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Origin of Complex Order in Biology:
Abdu'l-Baha's concept of the originality of species compared to concepts in modern biology

by Eberhard von Kitzing

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Chapter 2

Evolution of the Term Species in Occidental Biology

In chapters 46 and 49 of SAQ and pages 355-361 of PUP `Abdu'l-Bahá rejects the theory of the modification of the human species during the evolution of life on this planet developed by "European philosophers". /1/ To understand the arguments of `Abdu'l-Bahá in favor of the originality of the humans species some knowledge of the development of the theories of evolution, of concepts about the origin of complex biological order during the development of this planet, and of the meanings of the term species in Europe before and after Darwin is required. A comprehensive presentation of the Growth of Biological thought towards modern biology is given in Mayr's book (Mayr, 1982) about the history of modern biology.

Between the beginning of the 19th century until the middle of the 20th century the classical concept of the biological species, assuming that the particular members of a population derive their outer form, i.e., their phenotype, from timeless species essences, was replaced by a modern definition, where the biological species are defined by the population of particular individuals, i.e., a gene pool common to a group of interbreeding beings. A careful inspection of the evolution of the definition of the term species in biology indicates that the arguments of `Abdu'l-Bahá in in Haifa around 1905 and in the States around 1912 specifically address the European discussion which was known in the Orient by translations of popular books of Büchner, Haeckel or Spencer, and were discussed in few arabic journals (see Keven Brown's article).

In this section the development of the term species beginning with Plato and Aristotle is presented. Specifically, the evolution of this term in the occident is addressed, whereas the diverse concepts current with oriental philosophers are covered in the accompanying essay by Keven Brown (see Keven Brown's article). First the species concept of classical biology is considered, followed by the respective views in modern biology.

2.1) Classical concept of the species

The discussion about a correct understanding of the existence of stable and clearly distinct biological populations was initiated by Plato and Aristotle. These populations are stable, e.g., horses remain horses over many generations. And they are distinct, a cat can produce fertile offsprings only with other cats, but not with dogs. This stability and distinction suggests to consider cats and dogs as clearly distinct entities.

Plato was interested in the order on which our cosmos is built, in unchanging realities. He was looking for the reality behind all the particular events. He believed in the existence of ideas, of essences representing the true timeless reality behind our everyday experiences. For Plato the prototypes of essences were geometric objects such as triangles, squares, tetrahedra or cubes (i.e., the Platonic ideal bodies). These objects are clearly distinct and there exists no "smooth" way to transform a triangle into a square, or a tetrahedra into a cube. Because animals and plants form distinct classes, such as roses, cats etc., Plato assumed the existence of timeless essences for each of those classes, the species. These essences were believed to assure the stability of the species, i.e., that cats can give birth only to cats and not to cows or birds. Such species essences are assumed by Plato to represent the timeless reality of the biological populations independent of the existence of particular members.

In contrast to Plato, Aristotle was particularly interested in biology and invented many biological disciplines. Much of his work is still valid today. He did not believe in the existence of essences, but assumed that the existence of particular beings of a biological population is sufficient to maintain the existence and the stability of its kind. Because Aristotle had a static picture of the world and consequently assumed the eternal existence of the different species, he rejected any idea of biological evolution.

Plato's concept of essences and Aristotle views particularly of biology built the early foundation of occidental science and philosophy. Today the development of Western sciences is often presented as an emancipation from those concepts. In the 18th and 19th century the belief in essences was firmly established in nearly every branch of the sciences. Even today, physics is basically essentialistic, whereas in modern biology essentialism is discarded because species essences are assumed to contradict the facts of evolution (Dennett, 1995; Mayr, 1982).

2.1.1) Essentialism in physics and chemistry

The statement of Newton about the relation between God and Nature gives a good account of the general belief of his time about the origin of complex order in the biosphere:
We know Him only by His most wise and excellent contrivances of things, and final causes; we admire Him for His perfections; but we reverence and adore Him on account of His dominion; for we adore Him as His servants; and a God without dominion, providence, and final causes, is nothing else but Fate and Nature. Blind metaphysical necessity, which is certainly the same always and everywhere, could produce no variety of things. All the diversity of natural things which we find, suited to different times and places, could arise from nothing but the ideas and will of a Being necessary existing. (Mayr, 1982, p. 141)
Nature was understood to be a realization of God's ideas, an expression of His eternal plan. According to Newton accidental and necessary forces cannot produce the diverse complex order found in biology, but can repeat only the same things again and again. The diversity found in Nature, therefore, was assumed to require a Creator. This type of argument remained nearly unchallenged until the establishment of Darwinian evolution. /2/

In physics and chemistry the concept of timeless essences is generally accepted until today with only few exceptions. They are, however, designated "natural laws" and their explicit form changed considerable throughout time. At the beginning the essences in physics were concrete, until today they became rather abstract. After the discovery of the chemical elements, these elements were considered to be the expression of time invariant essences. Chemical elements cannot be transmutated by chemical means. Within chemical reactions they would modify their properties, but one can always get them back afterwards completely unchanged. The smallest units of these elements are the atoms. Later Rutherford discovered that the atoms themselves were composed of a nucleus and an electron shell. Nuclear physics revealed that the nucleus is composed of subatomic particles. For some time those subatomic particles were considered to be elemental, designated elemental particles, i.e., direct representations of timeless essences. But the growing zoo of "elemental particles" and the possible transmutation of one type of particle into other ones questioned their elementary status. At present quarks (Gell-Mann, 1994) are generally considered to be the elemental subunits in the physical world, representing timeless units, essences, on which all the higher levels of existence depend.

