What Evolution Is

The story of evolution is so diversified that it poses innumerable questions to whoever first encounters evolutionary problems. Even though I have attempted, in the first twelve chapters, to answer these questions in considerable detail, I now try to provide a concise answer to the most frequently posed of these questions.

Evolution is not merely an idea, a theory, or a concept, but is the name of a process in nature, the occurrence of which can be documented by mountains of evidence that nobody has been able to refute. Some of this evidence was summarized in Chapters 1–3. It is now actually misleading to refer to evolution as a theory, considering the massive evidence that has been discovered over the last 140 years documenting its existence. Evolution is no longer a theory, it is simply a fact.

The answer is an emphatic “No.” In earlier periods many authors thought that a perfection-giving process was involved in evolution. Before the discovery of the principle of natural selection, one could not imagine any other principle than teleology that would lead to such seemingly perfect organs as the eye, annual migrations, certain kinds of disease resistance, and other properties of organisms. However, orthogenesis and other teleological explanations of evolution have now been thoroughly refuted, and it has been shown that indeed natural selection is capable of producing all the adaptations that were formerly attributed to orthogenesis (see Chapters 6 and 7).

This is the wrong question. In On the Origin of Species and in his later publications, Darwin advanced numerous theories, among which five are most important (see Chapter 4). Two of them, evolution as such and the theory of common descent, were accepted by biologists within a few years of the publication of the Origin in 1859 (see Box 5.1). This was the first Darwinian revolution. The other three theories, gradualism, speciation, and natural selection, were widely accepted only much later, during the time of the evolutionary synthesis in the 1940s. This was the second Darwinian revolution.

Yes, up to a point. The movement of planets can be observed directly. By contrast, evolution is a historical process. Past stages cannot be observed directly, but must be inferred from the context. Yet these inferences have enormous certainty because (1) the answers can very often be predicted and the actual findings then confirm them, (2) the answers can be confirmed by several different lines of evidence, and (3) in most cases no rational alternative explanation can be found.

If, for instance, in a chronological series of geological strata a series of fossil therapsid reptiles is found that become more and more similar to mammals in successively younger strata, finally producing species about which specialists argue whether they are still reptiles or already mammals, then I do not know of any other reasonable explanation than that mammals evolved from therapsid ancestors. Actually, there are thousands of such series in the fossil record, even though admittedly there are occasional breaks in most of these series, owing to breaks in the fossil-bearing stratigraphy.

Frankly, I cannot see why such an overwhelming number of well-substantiated inferences is not scientifically as convincing as direct observations. Many theories in other historical sciences, such as geology and cosmology, are also based on inferences. The endeavor of certain philosophers to construct a fundamental difference between the two kinds of evidence strikes me as misleading.

It is obvious, for example, that we cannot experiment with the extinction of the dinosaurs. Instead, one applies the method of “historical narratives” to explain historical (including evolutionary) processes. That is, one proposes an assumed historical scenario as a possible explanation and tests it thoroughly for the probability of its correctness. In the case of the extinction of the dinosaurs, a number of possible scenarios were tested (such as a devastating virus epidemic or a climatic disaster) but rejected because they were found to be in conflict with the evidence. Finally, the Alvarez extinction theory (caused by an asteroid impact) was so convincingly supported by the existing evidence and by all subsequent research that it is now universally accepted (see Chapter 10).

All theories of science, including Darwinism, are vulnerable to rejection if they are falsified. They are not unalterable, in contrast to revealed dogmas of religions. There are numerous cases in the evolutionary literature of provisional evolutionary theories that were eventually rejected. The belief that a gene can be the direct object of selection is one such refuted theory. The formerly widely adopted theories of transmutationism and transformationism were also rejected.

Evolution is subject to a large number of interactions. Different genotypes within a single population may respond differently to the same change of the environment. The changes of the environment, likewise, are unpredictable, particularly the arrival at a locality of new predators and competitors. Finally, there are occasionally very drastic changes in the global environment, resulting in so-called mass extinctions. In such mass events, chance may play a large role in survival. Owing to the unpredictability of all of these situations, the nature of the evolutionary change by which a population will respond is necessarily also unpredictable. Nevertheless, a knowledge of the potential of a genotype and of the nature of constraints permits in most cases a reasonably accurate prediction.

