What is the theoretical framework of modern biology? If we would question a professional biologist, or even an eager reader of popular books on biology, we would await the answer: Darwinism, that is, Charles Darwin’s theory of evolution by natural selection. But this answer is not entirely precise. Of course, Darwin’s position in the history of biology can be compared to that of Galileo Galilei in the history of physics. But it would be surprising if the nearly 150 years since the first publication of Darwin’s main work, On the Origin of Species (1859), had not brought forth new discoveries and fresh theoretical ideas that changed the original version of Darwinism. And, indeed, this was the case. Therefore, a terminologically more precise answer to our initial question would be that Neo-Darwinism is the theoretical framework of modern biology.
What are the most important theoretical developments after Darwin that shaped Neo-Darwinism? The title Neo-Darwinism was used in the history of evolutionary thought to denominate, above all, the following three theoretical developments:
- August Weismann’s panselectionism assumed that natural selection works, not only on the level of organisms, but also on the levels of chromosomes, cells, and populations (since the 1870s).
- Theoretical population biology constructed mathematical models for the workings of natural selection in the history of life (since the 1910s).
- The Modern Synthesis integrated all subdisciplines of biology into an overall picture of Darwinian evolution (since the 1930s).
Contemporaries called the evolutionary thought of the German biologist August Weismann (1834-1914) Neo-Darwinism (or even Ultra-Darwinism) because he extended the range of the Darwinian principle of natural selection so that it comprised the struggle for existence, not only between organisms, but also between populations, cells, and chromosomes. Weismann did much experimental research on specific biological phenomena in the light of evolutionary theory, and he interpreted his experimental results with the help of courageous conceptualizations. That is why Weismann can be called the second most important Darwinian of the 19th century (excelled only by Darwin himself).
We shall give a first impression of the importance of Weismann’s work by describing how he embedded the empirical falsification of a then widely accepted genetic theory into a new conceptual framework, and how he used this framework to sketch a radical Darwinian picture of the evolution of life.
Weismann tested experimentally, in the years 1875 to 1880, the theory of the inheritance of acquired characters. The idea that organisms transmit knowledge acquired by practice and learning genetically to their offspring was developed into a biological theory by the French biologist Jean Baptiste de Lamarck at the beginning of the 19th century. From edition to edition of On the Origin of Species, Darwin more and more became a Lamarckian in genetics. Weismann tested Lamarckism by experimenting with lines of mice, whose tails he cut generation after generation. But he did not succeed this way in breeding mice that have shorter tails as compared to mice of the same origin whose tails were never cut.
How to explain this result? Weismann developed, in the first half of the 1880s, the theory of germplasm. He distinguished two types of cells in every organism: germ cells and somatic cells. The germ cells (in sexually reproducing organisms, the male sperm and the female egg) secure the reproduction of an organism by means of the germplasm they contain in their nuclei. It is, in modern terms, nothing else than the carrier of genetic information. All other functions of the organism are fulfilled by somatic cells.
Then Weismann postulated that the differentiation of somatic cells during the development of an organism is controlled by the structure of the germplasm. Additionally, he proposed that somatic cells do not have any retro-effects on the germplasm, so that Lamarckian inheritance of acquired characters is not possible. The continuity of the germ line from generation to generation is thus declared to be the fundamental base of evolution. Phenotypical variation of somatic cells is caused by genotypic variation.
What does, in turn, cause genotypic variation? Because Weismann considered the components of the germplasm (which he could identify with the chromosomes) as being little self-replicating and metabolizing systems, he came to the conclusion that the struggle for existence, and thus natural selection on the molecular level of the germplasm (“germinal selection”), is the ultimate cause of genotypic variation.
In his famous lectures on evolutionary theory, which filled three voluminous editions that were published between 1902 and 1913, Weismann integrated his genetic determinism into a radical Darwinian picture of evolution. He extended the explanative scope of the principle of natural selection from the level of organisms, the level on which Darwin had studied it, to other levels of the hierarchy of biological organization. Weismann’s panselectionism assumes that natural selection works also on the level of populations, organisms, cells, and germplasms.