In physics one often searches for conserved entities. In his famous treatise Über die Erhaltung der Kraft published in 1847 Hermann Helmholtz (1821-1894) formulated the law of the conservation of energy. This discovery parallels the findings of Lavoisier of the conservation of mass and elements. Energy may change its form, but it is not created nor eliminated in any physical process. Consequently, the search for timeless properties became essential in physics and dominates most of its branches. This is best documented in the fundamental assumption that physics should be the same, yesterday, today and tomorrow. /3/ In other words, the general laws of physics have to be time invariant.

In the 19th century, physics and physical chemistry concentrated mainly on equilibrium and close to equilibrium systems. Such systems often are sufficiently simple to study their basic properties and to derive the necessary theoretical instruments for their proper quantitative description. Because living systems generally exist only far form equilibrium, 19th century concepts of physics and chemistry are with few exceptions rather inappropriate for the description of biological phenomena (Prigogine, 1979; Prigogine and Stengers, 1981). Therefore, the repeated attempts for the physicalization of biology, as for instance postulated by Helmholtz, generally failed and provoked counter reaction resulting in the development of vitalistic theories.

Only in the 20th century physics and chemistry became sufficiently mature that one can begin to study systems far from equilibrium (Land, 1991; Prigogine, 1979; Prigogine and Stengers, 1981). Today, the investigation of non-equilibrium systems, e.g., complex dynamic systems such as the weather, is at the fore front of modern sciences (Gell-Mann, 1994; Kauffman, 1995).

2.1.2) Essentialism in classical biology

Due to the introduction of biological evolution by Darwin the philosophy of biology changed drastically. In this sense one can speak about a pre- and post-Darwinian biology, here referred to as classical and modern biology.

The term species in classical biology was dominated by two concepts originating from Plato (Mayr, 1982): the phenotypes of the members of a population were assumed to be determined by their species essence; and the existence of a species requires a creative force, a demiurg. In Christianity and Islam (see Keven Brown's article) the origin of the creative force was equated with God, the Creator. These concepts were so firmly rooted in the scientific community that still in the middle of the 19th the biologist Louis Agassiz stated as a widely accepted view that "it is the task of the philosopher to reveal the blueprint of the creator." (Mayr, 1982, p. 305) The same author emphasized in his Essay on classification published in 1857:

All organized beings exhibit in themselves all those categories of structure and of existence upon which a natural system may be founded, in such a manner that, in tracing it, the human mind is only translating into the human language the Divine thoughts expressed in nature in living realities. (Mayr, 1982, p. 865)
According to Agassiz, discovering the order in nature is equivalent to translate the ideas of our Creator about nature into the human language. This credo was not a singular opinion of a somewhat obscure scientist; it represented the belief of a considerable number of his colleges.

Even in scientific circles it was widely assumed that the various species were directly created by God's command. The famous Swedish naturalist Carl Linné, who proposed in 1735 in his Systema Naturae a first attempt to systematize the manifold forms of life, stated: "Species tot sunt diversae, quot diversaes formas ab initio creavit infinitum ens." /4/ Also the influential french biologist Georges Cuvier, who for instance invented palaeontology as a branch of biology, assumed a creationistic concept of the biological species. According to him, all particular members of a single species root from the first couple of their species created originally by God:

We imagine that a species is the total descendence of the first couple created by God, almost as all men are represented as the children of Adam and Eve. What means have we, at this time, to rediscover the path of this genealogy? It is assuredly not in structural resemblance. There remains in reality only reproduction and I maintain that this is the sole certain and even infallible character for the recognition of the species." (Mayr, 1982, p. 257)
In particular cases, Cuvier considers it to be impossible to trace the genealogy of a member of a population back to its original couple. However, because only members of the same species can interbreed the ability to produce fertile offsprings is in itself a sufficient proof that both parents belong to the same species. What Cuvier thinks to be the consequence of Gods Creation is used today to define a biological species, i.e., the ability of its members to interbreed (see below).

The species is not only defined as a population of offsprings from an original couple. But, in agreement with Plato's concept of ideas, each species is defined by a "prototype", an archetype, by it's species essence. In his Histoire naturelle Georges Louis Buffon explained:

There exists in nature a general prototype in each species upon which all individuals are moulded. The individuals, however, are altered or improved, depending on the circumstances, in the process of realization. Relative to certain characteristics, then, there is an irregular appearance in the succession of individuals, yet at the same time there is a striking constancy in the species considered as a whole. The first animal, the first horse for example, was the exterior model and the internal mould from which all past, present, and future horses have been formed. (Mayr, 1982, p. 261)

The species essence was considered to be the unchanging concept in the mind of God about the ideal form of the members of a biological population. /5/ Because the particular members of a population were thought to be the direct representations of their species essences, also the phenotypes were assumed not to change within time. Michel Adanson stated 1769 "that the transmutation of species [e.g., biological populations] does not happen among plants, no more than among animals, and there is not even direct proof of it among minerals, following the accepted principle that constancy is essential in the determination of a species." (Mayr, 1982, p. 260, the text in square brackets is added by the author) The invariability of species according to classical biology is clearly stated by Mayr (Mayr, 1982, p. 404): "Each species had its own species-specific essence and thus it was impossible that it could change or evolve." In classical biology the biological population was assumed to directly reflect their species essence. These populations, therefore, were assumed not to change and to remain a direct constant realization of their species essence.

Why did classical biology reject the existence of evolution? What was the origin of this static world view? Classical biology bases heavily on the concepts of Plato and Aristotle. Although Aristotle had a rather modern concept of the species as a population he insisted in a purely static world view:

... Not so with Aristotle. He held too many other concepts irreconcilable with evolution. Movement in the organic world, from conception to birth to death, does not lead to permanent change, only a steady-state continuity. Constancy and perpetuity are thus reconcible with movement and with the evanescence of individuals and individual phenomena.

