Modern Darwinism, also known as the “modern synthesis” or “neo-Darwinism,” is a comprehensive theory of evolution that combines Darwin’s theory of natural selection with principles of Mendelian genetics. Although the theory was established in the 1920s to 1940s and biology has undergone profound and rapid changes since that time, neo-Darwinism is still considered to be a generally accepted paradigm of biological evolution.
A basic idea of neo-Darwinism is that it is a two-step process. The first step is a random generation of genetically determined variance in population of individuals, followed by second step, the selection of those individual variants by environments that are relatively more successful to survive and reproduce. In recent years, we have been witnessing an expansion of neo-Darwinian principles beyond biology: to cosmology, medicine, economics, computing, neurology, psychology, psychiatry, modeling of cultural development, and history of science. Neo-Darwinian algorithm is applicable not only to living organisms but also to any system (whether made of molecules, organisms, digital strings) if the following conditions are satisfied:
- There is a population of entities capable of multiplication and variation.
- There is a process of selection by a limited environment in which better-adapted entities multiply faster than others.
The Reconciliation of Mendelism and Darwinism
Soon after the publication of On the Origin of Species, the Achilles heel of the whole theory of evolution via natural selection was recognized in an inheritance theory used by Darwin. Darwin worked with the blended-inheritance theory commonly accepted by his contemporaries, which, however, logically undermines the very nature of the evolutionary mechanism as suggested by Darwin, an accumulation of a small inherited adaptive changes through a long time. If inheritance is blended, any accumulation of inherited variations, which is a key element in the gradual building of complex adaptive structures, is impossible. For if heredity is of a blending type, any new variation is halved in every generation, and in fact it disappears very soon. Darwin tried to answer the problem by his own ad hoc theory of heredity (called pangenesis), a totally speculative one. He flirted with his own version of the inheritance of acquired characteristics (despite the fact that he was strongly critical of Lamarckian theory of evolution), but he never was happy with it. Paradoxically, at the same time, the right answer was already there—in Gregor Mendel’s theory of inheritance. Mendel came up with the idea of hereditary “atoms” (he called them “factors”), which cannot blend, but can only combine in a particular rations. Mendel’s work was published in 1865, but unfortunately it remained unknown (although not inaccessible to Darwin, as well as to almost all his contemporaries) until it was rediscovered two decades after Darwin’s (1882) and Mendel’s (1884) deaths at the beginning of 20th century.
German biologist August Weismann (1834-1914) is now known as a forerunner of neo-Darwinism, since he recognized that the Darwinian mechanism of evolution can work perfectly without any kind of Lamarckian inheritance of acquired characteristics, and all it needs is a generation of random changes in hereditary material and selection of those individuals who carry not the best possible adaptations, but better than their competitors.
When principles of Mendelian genetics were rediscovered and accepted by biologists during the first two decades of the 20th century, they were first completely misunderstood as a crucial argument not in favor but (paradoxically from today’s perspective) against Darwinism. The argument went like this: Mendelian ratios of inherited characteristics display nonblended discrete inheritance, which is also responsible for discrete differences not only between individuals within a species but also between species. New species are generated by a big change in hereditary material, which is responsible for large differences in phenotypes, and therefore natural selection is not needed to explain speciation.
However, the schism between Mendelian genetics with mutational saltationisms at one side and natural selection with gradualism at the other side did not last for a long time. A reconciliation between Darwinism and Mendelism was achieved by the independent theoretical works written around 1930 by Ronald A. Fisher (1890-1962), John B. S. Haldane (1892-1964), and Sewall Wright (1889-1988). As early as 1918, R. A. Fisher published a paper in which he showed that continuous character distribution in populations as observed by biometricians can be explained by discrete Mendelian genetics. He expanded his mathematical exploring of evolution in the book The Genetical Theory of Natural Selection (1930). In a similar way, J. B. S. Haldane summarized a series of his theoretical papers on evolution in his book The Causes of Evolution (1932), and S. Wright published a long paper called “Evolution in Mendelian Populations” in 1931.
These works inspired other biologists in the 1930s to 1940s from a variety of different biological disciplines to contribute to this project of reconciliation: from field and laboratory studies in population genetics (T. Dobzhansky, E. B. Ford), paleontology (G. G. Simpson), and systematics (E. Mayr). The reconciliation is now known under several names: neo-Darwinism, the synthetic theory of evolution, and the modern synthesis.
