristics gradually disappear.
4. The process continues indefinitely. So far we do not take exception, for this reasoning is strongly supported by observational evidence gathered over the past century. The situation is entirely different with the suggestions that follow, however:
5. Indefinitely continued, the process leads not only to the appearance of new races and new species but to new kinds of animals and plants, covering all the variation possible in nature.
6. All the differences seen between fossil species and living organisms are to be accounted for by the gradual accumulation of changes retained by plants and animals that were successful in the continual struggle for survival.
Darwin was well aware of the differences that appear between members of the same species or even between members of the same litter but it was decades later before it was realized that heritable differences are of three kinds—point mutations (errors that occur at particular points in the genes), chromosome aberrations (rearrangements or insertions or deletions of whole chains of genes on chromosomes), or novel gene combinations (occurring when particular sets of genes from the male and female parents appear together for the first time in the same individual). All three kinds of heritable change qualify for inclusion in Darwin's theory—unlike "acquired characters," which some of Darwin's contemporaries thought could lead to evolution but that cannot be passed on from one generation to another.
Population genetics is now a mature science, and examples of these three kinds of variations and of the advantages and disadvantages they may have in adaptation are readily available.
Point mutations have been observed in every species that has been subjected to study. As befits their origin as errors in the genetic code, their effects are almost always deleterious. A gene is a sequence of chemical code letters that the cell must decipher to find out the sequence of amino acids to be used in constructing an enzyme. But enzymes are very intricately designed and are likely to be seriously impaired if there is a change in any of the hundreds of amino acids that must be specified, each one to its exact location, along with the enzyme chain. Naturally, this makes point mutations poor candidates for providing the kind of variation from which evolutionary improvement could be built.
A very well-known example of the effect of mutations on a species is provided by the history of the peppered moth, a we 11-camouflaged species found widely distributed on the earth, especially in cooler countries like England and Scotland. Naturalists have collected the moth for centuries, noting every now and again the appearance of a dark-colored moth among the common off-white members of the species. The off-white camouflage blends extremely well with the lichen-covered trunks of trees in England and prevents the moth from being taken by birds while resting during the day. This advantage is not possessed by the dark-colored mutants, which, because of the ease with which they are discovered by birds, rarely survive. However, this disadvantage was suddenly reversed in the nineteenth century as factory smoke began to blacken the tree trunks and kill the lichens in the English midlands, and it is now the light-colored form that lacks the protective coloring and the dark-colored moth that survives. The mutant moths produced offspring with the same characteristics, and in the polluted environment gradually these became the dominant form of the species. A better example of natural selection could hardly be found.
Another example of natural selection involving mutants is the case of the inherited disease that affects the oxygen-carrying protein of the blood, hemoglobin. Due to a mutation in the hemoglobin gene, two of the amino acids of the 600-odd that are required to build this important protein are wrongly specified. In place of normal behavior, the molecules clump together within the red blood cell, deforming it badly. An early research worker, peering down his microscope, described these cells as "sickle-shaped," which led to the mutation-induced disease being called "sickle-cell anemia."
This disease is so serious that individuals cannot survive if the hemoglobin genes from both parents have been affected; even if the disease is inherited from only one parent the impairment is considerable. However, it happens that the malarial parasite is unable to live in sickle cells, with the result that in places where malarial infestation is high the disadvantage of sickle cells is greatly offset by the advantage of resistance to malaria. This is undoubtedly the explanation for the high incidence of sickle-cell disease among Africans in malarial areas of Africa, compared with the relatively low frequency of the trait among blacks in America.
Not all point mutations have such devastating effects on organisms as the one that produces sickle cells, and some must be expected to affect survival without major damage to the original design. Mutations affecting color or appearance, like the one that blackens the peppered moth, are good examples, and though trivial, are frequently of survival value. But if they are trivial they are not likely to lead to genuine evolutionary novelty of the kind that separates the major groups of living things. Superficial changes of this kind might well be adaptive and lead to the formation of races within a species and, as the result of many such happenings, new species; but such species diverge only because of accumulated trivialities and never in a way that could explain how fish produced reptiles or reptiles, birds. Contrariwise, nontrivial changes, like the sickle-cell mutation, are also incapable of explaining major evolutionary improvements, because of their very nature as errors in an already complex mechanism. It is only an unusual situation that permits them to be tolerated at all.
To be harmless, mutations must also be trivial; but to be trivial they must renounce evolutionary importance. The examples given above are among the best-documented examples known of natural selection in action. If typical, they are very instructive of its limitations. No one would seriously suggest that changes like the blackening of moth wing-patterns or the destruction of the major function of a vital enzyme could lead to evolutionary improvement, however long continued.
Mutated genes of a trivial kind are found in large numbers in the natural world, where they contribute, along with the other kinds of variation, to the continual processes of adaptation, race formation, and speciation. They account for the huge diversity of living things within the major kinds of organisms that inhabit the globe, but the evidence for the extension of Darwin's theory to explain the origin of the major kinds themselves is still lacking.
Chromosome aberrations are not readily studied in a species unless it is possible to draw a chromosome map. This is no easy undertaking, but it has been done with success in some species. One of the reasons why the vinegar fly, Drosophila, is so much used for studies of genetics is that, by some quirk of nature, it possesses a giant set of chromosomes in its salivary glands, which make it possible to draw a chromosome map with nothing more than a good microscope, skill, and patience. The salivary glands of a fly whose total length is only one-eighth inch are not as big as one might wish, but geneticists are grateful, nonetheless.