As a naturalist, he found everywhere well-defined species, fixed and unchanging, and in spite of all his stress on continuity in nature, this fixity of species and their forms (eide) had to be eternal... There is order in nature, and everything in nature has its purpose. He stated clearly (Gen. An. 2.1.731b35) that man and the genera of animals and plants are eternal; they can neither vanish nor have they been created. The idea that the universe could have evolved from an original chaos, or the higher organisms could have evolved from lower ones, was totally alien to Aristotle's thought. To repeat, Aristotle was opposed to evolution of any kind. (Mayr, 1982, pp. 305-306)

This static view of the universe agreed with Plato's idea that our universe is harmonious and perfect from the very beginning (Mayr, 1982, pp. 305). The combination of Plato's timeless essences, his concept of a perfect, harmonious universe, Aristotle's static world view and the biblical cosmology taken literally led to a concept of species essences which was applicable only to a static world. In a perfectly harmonious world there can exist per definitionem no process which increases this perfection. Any change could only decrease the degree of perfect harmony.

Within such a concept, the appearance of a new biological form could only result from the creation of a new species essence. According to Mayr all theories of biological change before Lamark (see below) were more or less variants of this idea. Because the invention of the new species in this concept is not gradual, such theories are designated saltational evolution:

Saltational evolution is a necessary consequence of essentialism: if one believes in evolution and in constant types, only the sudden production of a new type can lead to evolutionary change. That such saltations can occur and indeed that their occurrence is a necessity are old beliefs. Almost all theories of evolution described by Osborn in his history of evolution, From the Greeks to Darwin (194), were saltational theories, that is, theories of the sudden origin of new kinds. (Mayr, 1991, p. 42)
To summarize, classical biology was rooted in the concept of creation and the assumption of a static world:
It had two major theses. The first was the belief that the universe in every detail was designed by an intelligent creator. This together with the other one, the concept of a static, unchanging world of short duration, were so firmly entrenched in the western mind by the end of the Middle Ages that it seemed quite inconceivable that they could ever be dislodged. (Mayr, 1982, p. 310)
According to Mayr, the erosion of these principles was required before a "real" theory of evolution could be developed.

2.1.3) The mechanization of biology

With the publication of the Principia Mathematica in 1687 Newton "unified" terrestrial with celestial mechanics. Newton's theory explains the falling of apples on earth as well as the path of the planet venus around the sun. That apples falling to the ground should suffer the same kind of forces than the venus circling around the sun was by no means self-evident at that time. This achievement and many others made mechanics a sciences par excellence. Until the beginning of the 20th century the quality of a science was often equated with the degree this science was based on mechanics.

In the renaissance the mechanization of Nature had by no means atheistic tendencies, as shown in the quote of Newton given above. Two opposing views about Nature were established. In the mechanistic world view the universe was considered to be created by God. It runs on the basis of a few natural laws, /6/ e.g., Newton's laws, with only minor inventions by the Creator. The living creatures were considered to be nothing but mechanisms.

The mechanistic view that the world is based on a few secondary causes was at variance with the abundance of life. A reaction on such mechanization tendencies was natural theology which considered nature to be the result of a direct, detailed providence and care from the Creator:

Everything in the living world seemed to be so unpredictable, so special, and so unique that the observing naturalist found it necessary to invoke the creator, his thought, and his activity in every detail of the life of every individual of every kind of organism... John Ray's The Wisdom of God Manifested in the Works of the Creation (1691) is not only a powerful argument from design but also a very sound natural history... Natural theology was a necessary development because design was really the only possible explanation for adaption in a static "created" world. Any new finding in this early age of natural history was grist on the mill of natural theology. The supposedly idyllic life of the inhabitants of the tropics, in particular, was seen as evidence for the providential design by the creator. (Mayr, 982, p. 104f)
In Britain natural theology was rather influential until the middle of the 19th century. No contradictions were found between biology and theology. Biology was considered to prove the Glory of its Creator. At that time most of the British biological scientists were theologians. In France and in Germany natural theology lost its importance much earlier, around 1780. In Germany in the 18th and 19th century various romantic movements determined the schools of thought. Those movements were in part reaction on mechanistic concepts. The names of Herder and Goethe are related with these schools, culminating in the Naturphilosphie as developed by Schelling, Oken and Carus.

The 19th century experienced an explosive development of the natural sciences. Mechanics as formulated by Newton and developed by Euler, Hamilton, Lagrange, Laplace and Poincaré (to name only a few) was considered as the natural science par excellence. Important discoveries of "modern" sciences were the conservation of matter in 1789 by Lavoisier (1743-1794) and the conservation of energy in 1842 by Robert Mayer (1814-1878) and 1847 by Helmholtz. The high esteem for the physical sciences and the influence of vitalistic schools gave rise to a strongly reductionistic physicalism in physiology in the middle of the 19th century in Germany. A considerable number of prominent scientists expected any good science to explain its phenomena by mechanistic causes, at least on the long run. One of the most prominent advocates of the physicalization of physiology was the German physician and physicist Helmholtz. /7/ During the opening lecture at the meeting of German naturalists and physicians in Insbruck 1869 he outlined his scientific program: "The ultimate objective of the natural sciences is to reduce all processes in nature to the movements that underlie them and to find their driving forces, that is, to reduce them to mechanics." (Basfeld, 1992; Mayr, 1982, p. 115). According to Büchner, the sciences more or less prove "...that the macroscopic as well as microscopic existences in all aspects of its growth, life and decay follow only mechanic laws, grounded in the things themselves." (Büchner, 1904)