Changes of Gene Frequencies in Gene Pools
The key concept for the modeling of evolutionary dynamics is the gene pool, a concept that comes from quantitative population genetics and became a main conceptual tool of reconciliation between Mendelism and Darwinism, mentioned above. A gene pool is a collection of all genes, in fact, all gene variants (alleles), shared by all members of population. From a philosophical point of view, Darwin is thought to have brought population thinking into species conceptualization in order to replace traditional typological species concept. For him, species were populations of different individuals.
A gene pool is a scientific abstraction, since genes do not actually exist freely in some sort of pond, but they always exist within living organisms. As with every abstraction, the gene pool model presupposes a set of ideal conditions, which are hardly met in nature. Despite this, the model is no less successful than other useful population models in descriptions of a real population of organisms and in predictions of the dynamics of its change, as for example, a concept of ideal gas in physics. In fact, ideal conditions of the gene pool helped biologists to recognize quantitative limits for stability and evolutionary change of real populations. These conditions are as follows: the absence of mutation; no migration of genes from other gene pools; the population size is large enough (effectively infinite), so there is no a sampling error in a process of setting up a new gene pool by reproduction; combinations of gene variants into genotypes (as result a of mating) are random; and finally, there is no selection—all gene variants have the same probability to have their own copies in next generation. As it was shown independently in 1908 by English mathematician G. H. Hardy and German physician G. Weinberg, if these conditions are met, frequencies of gene variants in gene pool are stable through generations and there is no evolution, the statement known as the Hardy-Weinberg theorem. It can be demonstrated that the stability of gene frequencies in successive gene pools is a result of bionomic distribution.
Slower or rapid changes in gene frequencies of population are the results of parameter values of processes destabilizing the Hardy-Weinberg equilibrium: mutation, migration, nonrandom mating, limited size of population, and selection.
An objection can be raised that all changes of allele frequencies in a gene pool are only of small importance for evolutionary theory, because they explain what is called “microevolution,” or changes within borders of the same species. But “true” evolution has to do with the origin of a new species, and big changes. The neo-Darwinian answer to this objection is that gradual accumulation of microevolutionary changes over a long period of time leads to big changes, so big that new species arise. For example, a new gene pool can be set up by the gradual accumulation of microevolutionary processes after an original population divides into two subpopulations that are isolated geographically. In such a way, they in fact form two separate gene pools, which differ in their gene compositions so much that they cannot fuse together later when geographic barriers disappear and populations physically unite (allopatric speciation). Or a breeding barrier can arise within the original population in a way that a new gene pool is set up inside the old one (sympatric evolution), without geographic isolation. Evolutionary biologists spent some time quarreling over which type of speciation is the right one, until they found that both are present in nature.
Impact of Molecular Approach to Modern Darwinism
Since early the 1970s, the use of modern molecular biology techniques such as gel electrophoresis, restriction fragment length polymorphism (RFLP) analysis, and recently, DNA fingerprinting with the help of polymerase chain reaction (PCR), protein, and mostly DNA sequencing, all together with the use of computer power, have revolutionarized our understanding of past evolutionary history and Darwinian mechanisms of the evolutionary process.
Molecular phylogenetic studies have helped to settle some issues of phylogenetic relationships that were impossible to solve using traditional methods of comparative morphology, anatomy, physiology, or behavior. The range of molecular approaches in reconstructing phylogenies is incredible—from very rapid recent evolution of viruses like HIV to the origin of the three domains of life (bacteria, archea, and eukaryotes). New phylogenetic trees based on protein or DNA sequence homology were constructed. In many cases, they were in good agreement with trees constructed by traditional methods; however, in some important cases, they revealed surprising differences, as, for example, in the case of human evolution. Based on various molecular data, scientists estimated that African apes and humans diverged from each other about 5 million years ago, not 15 or even 30 million years ago, as it had been estimated from fossil record evidence. These data suggest that human lineage diverged after the separation of the orangutan lineage, but before the separation of chimpanzees and gorillas.
Analysis of sequence differences in mitochondrial DNA coming from recent human populations from around the world led to a reconstructed phylogenetic tree with a single root, which represents a common maternal ancestor (“mitochondrial Eve”), living 120,000 to 300,000 years ago. A new powerful technique (mostly PCR) enables scientists late in the 20th century to isolate fragments of mitochondrial DNA from the Tyrolean Ice Man, an Egyptian mummy, or a Neandertal skeleton (so-called molecular paleontology). Since Neandertal DNA differs from human DNA about 4 times more than the difference between living humans, it is reasonable to conclude that Neandertals must have diverged from a common ancestor with humans about 550,000 to 690,000 years ago, which means that Neandertals were probably a separate species.