Three accomplishments of the synthesis are particularly important. First, it effected the universal rejection of the three evolutionary theories competing with Darwinism, orthogenesis (finalism), transmutationism (based on saltations), and inheritance of acquired characters; second, it produced a synthesis between the thinking of the students of adaptation (anagenesis) and those of organic diversity (cladogenesis); and third, it confirmed the original Darwinian paradigm of variation and selection while refuting all criticism of it.

Molecular biology has made great contributions to our understanding of the evolutionary process. However, the basic Darwinian concepts of variation and selection were not affected in any way. Not even the replacement of proteins by nucleic acids as the carriers of the genetic information required a change in the evolutionary theory. Indeed, an understanding of the nature of genetic variation has contributed greatly to strengthening Darwinism. For instance, it confirmed the finding of the geneticists that an inheritance of acquired characters is impossible. Also the use of molecular evidence when added to the morphological evidence has led to the solution of many phylogenetic puzzles.

No, evolution is a much broader concept. Phylogeny refers only to one of many evolutionary phenomena, the pattern of common descent. However, properly considered, phylogeny means not only the pattern of branching points, but also the changes between these nodes.

Are phylogenetically later organisms “higher” than their ancestors? Yes, they are higher on the phylogenetic tree. But is it true that they are “better” than their ancestors? Those who make this claim list a number of characteristics of “higher” organisms, purporting to demonstrate advance, such as division of labor among their organs, differentiation, greater complexity, better utilization of the resources of the environment, and in general better adaptation. But are these so-called measures of “progress” truly valid evidence for an advance?

It seems that those who deny any signs of evolutionary progress in the advance from bacteria to higher organisms give a teleological or deterministic aspect to the idea of progress. Indeed, evolution seems highly progressive when we look at the lineage leading from bacteria to cellular protists, higher plants and animals, primates, and man. However, the earliest of these organisms, the bacteria, are just about the most successful of all organisms, with a total biomass that may well exceed that of all other organisms combined. Furthermore, among the higher organisms there are lineages such as parasites, cave animals, subterranean animals, and other specialists that show many retrogressive and simplifying trends. They may be higher on the phylogenetic tree, but they lack the characteristics always listed as evidence for evolutionary progress. What cannot be denied, however, is that in every generation of the evolutionary process, a surviving individual is on the average better adapted than the average of the nonsurvivors. To that extent, evolution clearly is progressive. Also, throughout evolutionary history innovations were introduced that made functional processes more efficient.

Once a species has acquired effective isolating mechanisms, it may not materially change for millions of years. Indeed the so-called living fossils have hardly changed for hundreds of millions of years. How can this be explained? It has been argued that this stasis was due to the operation of normalizing selection, which culls all the deviations from the optimal genotype. However, normalizing selection is equally active in rapidly evolving lineages. Stasis apparently indicates the possession of a genotype that is able to adjust to all changes of the environment without the need for changing its basic phenotype. To explain how this is done is the task of developmental genetics.

It is now clear that the seemingly sudden origin (within 10–20 million years) of so many animal types in the early Cambrian (beginning 544 million years ago) is an artifact of preservation. By use of the molecular clock, the origin of the animal types can be placed at about 670 million years ago, but the animals living between 670 and 544 million years ago are not preserved as fossils because they were very small and without skeletons.

The reason why no major new types originated in the ensuing 500 million years is more complex and only partly understood. However, molecular genetics has led to an explanatory suggestion. Development is tightly controlled in the now living organisms by very precise “working teams” of regulatory genes. In the Precambrian, there were apparently only a few such genes, which did not control development as tightly as later on. This allowed a frequent occurrence of rapid major restructuring of the structural types. By the end of the Cambrian, the dominance of these regulatory genes had been fully established and the origin of completely new structural types had become difficult, if not impossible. One must always remember that the changes prior to the Cambrian did not occur suddenly, but over a period of several hundred million years, even though not documented in the fossil record.

Mutation is the principal source of new genetic variation in a population. Most mutations are due to errors in the replication process during meiosis that are not corrected by any repair mechanism. There is no mutation pressure. Most of the variation of genotypes available for selection in a population is the result of recombination, not of new mutations.