The existence of independent higher qualities like free will was denied. Haeckel (Haeckel, 1984, p. 27) describes free will as a dogma consisting in delusion: "Free will is not an object of scientific investigation, because as a mere dogma it is based on illusion and does not exist in reality." The complexity of our universe including all levels of life was considered to emerge from the laws of physics and chemistry. Matter was thought to obey the laws of classical mechanics. /8/ Such ideas were popularized by Ludwig Büchner, Ernst Haeckel, Johannes Müller, Jacob Moleschott, /9/ Wilhelm Ostwald and Karl Vogt. These ideas became known as positivism (and should not be mistaken with neopositivism from the Vienna School (Kraft, 1968)). To develop and distribute a scientific view of life, Büchner cofounded in 1881 the Deutschen Freidenkerbund and until his death he was the head of this society. Haeckel promoted in 1906 in Jena the Monistenbund. Those movements did not have precise philosophies, many lines of thought were subsumed under the name of positivism. Their central goal was to develop a scientific view of life.

2.1.4) Orthogenetic evolution

Most early concepts of biological evolution were based on essentialism and mostly assumed a plan, a purpose of evolution "implemented" by the Creator. Such goal directed evolution concepts are sometimes designated orthogenetic evolution. Many of the early philosophical approaches to evolution such as proposed by the German Naturphilosophen were essentialistic and goal directed, they had, however, nearly nothing to do with biology. According to Mayr "Teleological thinking was extremely widespread in the first half of the nineteenth century. For Agassiz and other progressionists the sequence of fossil faunas simply reflected the maturation of the plan of creation in the mind of the creator." (Mayr, 1982, p. 528)

The first scientifically consistent theory of biological evolution was proposed by Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamark (1744-1829). From his studies of huge amount of living and extinct moluscs he drew the conclusion that biological systems are endowed with the ability to accumulate complexity. In his Philosophie Zoologique published in 1809, i.e., 50 years before Darwin's Origin, he stated: "Nature, in successively producing all species of animals, beginning with the most imperfect of the simplest, and ending her work with the most perfect, has caused their organization to become more complex" (Mayr, 1982 p. 353). For Lamark a central force for the evolution of life was the principle that animals are in perfect harmony with their environment. /10/ This principle goes back to Plato. Such harmony can be discovered nearly everywhere in Nature and was always emphasized by natural theologians. Because the findings in geology document drastic changes within the environment during geological history the animals must have evolved, i.e., adapted to the new situation, simply to maintain the harmony. This concept has parallels in the principle of evolution proposed by Gell-Mann (Gell-Mann, 1994) that adaption means to reduce the information differences between biological populations and their environment.

Especially in the first half of the 19th century the belief in orthogenetic evolution, i.e., Nature following the plan and goals given by our Creator, was widely spread. The belief in a force directing evolution towards increasing complexity was often the result of a teleological world view, the direct sign of Gods purposeful plan. For instance the embryologist von Baer stated in a review of Darwin's Origins: "My goal is to defend teleology... Natural forces must be coordinated or directed. Forces which are not directed--so-called blind forces--can never produce order... If the higher forms of animal life stand in causal relationship to the lower, developing out of them, than how can we deny that nature has purposes or goals?" (Mayr, 1982, p. 529). Von Baer argues very similar as Parley did in his watchmaker example. Accidental influences cannot produce order. For him the existence of orthogenetic forces creating increasing complexity was required by the fact of evolution and for him constituted a direct proof for the existence of purpose in nature.

Orthogenetic theories were defended until the middle of this century. In 1926 L. Berg wrote: "Evolution of organisms is the result of certain processes inherent in them, which are based upon law. Purposive structure and action are thus a fundamental property of living being." (Mayr, 1982, p. 530) A recent prominent advocate of orthogenetic evolution was Teilhard de Chardin with his omega principle (de Chardin, 1947). He considers evolution as the result of a goal directed plan which will eventually lead to the unification of mankind. Recently, Hatcher (Hatcher and Hatcher, 1996) and Ward (Ward, 1996) discussed concepts of orthogenetic forces (see below).

Since early human history our world was often understood to follow a final goal given by its creator. Such kind of directedness, however, is mostly rejected in modern philosophies related directly to natural sciences:

From the Greeks on, there was a widespread belief that everything in nature and its processes has a purpose, a predetermined goal. And this processes would lead the world to evergreater perfection. Such a teleological worldview was held by many of the great philosophers. Modern science, however, has been unable to substantiate the existence of such a cosmic teleology. Nor have any mechanisms or laws been found that would permit the functioning of such a teleology. The conclusion of science has been that final causes of this type do not exist. (Mayr, 1991, p. 67)
Even today, in presentations of biological evolution to the general public evolution is often depicted as a directed process. Invertebrates are followed by fishes, amphibian, reptiles, mammals, and finally homo sapiens. The existence of evolution directed from the simple towards the complex as a general principle would be a good argument in favor of orthogenetic theories. According to Gould and others (Mayr, 1982), however, no directionality can be found in evolution, if studied in detail:
Our impression that life evolves toward greater complexity is probably only a bias inspired by parochial focus on ourselves, and consequent overattention to complexifying creatures, while we ignore just as many lineages adapting equally well by becoming simpler in form. The morphologically degenerate parasite, safe within its host, has just as much prospect for evolutionary success as its gorgeously elaborate relative coping with the slings and arrows of outrageous fortune in a tough external world. (Gould, 1994)
Today orthogenetic theories are no longer accepted by most biologists because a clear overall tendency or direction cannot be detected in the development of or universe or the evolution of the various species.

