What do you think of Charles Darwin

Charles Darwin: The theory of evolution put to the test

Ultimately, it took a new generation of researchers to break the dogma. One of the most important theoretical considerations here was that there will be competition in a population that has optimally adapted to an environment. Most individuals then wear the same adaptation and thus compete for the same resources. That makes the actually advantageous adjustment a disadvantage again. You can compare it to gold diggers who have discovered a new vein. The first have great prospects of profit, but when many more are added, the costs of the mutual rivalry eventually outweigh the costs. It can then make sense for individuals not to look for gold anymore, but to open a saloon or a shop, for example.

Something similar happens in natural populations. Individuals with genetic variances that allow them to utilize new resources have an advantage over the crowd, which depends on the main resource. However, because of the constant flow of genes, this does not immediately lead to a split into separate species. Another factor must be added - such as the preferred pairing of identically adapted individuals (the »assortative pairing«) or an ecological gradient, for example spatial differences in ambient temperature, humidity or soil nutrients. Such factors can drive the separation of gene pools and thus enable what is known as adaptive speciation, much faster than the allopatric mechanism. This explains why numerous species are constantly splitting into new ones without any strict spatial separation. Species formation is therefore not only a passive spatial phenomenon, but also an actively ecological one - just as Darwin and Wallace had imagined.

Return to Lamarck?

Even before Darwin, the French naturalist Jean-Baptiste de Lamarck (1744–1829) was one of the first to recognize that species are not immutable, but that they evolve through adaptation to the environment. He assumed that organisms would acquire properties during their lifetime which they would then pass on to their offspring.

Darwin also assumed such a mechanism and brought it into connection with the cell theory. He suspected that every cell in the body would secrete small germs, so-called gemmulae, which circulate in the organism, are passed on to the next generation and thus shape the offspring. The discovery of the germline in the late 19th century by the German doctor August Weismann (1834–1914) contradicted this. According to this, the germ cells (sperm and egg cells), as the founders of the next generation, are clearly separated from the body cells. The Mendelian rules and the knowledge on which mechanism inheritance is based also seemed to refute the passing on of individually acquired traits to the offspring.

But this picture has changed fundamentally in the meantime. We are now familiar with molecular processes that ensure that acquired characteristics are carried into the next generation: the so-called epigenetic imprints. They do not affect the sequence of letters in the DNA itself, i.e. its base sequence, but occur through chemical changes to the DNA - such as the transfer of methyl groups to DNA building blocks. In a sense, this flips a regulation switch that determines how active the affected gene is.

Epigenetic imprints ensure, among other things, that blood, liver or nerve cells perform different functions in the organism, even though they all carry the same DNA sequence. As Oliver Rando from the University of Massachusetts and Rebecca Simmons from the University of Pennsylvania demonstrated in a systematic review in 2015, epigenetic imprints that arise from environmental influences - for example from a changed food supply - can enter the germline. They then persist in the next generation, even if their trigger has ceased to exist.

Probably this is done in a way that is reminiscent of Darwin's Gemmulae. All body cells secrete small vesicles called exosomes that migrate through the organism. They contain parts of the cells from which they originate - including regulatory RNA molecules that can epigenetically modify DNA. In the meantime, at least for sperm, it is known that they ingest such vesicles during their maturation, which makes it possible to transfer epigenetic information to the offspring. In 2019, researchers led by Wei Zhou from the University of Newcastle, Australia, showed that exosomes from the epididymis of mice couple to maturing sperm and transfer molecules to them. Another team headed by Lucia Vojtech from the University of Washington demonstrated in 2014 that human sperm contains countless exosomes that are loaded with RNA molecules that are believed to have a regulatory function. While this is not exactly the mechanism Darwin envisioned, it does make it clear that the body and the germ line are not entirely separate. In a systematic review from 2019, the biologist Upasna Sharma from the University of California, Santa Cruz, compiled more than 150 studies that support the concept of cross-generational transmission of epigenetic information.

In view of these findings, the advocates of an "extended synthesis" call for the theory of evolution to be fundamentally revised. Their argument: Epigenetic imprints should significantly accelerate the adaptation to new environmental conditions, which requires that Fisher's population genetic formulas be adapted. However, it is in the nature of epigenetic changes that they do not affect the DNA sequence. Therefore, their effects can usually only be demonstrated for a few generations, after which they are lost again, as a team led by neurobiologist Leah Houri-Zeevi from Tel Aviv University has shown in 2020. Long-term effects are therefore not possible.