By so doing, Weismann could not only analyze separately the constraints on selection for every level and describe the role of each level in the development of an organism and in the history of life, but also have a synthetic look on the interactions of all those levels in an ecological environment. So, events concerning whole populations can strongly influence the dynamics of selection on lower levels, such as if a natural catastrophe were to isolate a population. Therefore, every level constitutes a boundary condition on the mechanism of natural selection on all other levels.
Weismann represented selection-driven evolution as the natural dynamics of a self-regulating hierarchical system of biological organization. This system has a center of gravity: germinal selection between chromosomes. Weismann’s emphasis on genetics lets us understand why the most radical of today’s Neo-Darwinians, like Richard Dawkins, still accept the name “Weismannism” as an adequate denomination of their evolutionary thought.
Whereas the overall architecture of Weismann’s evolutionary thought of evolution has an up-to-date outlook, important parts of his theory of inheritance were falsified by empirical research on molecular genetics. The functional distinction between germ cells and somatic cells had to be reshaped into a difference between nucleic acids and proteins, which are components of every cell. Weismann’s idea of germinal selection as resulting from a direct struggle for survival between chromosomes is, taken literally, completely false.
But the main obstacle to Weismann’s project of constructing a panselectionistic picture of evolution centered in genetics was not his lack of biochemical knowledge. He simply knew too little about formal genetics. Weismann learned about Gregor Johann Mendel’s genetics of discrete hereditary units, first published in 1866, just when it was rediscovered in 1900, although he had prepared the ground for this rediscovery by postulating the existence of such units in the germplasm.
Weismann lived too early to get the chance to base the Darwinian theory of evolution on Mendelian genetics. The feat of doing this and of developing a new kind of Neo-Darwinism was performed by the three founding fathers of theoretical population genetics, Ronald Aylmer Fisher (1890-1962), John Burdon Sanderson Haldane (1892-1964), and Sewall Wright (1889-1988), who published their foundational works between World Wars I and II.
Theoretical population genetics is the biological discipline that tries to construct a formal theory of evolution, which must be inspired and controlled by empirical research. A theoretical population geneticist understands natural selection as a hypothesis that can be established by mathematical models of the origin of evolutionary mechanisms.
Fisher, Haldane, and Wright began to describe the most various biologic phenomena by their quantitative effects on the frequency of genes in populations with Mendelian inheritance. Thereby, they founded theoretical population genetics as the core discipline of this second type of Neo-Darwinism. Because a complete survey even of the works of those founding fathers is not possible here, I shall concentrate on Fisher’s theory, who is well known for his work not only in mathematical genetics, but also in statistics and experimental design.
Fisher’s fundamental insight in evolutionary theory was that natural selection is not a principle that must be axiomatically posited instead of being deduced from natural laws. If biologists knew the laws of variation and heredity, could embed them in a model of ecological dynamics, and had sufficient data about present-day biodiversity at their disposal, they should be able to deduce natural selection as the resultant mechanism by which life on Earth has evolved.
The first sentence of Fisher’s main work in genetics, The Genetical Theory of Natural Selection (1930)— “Natural selection is not evolution”—contains implicitly his fundamental insight. Fisher saw that natural selection is an abstractly describable mechanism that can be described formally without referring to the history of life on Earth. But its mathematical description also states which conditions real evolution must meet if natural selection is to be accepted as an explanatory rule in biology.
On the basis of his fundamental insight, Fisher formulated a theorem that he called “the fundamental theorem of natural selection.” Why fundamental? Fisher drew an analogy between his theorem and the second law of thermodynamics, which describes statistically how a measurable property of an aggregate of units, the specific nature of which is not important for the formulation of the law, increases constantly. And the same is true for Fisher’s fundamental theorem. It describes the adaptive improvement of organisms that is caused by natural selection in a population, which is supposed to live in a constant environment and within which do not happen any arbitrary fluctuations in the gene pool.
Fisher’s theorem states that “the rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time.” What does this mean in more informal terms?