2.2) "Species" in modern biology

Today, Darwinism is considered to be the central theory in biology. All concepts developed in modern biology have to be compatible with evolution as clearly stated in 1973 by Theodosius Dobzhansky in The American Biology Teacher: "Nothing in biology makes sense except in the light of evolution." (Dobzhansky, et al., 1977) The philosophical implications of Darwinism, of course, strongly influence the definitions in the biological nomenclature. This is particularly true for the term "species". But before modern species concepts are considered some background in neo-Darwinism is given.

2.2.1) Neo-Darwinian theory of biological evolution

The commonly proposed scientific model for the biological evolution of life on earth starts with the pre-biotic soup (Orgel, 1994). The soup is believed to have provided our planet with preliminary forms of life. The exact details of this process are largely unknown. The historical details may resist any attempt to become uncovered (Eigen, 1992; Orgel, 1994). The oldest fossils are between two and four billion years old, originating from single celled organisms (Alberts, et al., 1989; Barghoorn, 1971; Schopf, 1993). Multicellular organisms appeared at the beginning of Cambrium about 600 million years ago (Gould, 1994).

According to neo-Darwinian theory, which here is summarized for single celled organisms, the target of evolution is the genome, the genotype (Dawkins, 1989). It consist of a "program" how to run the cell, how to find food, how to react in difficult situations, how to interpret the program, to make it short, the genome is translated into a living cell, the phenotype. The genome consists of long polymer chains of RNA, for few primitive organisms, or DNA for all higher organisms using four different monomers, the elementary building blocks. The four elementary units, the nucleosides, are designated by the characters A, C, G and T (U for RNA). These four characters stand for the bases adenine, cytosine, guanine and thymine (uracil for RNA). The whole genome is made up of these four letters and the precise sequence of these letters defines the genomic message and its translation product the phenotype, the particular living organism. The total chain length for bacteria is typically 5 million and for humans 3 billion nucleotides (Alberts, et al., 1989). DNA and RNA are the genetic material common to all known living system on earth (Orgel, 1994). Even the rules of translation are the exactly the same in all living cells with only rare exceptions.

For single celled organisms reproduction means cell division, a mother cell divides into two daughter cells. To provide both daughter cells with the necessary genetic information, the DNA must be copied. Although the fidelity in gene-reproduction is very high, /11/ occasionally errors occur, e.g., a single letter is replaces by one of the three others. Such a type of mutation is designated point mutation. But also deletions or insertions of parts of sequences are possible. After cell division there is a certain probability that the genes of the two daughter cells are different. Because the positions and directions of the mutations are unpredictable, they are considered to be random.

Many alterations in the genomic sequence will be lethal or will reduce the ability of the cell to face the needs of life. In rare cases, however, a mutation will improve the cell's capability to survive and to reproduce in its given or in a neighboring environment. Cells with the highest reproduction rates have a good chance to spread their genes also in future. This rule is often designated as natural selection or the survival of the fittest. Evolution in terms of neo-Darwinism can be considered as the "diffusion" of the DNA sequences through the space of possible sequences using a four letter code accumulating increasingly potent genes. In principle, very similar rules apply for multicellular, sexual reproduction (Dawkins, 1989; Sober, 1993).

In practice it is not possible to follow the evolution of higher forms of life like the human species (Gould, 1994). There are in vitro experiments, where important aspects of evolution can be studied (Biebricher, et al., 1993; Biebricher and Luce, 1993; Spiegelman, 1967). Also some viruses, like the influenza and the aids virus, utilize high mutation rates to outwit the human immune system (Dopazo, et al., 1993; Eigen, 1993). These examples indicate certain areas where biological evolution can be studied "at work" in nature.

2.2.2) "Natural Selection" as a two step process

Mayr and others describe natural selection as a two step process. During the first step mutations and recombination /12/ produce a wide range of variations. Random changes are of course a good way to achieve this goal. The second step consists in the selection for the most potent organisms which are best adapted for their particular environment. Mayr explains this view by contrasting it to his perception of the essentialistic view:
Selection, for an essentialist, is a purely negative factor, a force which eliminates deleterious deviations from the norm. Darwin's opponents, therefore, insisted in the spirit of essentialism that selection could not create anything new. By saying this, they revealed that they has neither understood the two-step process of selection nor its populational nature. The first step is the production of an unlimited amount of new variation, that is, of new genotypes and phenotypes, particularly through genetic recombination rather than by mutation. The second step is the test to which the products of the first step are subjected by natural selection. Only those individuals that can pass this scrutiny became contributors to the gene pool of the next generation. (Mayr, 1982, p. 591)
Mayr decomposes the process of evolution into two steps: (1) creating random variations in the genotypes and (2) selecting the phenotypes according to their ability to cope with the odds of their environment. But the question still remains how random changes in the genotype can lead to such "well designed" adaptions found in nature.