But precisely in these lies the key to understanding evolution. Opponents of the theory of evolution have always argued that a number of adaptations are far too unlikely to come about through natural selection alone. But Fisher demonstrated as early as 1954: A combination of selection and stable inheritance over many generations makes even the most unlikely events possible. He compared this to the task of finding a specific proton in the entire universe, for which he has an extremely low chance of success of 10-79 stated. The natural selection of a genetic variant, which only brings an advantage of two percent per generation, can bring about such an unlikely adaptation within 10,000 generations. Consequently, evolution can only be explained by means of combined selection and stable inheritance over long periods of time.

Epigenetic imprinting is retained for a few generations at best and therefore does not require any fundamental expansion of the theory of evolution. But it may be useful to explain what is known as phenotypic plasticity. What is meant is the fact that it is not only the DNA sequence that determines the appearance of the organism, but also the environment. It is not clear whether phenotypic plasticity accelerates or slows down evolutionary adaptation. Both seem conceivable. Thanks to epigenetic changes, entire populations could adapt more quickly to new environments. On the other hand, epigenetic mechanisms may reduce the selection pressures on the DNA sequence and make long-term adaptations less likely.

The genotype-phenotype relationship

Another argument of the advocates of an extended evolutionary synthesis concerns the question of how evolutionary leaps in development of the phenotype arise. This is also a well-known problem, but with the discovery of developmental control genes it has been given new nourishment. A whole line of research, evolutionary developmental biology, Evo-Devo for short, deals with the role that individual development plays in evolutionary processes. Ultimately, she is concerned with the - largely unanswered - question of how the linearly arranged, one-dimensional information of the DNA sequence produces the three-dimensional structure of the phenotype. Experts refer to this as the genotype-phenotype relationship.

Evo-Devo researchers have shown: A changed activity of development control genes can lead to new body shapes, even to new body plans - for example, eyes instead of antennae in flies. Is this the key to understanding what is known as macroevolution, the sudden emergence of new forms of life as some fossil sequences seem to suggest? Does macroevolution follow different laws than we have known before? Even after around 30 years of intensive Evo-Devo research, there is still no proof of this. A possible counter-argument would be: Mutations in development control genes usually lead to so many changes at the same time that they almost always have a harmful effect on the affected individuals. Large changes that are beneficial would therefore be more likely to be achieved in many small steps. Gradually, as postulated by Darwin and Wallace.

There will only be more insight into this problem when the genotype-phenotype relationship is better understood. A large scientific consortium led by bioinformatician Andrew Wood from the University of Exeter came to the conclusion in 2014: Individual genes with their natural variants hardly influence the phenotype. Instead, it is combinations of thousands of genes that determine the appearance of the organism, such as body size in humans. Because their variants are randomly thrown together in each generation, characteristics such as body size are normally distributed in populations. This is known as the polygenic determination of the phenotype. Darwin's cousin Francis Galton (1822–1911) had already anticipated it in his work "Natural Inheritance" (1889), and Fisher developed the mathematical principles to describe it.

It is estimated that there are so many possible combinations of natural gene variants in humans that a body height of six meters could be achieved if the corresponding genetic makeup were combined in one individual, which, however, could only be achieved through selection over thousands of generations. New mutations or epigenetic imprints would not be necessary for this. The natural variation, together with the possibilities of combinatorics, thus holds a huge reservoir for phenotypic innovations, which is the basis of animal and plant breeding. Darwin also had this insight, partly because of his interest in pigeon breeding.

Because geneticists have focused on the effects of individual genes for many years, the polygenic determination of the phenotype has meanwhile received little attention. Fisher's mathematical concept faded into the background; Instead, many researchers preferred an alternative model of his, in which one or a few hereditary factors significantly shape the phenotype, while the others only contribute small modifications. However, genome research provides more and more evidence that the genotype-phenotype relationship is generally polygenic and that a large number of natural gene variants contribute almost equally to the appearance.

Some even speak for an omnigenic mechanism in which the variants of all hereditary factors that are active in a certain stage or a certain organ contribute to the phenotype - according to the finding of a team led by geneticist Jonathan Pritchard from Stanford University in 2017. Scientist are working on adapting the mathematical models accordingly, which is complicated. Whether this will require a fundamentally new theory cannot be answered at the moment.

Do we need an extended synthesis?

It is clear to all evolutionary biologists that, then as now, their subject faces unresolved challenges. In addition, there are constantly new biological discoveries that require classification. An expansion of the theory of evolution has already taken place several times and will continue to do so. However, this is not a matter of clearly definable theoretical leaps. Even the modern synthesis was not a uniform conceptual step, but the sum of many new insights. Therefore, quite a few researchers see no compelling reason to now create an "extended evolutionary synthesis" which itself tried to explain various different phenomena. The proponents of such a synthesis, however, point out points that have not been conclusively clarified. In retrospect, however, Darwin's ideas have proven to be an extremely stable basis.