We have to remember that the fitness of an organism is defined as the number of its adult offspring. If an organism is better adapted to its environment than its rivals, its chances are better to also have a higher fitness. Transposed to the genetic level of a population, this means that the fitter an organism, the higher the relative frequency of its genes in the genetic pool of its population.
Now, natural selection causes the average fitness of our idealized population to always increase over time. But how fast? Fisher’s fundamental theorem gives the answer. The more the organisms in the population vary in fitness, the faster this increase will turn out. This is intuitively evident. If there is, for example, a difference in fitness between two subpopulations, the subpopulation with higher fitness will have more offspring than the one with lower fitness proportional to their difference in fitness. Eventually, the offspring of the fitter subpopulation will make up the whole population. The average fitness of this new population will then be, at least, the average fitness of the former subpopulation with higher fitness.
Yet Fisher was not a naive selectionist. From his point of view, the ecological environments in which natural selection is working permanently influence considerably the evolutionary process. There are, for example, environmentally caused mutations that will probably have negative effects on the fitness of organisms. In general, all ecological changes and the simultaneous evolution of competitors in the struggle for existence set up new conditions on the adaptedness of an organism and change the average fitness of its population. Fisher called this ecological component of the total change in fitness of a population over time the “deterioration of the environment,” because it represents destructive environmental effects on well-adapted organisms.
After Fisher’s generation came to an understanding of the hypothesis of Darwinian selection as a deductive consequence of a formally describable natural dynamics, the history of theoretical population genetics has led to mathematically more and more complex models of natural history. Thereby, this second type of Neo-Darwinism formalized the Weismannian picture of evolution, which is to be explained by the dynamics of the hierarchy of biologic organization with a center of gravity on genetics.
But the theoretical population geneticists also found astonishingly simple formulations of fundamental evolutionary laws. For example, Fisher’s separation of two types of evolutionary factors, natural selection and environmental effects, can be mathematically expressed in a plain formalism using the statistical concepts of covariance and expectation value. Such an example shows that theoretical population genetics still is a very important part of evolutionary thought because it uses a high level of abstract reasoning, comparable to the use of mathematics in physics, to convey a vivid understanding of the fundamental processes of the history of life. So, it is not surprising that the most important Neo-Darwinians after World War II tried to base their theories on models from theoretical population genetics.
The formal kind of Neo-Darwinism founded by Fisher, Haldane, and Wright sometimes seems to be more similar to a mathematical than to a biological discipline. One has to learn a lot of calculus and be able to program computers in order to test the mathematical models by simulations. These models show how natural selection causes the gradual evolution of populations by calculating the change in gene frequencies. But how can they be related to the empirical reality of living populations, which the naturalist observes? What do they tell us about the origin of today’s biodiversity, in which the field biologist is interested? Isn’t theoretical population biology just a too-impoverished picture of nature—even on the genetic level? Does it think too mechanistically by assuming that everything is built on the base of single genes, the effects of which have to be simply added in order to generate a complex organism?
One of the men who asked all these questions very intensely since the 1930s was an ornithologist with extensive experience in field research, a staunch Darwinian with a broad knowledge of the history of biological thought, and a theoretical biologist with a deep interest in the foundations of evolutionary thinking. His name is Ernst Mayr (born 1904 in Germany), and he is one of the builders of the “Modern Synthesis,” which was seen as being a renewal of Darwinian thought—a modern kind of Neo-Darwinism.
The English biologist Sir Julian Sorell Huxley (1887-1975), the grandson of Darwin’s “bulldog,” Thomas H. Huxley, and the brother of the famous writer Aldous Huxley, published in 1942 his book Evolution: The Modern Synthesis and established thereby the “official” title for this kind of Neo-Darwinism. But why “Synthesis”?
The aim of Mayr and his colleagues was to counteract the specialization of biological disciplines by showing how the fundamental ideas of Darwinism were able to bring them all back in fertile connection. For example, the gap between abstract models of theoretical population genetics and naturalist knowledge should be bridged. This could be done only if, on one hand, the naturalists would learn the theoretical geneticists’ models of gradual evolution, and only if, on the other hand, the theoreticians would take into account the naturalists’ knowledge about how populations in the wild behave.