The chance to obtain the DNA sequence of an efficient enzyme within a few large mutation steps from scratch is by far too small that such an event can practically be excluded by simple probabilistic estimates. Only if it is possible to split up the few large evolutionary steps into many small gradual steps evolution becomes plausible. Dawkins designates this concept as cumulative selection (Dawkins, 1986):

We have seen that living things are too improbable and too beautifully `designed' to have come into existence by chance. How, then, did they come into existence? The answer, Darwin's answer, is by gradual, step-by-step transformations from simple beginnings, from primordial entities sufficiently simple to have come into existence by chance. Each successive change in the gradual evolutionary process was simple enough, relative to its predecessor, to have arisen by chance. But the whole sequence of cumulative steps constitutes anything but a chance process, when you consider complexity of the final end-product relative to the original starting point. The cumulative process is directed by nonrandom survival. The purpose of this chapter is to demonstrate the power of this cumulative selection as a fundamentally nonrandom process.
Dawkins particularly emphasizes the cumulative character of evolution. Small random favorable mutation steps are conserved in the surviving DNA chains. Each little improvement becomes subject to further gradual success. /13/ Only if evolution can be decomposed into a sufficient number of small gradual progresses neo-Darwinism becomes reasonable. /14/

2.2.3) Relationships between species

In classical biology the similarity between species was understood to result from a unique "construction" plan of God resulting in the appearance of similar kinds of design several times in nature. The scala naturae was considered to represent a continuous spectrum of increasingly complex species. Although there was this scale of species, each species was seen to be distinct from all others from the very beginning, e.g., from the time point of creation. Breeding was known to be possible only within species but not across species boundaries. Because in classical biology the species was defined by its timeless essence, the resulting populations were likewise thought to be unchanging in time. In the Darwinistic view the situation is radically different. Here species don't depend on timeless essences, they are uniquely defined by their respective population, and due to evolution populations change over time. If we go back in time two closely related species which are clearly distinct today at some time merge in their common predecessor. The scale of originally distinct species was replaced by a phylogenetic tree. At branch points species split up into two separate populations to become distinct in future.

We are not in the position to directly follow the tree of evolution down to its roots. But how can we infer the biological degree of relationship between putative cousin species? There are several levels on which the biological "distance" between species can be estimated. The classical method is to compare the morphology. The form, size and existence of various organs can be compared. For parts of the body preserved in fossil records such comparison can be made even through history. Darwin's theory was based on such kind of data. Comparing modern and ancient species relicts Darwin arrived at a treelike relationship. Species can also compared on the level of cellular organization. /15/ The most quantitative measure of biological relationships is RNA, DNA and protein sequence analysis. Different parts of the genome of an organism have very different mutation rates. Genes coding for fundamental processes inside the cell, such as translating the DNA into protein sequences, are generally well conserved (Dawkins, 1986; Dayhoff, 1969; Eigen, 1992). Because no cell can live without those fundamental processes they must have evolved very early during evolution. They are very similar through all organisms. Such sequences are used to estimate "long distance" relationships. Parts of the genome subjected to intermediate mutation rates are used to estimate relationships of intermediate distances, e.g., among mammals. /16/ Certain parts, such as mitochondrial DNA, have very high mutation rates. Those are analyzed to understand the relationships within species, e.g., between human races. /17/

Neo-Darwinism predicts a specific kind of relationship between the species: the "relationship distances" should clearly form a tree. If the sequence distances of many sequences are compared, one can distinguish mathematically (Dopazo, et al., 1993), whether this distance network forms a tree, as required for neo-Darwinian theories, a star, i.e., all sequences originated from a very early single origin and since then developed independently in parallel, or some arbitrary network, which would indicate no evolutionary relationship at all between the sequences. The comparison of t-RNA, RNA, DNA or other protein sequences generally leads to in a tree like relationships between distantly related species (Dawkins, 1986; Dayhoff, 1969; Eigen, 1992). This treelike form of sequence distances is a strong argument in favor of neo-Darwinism. /18/

2.2.4) Population thinking as the basis for modern species definitions

A major distinction between classical and modern definitions of the biological species is the apparently complete rejection of essentialistic concepts in modern views. According to Ernst Mayr (Mayr, 1991): "Essentialism was not the only ideology Darwin had to overcome." Consequently, a new fundament of a species definition was adopted which does account for evolution:
The old species concept, based on the metaphysical concept of an essence, is so fundamentally different from the biological concept of a reproductively isolated population that a gradual changeover from one into the other was not possible. What was required was a conscious rejection of the essentialist concept... The first [difficulty to apply essentialistic concepts to life] was that no evidence could be found for the existence of an underlying essence of "form" responsible for the sharply defined discontinuities in nature. In other words, there is no way of determining the essence of a species, hence no way of using the essence as a yardstick in doubtful cases. The second difficulty was posed by conspicuous polymorphism, that is, the occurrence of strikingly different individuals in nature which nevertheless, by their breeding habits of life histories, could be shown to belong to a single reproductive community. The third difficulty was the reverse of the second one, that is the occurrence in nature of "forms" which clearly differed in their biology (behavior, ecology) and were reproductively isolated from each other yet could not be distinguished morphologically. (Mayr, 1982, p. 271, Text in square brackets added by the author)
For the classification of the different life forms no clearcut feature could be discovered which defines a species and necessarily distinguishes it from all others if not only present populations are considered but also the ancestors of the present ones. In contrast, one can clearly give a set characteristics which uniquely defines an electron. If all those characteristics are found for a certain particle one can be sure that it is an electron. These characteristics are timeless. They would have applied a billion years ago and will be the same within billion years. Such unchanging characteristics are not found in living systems. This situation becomes even more complicated by the existence of species where members show an extreme variability of their appearance, and also by others where the members of morphologically indistinguishable individuals belong to different reproductive communities, i.e., to different species!

A characteristic of important physical features is their time invariance; e.g., the law of the conservation of energy, the time invariance of fundamental laws, etc. In contrast, most important biological characteristics are the product of a long history. The physicist Max Delbrück states: "A mature physicist, acquainting himself for the first time with the problems of biology, is puzzled by the circumstance that there are no `absolute phenomena' in biology. Everything is time-bound and space-bound. The animal or plant or micro-organism he is working with is but a link in an evolutionary chain of changing forms, none of which has any permanent validity." (Mayr, 1982, p. 69) Such a dependence of populations on their own particular history is alien to a concept of a static world. Species which in classical biology were assumed to have perfectly been created by means of a first original couple have no history. They are perfect from the beginning, living in a harmonious, perfect universe. Only minor adaptions within a population are possible in such a view.