Chromosome maps of the different kinds of Drosophila found around the world show differences that can only be explained by assuming that chromosome aberrations occurred in individuals that became separated from the rest of the species and founded a new race. Changes within the new race continued to occur until it became not only geographically isolated from the original species but reproductively isolated as well. This means that the chromosomes of the new race, although comprised of genes from the same gene pool as in the parent species, were arranged in a way that made them incompatible with the chromosomes of the parent group, and interbreeding of the two forms could never be successful. Thereafter, of course, changes of any kind in one group could never be transmitted to the other, and the continued accumulation of them forces us to call them different species.
Sometimes new species arise quite suddenly, as in the chromosome doubling aberration (polyploidy) that occurs in plants, which has produced many of the large garden varieties of vegetables and flowers.
Chromosome aberrations are undoubtedly responsible for a large number of the changes that are found between different races and species of insects that possess so many distinct but basically similar species. The distinctions are not always obvious to the casual observer, but they are profound to the animals themselves, and would be difficult to understand without a knowledge of the way chromosome aberrations occur.
Chromosome aberrations do not often produce effects in organisms as readily identified as point mutations. In the most common types the genes are the same; it is in the order of their arrangement—and, therefore, probably in the cell's control of their operation—that they differ. Changes in color, size, behavior, food, activity, et cetera, have all been reported in insects as a result of this kind of variation, and the suddenness of the changes makes it a ready explanation for the origin of differences in animals, such as Drosophila (2,000 species around the world), and in many plants.
As in the case of gene mutations, changes of this kind affect a well-developed and intricately controlled system of genes already in existence, and can hardly be used to explain the origin of these systems.
Most of the variation observed be tween different races of the same basic kind is owing to new combinations of genes that already existed in the gene pool of the species but which, in the process of bisexual reproduction, are combined in one individual for the first time. Modern studies of the gene pools of species have revealed that there is an unexpectedly large reserve of variability in the different gene combinations of the individuals—new combinations are constantly being produced, and the possibilities are endless. Although endless in number, they are not unlimited—genetic combinations, like chromosome arrangements, cannot produce any more novelty than the original sources of the variability (gene mutations and chromosome aberrations) permit. The shuffling of genes is, therefore, a means of long-term adaptation to the pressures of the natural environment and accounts for the way each new generation of a species can throw up new solutions to the challenges of its competitors and its enemies. New gene combinations are constantly being "selected" by nature, thereby gaining the ascendancy over the previously existing combinations.
What has been rehearsed so far rep resents the kind of research carried out by zoologists and botanists during the past fifty years. More recently, of course, the discoveries of the "molecular biologists" have given us the tools to test the evolutionary hypothesis within living cells, and this is now a very active field of research. Now that the actual sequence of amino acids in enzymes and the actual sequences of DNA codons of the corresponding genes are known in so many cases, it is possible to detail the effect of gene mutations on the enzymes and relate the adaptive success or failure of the organism to chemical changes in individual cells.
These discoveries, opening up the new field of "molecular evolution," make it a lot easier to estimate the limits of evolutionary changes. On the one hand, it is now more clear just how sophisticated the cellular machinery really is (the kinds of change in living cells that would have been necessary if these bio chemical machines had evolved are quite staggering). On the other hand, the adaptability of living cells is found to be much greater than was even suspected before—consequently there are better explanations of microevolution and much less opportunity to categorically deny the overall importance of the mutation/selection concept in changing populations of viruses, bacteria, fruit flies, and so forth.
One idea being explored at present concerns the possibility that new genes may arise by duplication (a chromosomal aberration) and, after subsequent mutation to a functionless copy of the original gene, mutate further to produce a new gene coding for a new and completely different enzyme. Confirmation of part of this hypothesis has been obtained: enzyme modification and gene duplication have both been observed in bacteria forced to grow on foodstuffs not found in the natural world. But it is still not possible to extend the microevolutionary mechanism to account for the original enzyme systems on which these beneficial changes act.
As to the actual effects of mutations on individual cells, there is now a great deal of experience with many thousands of bacterial mutants that are widely used in biochemical research. (Organ isms that possess functionless enzymes make it possible to trace the extremely complicated enzyme pathways of the normal forms.) It is still clear that accumulation of single mutations is in sufficient to account for the change from one distinct kind of enzyme to another, much less of one enzyme system to an other. However, it is no longer possible to assert that all mutations are deleterious to organisms—many are neutral, stemming from certain types of amino-acid substitution in parts of the enzyme chain that are remote from the active site. In some genes there are mutations capable of modifying the structure of proteins. In bacteria these mutations lead to such consequences as changed resistance to antibiotics, changed ability to metabolize food stuffs, and changes in the type of organ ism they may attack.
In summary, the raw material needed to make Darwin's theory of natural selection a credible mechanism for evolutionary improvement is unavailable except on a very small scale. There is no evidence for any kind of change that could permit it to operate much above the race/species level. Nor does the fossil evidence provide support for continued change by the mutation/selection mechanism. The comparison of fossils between different levels shows two things. The differences are either the same limited variations as occur within species living today or they are the same major differences, without intervening stages, found between the major groups of living animals today. The two are quite distinct. The fact that Darwin's hypothesis is useful to explain the first is insufficient justification for using it to explain the second.