The unification of theoretical modeling and empirical research began in the 1930s. The Russian experimental geneticist Theodosius Dobzhansky (1900-1975), who worked since 1927 in Thomas Hunt Morgan’s famous Drosophila research group first at Columbia University and then at the California Institute of Technology, connected his empirical research to Darwin’s evolutionary theory in his book Genetics and the Origin of Species (1937). This publication has to be honored as the starting signal of the Modern Synthesis. The programmatic allusion of the title of Dobzhansky’s book to the main work of Darwin is clear. The same is true for the following important document of the Modern Synthesis.
Ernst Mayr, who worked since 1932 at the American Museum of Natural History in New York, wrote an important book on questions of taxonomy and gave it, in parallel to Dobzhansky’s, the title Systematics and the Origin of Species (1942). In it, he tried to answer the question of speciation: What are the processes responsible for the origin of new species? This question was not systematically treated by Dobzhansky.
It is, by the way, an interesting question whether it was necessary that two biologists of continental European origin and traditional taxonomic training were implanted into the Anglo-American environment of genetic thought in order to start the unification project of the Modern Synthesis.
That the Modern Synthesis was swiftly successful in the research community of evolutionary biology became evident at a 1947 conference in Princeton, New Jersey, where some of the most important biologists gathered. The proceedings of this conference were published under the title Genetics, Paleontology, and Evolution (1949), and they document that the Modern Synthesis ruled over the evolutionary Zeitgeist. This title brings together three main aspects of the history of life seen from the Neo-Darwinian perspective: first, the base for the transmission of biological information from one generation to the next, whereby evolution is made possible; second, the fossil data to reconstruct the concrete course of evolution on Earth; and third, the resulting process of evolution itself.
Today, Mayr considers the Modern Synthesis to have been confirmed in the past 60 years by innumerable empirical findings, especially in the field of molecular genetics (like the discovery of deoxyribonucleic acid as the carrier of genetic information). Yet the history of Darwinism did not stop after the Modern Synthesis found a codification at the end of the 1940s.
An important tendency was that the evolutionary biologists came, compared with the Modern Synthesis, to an even more radical understanding of the foundation of the history of life on genetic processes, so that we can even diagnose a return to Weismann. This “Neo-Weismannism” uses formal models of evolutionary processes (via theoretical population genetics), and its genetic theory is based on the knowledge of the biochemistry of inheritance (via molecular genetics).
Three kinds of Neo-Darwinism arose successively in the history of biology and are still important for current debates on evolutionary theory. They show many important differences but have also some main features in common, such as their emphasis on natural selection and their theoretical base in genetics.
The bold outlook of today’s Darwinism should not suggest that the foundational problems of biology are all solved. Nowadays, it is clear that mechanisms other than selection are effective in evolution. One important example is, on the molecular level, genetic drift, that is, the spread of selectively neutral genetic information in the gene pool of populations. How these mechanisms played, and still play, together in the history of life is a heavily debated topic.
Perhaps Darwin’s theory of natural selection need not be supplemented only by other evolutionary mechanisms. Perhaps it must be integrated together with them into a new conceptual framework. Then, Darwin would remain biology’s Galilei, but some day he would be succeeded by its Newton.
References:
- Dobzhansky, T. (1937). Genetics and the origin of species. New York: Columbia University Press.
- Fisher, R. A. (1999). The genetical theory of natural selection (variorum ed.). Oxford, UK: Oxford University Press. (Originally published 1930)
- Gayon, J. (1998). Darwinism’s struggle for survival: Heredity and the hypothesis of natural selection. Cambridge, UK: Cambridge University Press.
- Huxley, J. S. (1942). Evolution: The modern synthesis. London: Allen and Unwin.
- Jepsen, G. L., Mayr, E., & Simpson, G. G. (Eds.). (1949). Genetics, paleontology, and evolution. Princeton, NJ: Princeton University Press.
- Mayr, E. (1942). Systematics and the origin of species. New York: Columbia University Press.
- Weismann, A. (1904). The evolution theory (2 vols.). London: Edward Arnold.