The historicity of the fauna and flora clearly distinguishes most fields of biology from physics and chemistry. The phenomena of aging of materials and the behavior of non-equilibrium dynamic systems, however, require to introduce history into physics and chemistry. Only recently those subjects obtained specific interest in physics and chemistry (Gell-Mann, 1994; Land, 1991; Prigogine, 1979; Prigogine and Stengers, 1981; Ruthen, 1993). In biology, however, the reference to the history is the rule and not the exception:

There is hardly any structure or function in an organism that can be fully understood unless it is studies against this historical background. To find causes for the existing characteristics, and particularly adaptions, of organisms is the main preoccupation of the evolutionary biologist. He is impressed by the enormous diversity as well as the pathway by which it has been achieved. He studies the forces that bring about changes in faunas and floras (as in part documented by palaeontology), and he studies the steps by which have evolved the miraculous adaptions so characteristic of every aspect of the organic world. (Mayr, 1982, p. 69-70)
This explicit dependence of life on its own history makes it difficult to apply the classical concept of essences which assumes that the form of a particular cat is defined by a timeless reality independent of details of the particular history of the ancestors of this cat.

Instead of referring to a timeless species essence the concept of species in modern biology is related to actually existing populations. A species is defined by an existing community of interbreeding individuals. Only recently it was recognized that this concept of species has much in common with the respective ideas of Aristotle (Mayr, 1982). According to Mayr the major difference between essentialistic and populistic species concepts is the emphasis on the individual:

Population thinkers stress the uniqueness of everything in the organic world. What is important for them is the individual, not the type. They emphasize that every individual in sexually reproducing species is uniquely different from all others, with much individuality even existing in uniparentally reproducing ones. There is no "typical" individual, and mean values are abstractions. Much of what in the past has been designated in biology as "classes" are populations consisting of unique individuals. (Mayr, 1982, p. 46)
Modern definitions of a species are based on a group of individuals being able to produce common fertile offsprings (Mayr, 1982, p. 263): "A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature." Of course there also exist other modern species definitions. These differences in the species definitions, however, are irrelevant of the purpose of this essay.

2.3) "Species" in classical and modern biology

In classical biology the species were thought to be defined and maintained by their species essence. This concept parallels the modern idea that, for instance, the chemical characteristics of a molecule are solely defined by quantum mechanics independently from particularly existing molecules. The species present today were assumed to be the offsprings of the first couples originated from their Creator. In this view, only an intelligent Creator could have produced such a manifold of purposefully well adapted organisms. This view remained largely valid until the middle of the 19th century. Biologists such as Cuvier (1769-1832) easily won disputes about evolution in favor of this classical understanding of biology (Mayr, 1982, p. 363ff).

Because of the findings made in biology and palaeontology the classical concept of species became more and more questionable. The biological populations inhabiting the earth were not always the same. They changed drastically during the geological history of this planet (Gould, 1994). The increasing number of facts pointing towards evolution of life made it more and more clear that the classical concept of species essence cannot explain the fact of the development of various populations of living individuals.

This situation led to a complete rejection of the classical concept of species essences. Today, species are defined as reproductively isolated populations occupying an ecological niche. The ability to interbreed and produce fertile offsprings is a necessary condition to account two members of different sex to the same species. The particular characteristics of a species are thought to be entirely defined by its gene pool and are maintained by the high fidelity of gene reproduction. According to this definition, species in modern biology have no timeless, independent existence, they are names used by human scientists to classify an interbreeding population. Thus, the Darwin's theory of evolution not only changed the theory of the appearance of the different organisms on earth, but by replacing essentialism by a nominalistic school of thought, Darwinism modified the whole philosophy of biology.



Notes

    /1/ During the 19th century there was not much interaction between philosophers and biologists. The "philosophy of sciences" concentrated nearly exclusively on physics and chemistry. Metabiology, i.e., the philosophy of biology, accompanying the development of the theory of the evolution of life was therefore formulated mainly by biologists such as Ernst Haeckel or by interested physicians such as Ludwig Büchner or Hermann Helmholtz.

    /2/ Although Hume in 1779 criticized the design argument he could provide no mechanism for the generation of the diverse order of life (Dennett, 1995; Sober, 1993). But without such a mechanism the argument remains valid; the existence of the complex order of life requires an explanation. Dawkins states (Dawkins, 1986): "But what Hume did was criticize the logic of using apparent design in nature as positive evidence for the existence of God. He did not offer any alternative explanation for apparent design, but left this question open."

    /3/ Such time invariance of the "laws of nature" is required to reconstruct the past (e.g., in cosmology) and to make predictions for the future. A physical description of changing laws requires a metalevel of time invariant laws ruling the time evolution at the metalevel to maintain the ability of the theory for retro- and prospection. This question will be discussed more detailed below.

    /4/ Translated into English: "There are as many species as originally created by the infinite being."

    /5/ Today most biologists would reject such a concept. In other fields, however, similar views are still hold today. Many scientists, for instance, would assume that the chemical characteristics of a particular molecule are entirely determined by quantum mechanics (Dirac, 1929). Whenever this molecule is formed, it shows exactly the same physical and chemical properties. This means that in chemistry one assumes a time invariant reality in form of the quantum mechanical laws which defines "chemistry" independently from actually existing molecules. Consequently, the properties of a molecule potentially exist even before it is formed in this universe for the first time. In the same sense one can understand the existence of species essences as an independent timeless reality, defining the biological characteristics of populations of organisms independent of actually existing members.