Darwin already pointed out that the introduction of English plants and animals on New Zealand often resulted in the extinction of native species. Indeed, it has frequently been observed in other parts of the world that the success of one species may result in the downfall of another species. Authors have spoken of species selection, but this is a misleading term. Actually, selection acts on the individuals of the two species as if they were members of a single population. Therefore, the “struggle for existence” is between the individuals of the two species, but the individuals of one of them are in the long run more successful than those of the other species. Thus it is a typical case of Darwinian selection of individuals. The species as a whole is never the target of selection. However, one can admit that the differential success of entire species is superimposed on this individual selection. Misunderstanding can be avoided if one speaks of species turnover or species replacement, instead of species selection.

The individual in an asexually reproducing organism is the entire clone, that is, the totality of genetically identical individuals. Such an individual is replaced by selection at the moment at which the last member of the clone dies. Such an elimination is in principle the same as the elimination of an individual by natural selection in sexual organisms.

Why has there been so much controversy about the object of selection? At the time of the evolutionary synthesis, the geneticists believed that it was the gene, whereas the naturalists believed that it was the individual, as Darwin had always believed. Forty years of analysis have finally made it quite clear that the gene as such could never be the direct target of selection. However, in addition to the individual, a group can also be the target of selection if it is a social group and cooperation within this group enhances its survival. Finally, gametes are also directly exposed to selection and different gametes produced by the same individual may differ in their ability to achieve fertilization.

From the stage of the zygote on. Some evolutionists have neglected to take embryonic or larval life into consideration. These are often subjected to more selection pressure than the adults. However, the evolutionary effectiveness of selection ends with the end of the reproductive life. In the human species, for instance, diseases that manifest themselves only in postreproductive life are virtually unaffected by selection. Yet they may reduce the contribution to kin selection made by healthy grandparents (in social organisms).

20. Is the term “struggle for existence” to be interpreted literally? Definitely not! As Darwin already emphasized, the term is to be interpreted metaphorically. Plants at the edge of the desert may struggle for existence with each other, as few will survive while most of them will succumb to the desert conditions. However, a literal struggle is quite rare. It does occur in polygynous species of animals in which males fight with each other in territorial encounters, and it also occurs in struggle for space among marine benthic organisms, and in similar situations. It is most obvious whenever competition for space is involved. In social organisms, low-ranking individuals may struggle for resources with high-ranking individuals.

In evolutionary discussions, it is often stated that “selection pressure” resulted in the success or elimination of certain characteristics. Evolutionists here have used terminology from the physical sciences. What is meant, of course, is simply that a consistent lack of success of certain phenotypes and their elimination from the population result in the observed changes in a population. It must be remembered that the use of words such as force or pressure is strictly metaphorical, and that there is no such force or pressure connected with selection, as there is in discussions in the physical sciences.

The first step in selection, the production of genetic variation, is almost exclusively a chance phenomenon except that the nature of the changes at a given gene locus is strongly constrained. Chance plays an important role even at the second step, the process of the elimination of less fit individuals. Chance may be particularly important in the haphazard survival during periods of mass extinction.

Darwin already remarked that selection never produces perfection, but only provides adaptation to existing conditions. For instance, animals and plants in New Zealand had been selected to be adapted to each other. When English animals and plants were introduced to New Zealand, many of the native species, not being “perfect,” that is, not being adapted to the invaders, became extinct. The human species is highly successful even though it has not yet completed the transition from quadrupedal to bipedal life in all of its structures. In that sense it is not perfect.

This is a question that psychologists love to ask. The answer is actually quite simple: from animal consciousness! There is no justification in the widespread assumption that consciousness is a unique human property. Students of animal behavior have brought together a great deal of evidence showing how widespread consciousness is among animals. Every dog owner has had occasion to observe the “guilt feeling” a dog displays when, in the absence of its master, he has done something for which he expects to be punished. How far “down” in the animal kingdom one can trace such signs of consciousness is arguable. It may well be involved even in the avoidance reaction of some invertebrates and even protozoans. However, it is quite certain that human consciousness did not arise full-fledged with the human species, but is only the most highly evolved end point of a long evolutionary history.