Although there is still an ongoing debate about the methodology of molecular approach (for example, how accurate is “a molecular clock”), and much more data are needed, there is no doubt that molecular anthropology is receiving growing respect among scientists as their methods are rapidly improving.
Neutralist-Selectionist Controversy Over Molecular Evolution
One of the most important conceptual challenges to neo-Darwinian paradigm came from the theory of neutral molecular evolution proposed by Japanese geneticist Motoo Kimura (1924-1994) in November 1967, and published a year later. It started with the controversy over how to interpret available first indirect estimates of genetic variability based on protein polymorphism, which had started to be measured by gel electrophoresis. According to Kimura, the level of measured protein variability was too high to be explained by natural selection, and he believed that the overwhelming majority of genetic variability at the molecular level (DNA, RNA, and proteins) is due to random accumulation of selectively neutral mutations. With the help of population mathematical models, he tried to demonstrate that the way these neutral mutations are lost or fixed in gene pools is due to a sampling-error process (called “random genetic drift”). It has been known before that sampling errors are at work when offspring gene pools are created from gene pools of parents, but it was thought that it is significant only in small populations undergoing population bottlenecks. But Kimura pointed out that sampling errors are significant even in large but finite populations. He was not rejecting the role of natural selection on genes responsible for adaptations to the environment. But according to him, a process of neo-Darwinian selection is responsible for only a small fraction of observed genetic variability and the great majority of mutant substitutions at the molecular level are due to random fixations of selectively neutral mutants.
The support for the theory of neutral molecular evolution came later from the revealing of noncoding sequences of DNA as introns (nontranslated spacers within genes) and pseudogenes (a sort of gene wreck). These DNA sequences ignored by natural selection are accumulating mutations at a higher rate than coding sequences, as it was predicted by neutral theory. At present, the neutral theory is a widely accepted theory for the evolution of those parts of DNA that are selectively neutral.
Punctuated-Equilibrium Theory
In 1972, American paleontologist Niles Eldredge and Stephen Jay Gould opened the discussion about the tempo of macroevolutionary changes. They argued that the paleontological evidence does not support a classical Darwinian view of gradual transitions. They insisted that what is actually seen in fossil records are long periods of morphological stability (stasis), which are punctuated by short (in geologic terms) periods of rapid changes. They believed that this pattern cannot be explained in a satisfying way, referring to gaps in fossil records, but that in fact this fossil pattern reflects an evolutionary process itself in which new forms appear rather suddenly. They proposed a new model of evolution called “punctuated equilibrium.”
In the debate that followed, biologists discussed whether “revolutions” in phylogeny caused by micro-mutations (small changes in DNA sequences like base-pair substitutions, deletions, and insertions) or larger genetic changes (macromutations), such as various chromosomal rearrangements or movements of transponsable genetic elements across a genome, are needed in order to explain large morphological changes. Critics of the punctuated-equilibrium model argued that revolutionary changes are not in fact revolutionary and it all is a question of scaling. They were theoretically demonstrating that periods of “sudden” changes are long enough to be explained by the accumulation of micromutations and that gaps in fossils series really reflect incompleteness of the records.
A new perspective was brought to the debate by a recent advance in interdisciplinary research in evolutionary developmental biology, or “evo-devo.” On one hand, it supports the view that very small genetic changes can become amplified during organismal development into major morphological differences at the order of higher taxons. On the other hand, it also started to provide growing evidence showing how various macromutational changes, such as gene duplications, chromosomal rearrangements, and genome reshapings, could be responsible for major evolutionary events.
Neutral theory and punctuated-equilibrium theory were proposed as theories of “non-Darwinian” evolution, and they are a fundamental challenge to neo-Darwinism if we understand it in the narrow sense of a few limited ideas. But if we understand modern Darwinism in the sense of a broader framework for the evolutionary theory that combines randomness with selection, then both challenging theories can be incorporated into it. Randomly generated micro- or macromutations are fixed in gene pools either randomly, if they are selectively neutral, or by natural selection.
References:
- Lewin, R. (1997). Patterns in evolution. New York: Scientific American Library.
- Majerus, M., Amos, W., & Hurst G. (1996). Evolution: The four billion year war. Edinburgh: Longman. Ridley, M. (2003). Evolution. Oxford: Blackwell Science.
- Strickberger, M. W. (2000). Evolution. Sudbury, MA: Jones & Bartlett.
- Wen-Hsiung, L. (1997). Fundamentals of molecular evolution. Sunderland, MA: Sinauer.