    /6/ These natural laws were considered to be secondary causes. The Creator Himself was the Primary Cause, but by means of the secondary causes He was believed to rule the world. The mechanization of the world culminated in the concept of Laplace that the world started long time ago and is now following its world trajectory as predicted by Newton's laws such as a clockwork. In this picture, the only role God may have is that of a First Mover. Laplace thought that his model would work even without such a first mover.

    /7/ Hermann Helmholtz studied medicine as well as physics and mathematics. He made important contributions to physics, chemistry and medicine. In 1847 he wrote his famous treatise about the conservation of energy. 1850 he measured the velocity of neuronal excitation along nerve fibers. He therefore showed that the neurons work by material means and do not require some special vital substance for their functioning.

    /8/ The essential difficulties of mechanics as a universal theory are discussed by von Weizsäcker (von Weizsäcker, 1986).

    /9/ At a naturalist's meeting in Göttingen 1854 the Swiss physiologist Jacob Moleschott explained that thoughts are secreted by the brain, as urine is secreted by the kidneys. This statement provoked a comment from the philosopher Hermann Lotze: "Listening to college Moleschott, one gets the impression that he is right." (Bloch, 1972, p. 289)

    /10/ Today Lamark is mostly known for his assumption that learned characteristics can be inherited. This idea does not go back to Lamark, but is was generally accepted by the scientists of his time (Mayr, 1978; Mayr, 1982). Also Darwin did not exclude inheritance of learned characteristics. He considered them, however, not as important driving forces of evolution.

    /11/ The probability for replication errors in RNA viruses is approximately a single error per gene, in the case of DNA viruses and higher organisms it is in the order of 1 error in 1000 genes (Eigen, 1993)

    /12/ After conception, the male and female chromosomes are to some extend mixed up. Few genes on the male chromosomes are exchanged by those from the female chromosomes. By this mechanism of crossing over the different genes of a population are continuously mixed up (Alberts, et al., 1989).

    /13/ The huge effect of cumulative selection can be illustrated by throughing dice to get 100 times the 6. On the one hand, if I take 100 dice and try to get 100 times 6 in a single stroke, on the average I would have to through 6100 = 7x1077 times. If I through every second the time of the universe would not be sufficient to get the requested result only once. On the other hand, if I take each dice of the 100 individually, through it until it shows a 6 and keep it than, I would have to perform about 600 strokes. In the first case it was an all or none selection. Only if all 100 dice would show the 6 in a single stroke, it would be selected. In the second case, the 6's were sampled cumulatively, one 6 was accepted after the other. Although this game is certainly not a good example to show the evolution of complex biological order, it clearly shows the huge distinction between "all and none" and cumulative selection.

    /14/ Neo-Darwinistic evolution requires the mutation rate, i.e., the number of mutations per generation, to obey certain limits. If it is too large, the genetic information defining a species will get lost within a few generations. If it is too small, only the locally fittest sequence of a given species will survive, but there will be no further progress. At the optimal mutation rate, not only the locally fittest sequence does survive, but also a large number of closely related ones. This set of sequences forms the socalled quasi species (Eigen, 1993). An other important property is that the sequence path between different but closely related species must not be too long. The probability to progress in the sequence space to increasingly complex biological forms of life must considerably above zero. The requirements of the fitness landscape to favor the progress of evolution in the sequence space are for instance studied by Kauffman. It can be shown from first principles that, if the fitness-sequence relation would be quasi-random, evolution would become impossible (Kauffman, 1995).

    /15/ There exist two types of cellular organizations: the primitive prokaryonts without a nucleus and the more complex eukaryontic cells where the DNA is packed into the cell nucleus (Alberts, et al., 1989; de Duve, 1996). The eukaryonts are assumed to have organized by means of the fusion of prokaryonts. There exist still some relicts of these ancient precursors, some organelles such as the mitochondria until today have their own DNA. All higher taxa, plants and animals, are formed by eukaryont cells. The agreement in the complex organization of all eukaryontic cells is understood to indicate that all eukaryontic taxa originate from a small group of eukaryontic cells.

    /16/ Between homo sapiens and chimpanzees about 98% of the DNA sequences are identical. This is commonly interpreted that the higher primates and homo sapiens share a common ancestor. There are biological essays available to estimate the distances between DNA or RNA sequences directly. According to such a measure of degree of relationship the closest living non-human relatives to homo sapiens are the chimpanzees (Sibley, et al., 1990).

    /17/ Mitochondria are organelles, the "organs" of the cells, which produce energy rich molecules designated ATP (adenosine triphosphat). This chemical energy stored in those molecules is degraded in many energy demanding processes inside the cells such as copying DNA or contracting muscle fibers. Those mitochondria have their own DNA. Because mitochondria lack the sophisticated proof reading machinery of its host cell the mutation rate of mitochondrial DNA is large compared to the mutation rate of the host's DNA. Recently mitochondrial DNA has been used to estimate the biological relationship between humans around the world (Wilson and Cann, 1992). According to this study modern homo sapiens originated about 200,000 years ago in Africa.

    /18/ There exist examples where we can study evolution "at work". The analysis of viral DNA, where the mutation rates are sufficiently large to make evolution visible, favors the treelike relationship (Dopazo, et al., 1993; Eigen, 1993). Because in this case the time point of the outbreak together with the virus is well documented, in this case it is clear that indeed evolution has occurred.

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