evolution

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Under evolution (of Latin evolvere "roll out", "unwrap", "develop") refers to the German-speaking today primarily biological evolution . This is understood to mean the gradual change in the heritable characteristics of a population of living beings and other organic structures (e.g. viruses ) from generation to generation. The teaching and research area of ​​evolution is called evolutionary biology and, like many other sciences, is subject to continuous new insights, to which the discovery of new fossils or new methods alone can contribute. The subject of evolution was sometimes divided into evolutionary history , in which the changes in living beings in the course of the earth's history are described and where there are overlaps with paleontology , as well as the scientific explanations (hypotheses and theories) for the overall phenomenon of evolution within the framework of evolutionary theory . The two approaches are closely interwoven in science today and are mutually beneficial. Scientists also deal with biological evolution within the framework of theoretical biology . Theoretical biology as an interdisciplinary branch of biology develops mathematical models and carries out statistical hypothesis tests and laboratory experiments in order to increase the knowledge gained.

Basics of evolution

General and history

Family tree - representation of terrestrial organisms on the basis of molecular characteristics. The three domains (“realms”) are shown separately in color (blue: bacteria ; green: archaea ; red: eukaryotes ).

The characteristics of living beings are encoded in the form of genes that are copied during reproduction and passed on to the offspring. By mutations different variants (produced alleles ) of these genes that can cause altered or the emergence of new features. These variants and recombinations lead to hereditary differences in the form of genetic variability between individuals. Evolution takes place when the frequency of certain alleles in a population (the allele frequency in the gene pool ) changes and the corresponding traits in the population become rarer or more frequent. This happens either through natural selection (different survival and reproduction rates due to these characteristics), through sexual selection or randomly through genetic drift .

The importance of variability and the process that is kept going by natural selection in living beings was first presented in detail and well-founded by Charles Darwin in his book The Origin of Species , published in 1859 . The principles of heredity became known around 1900 (since the earlier investigations by Gregor Mendel had not been taken into account) and around 1910 the importance of chromosomes. As a result, the concepts of Darwinism , which emphasized the changes, and genetics , which taught the static transmission of traits, seemed to contradict each other. It was not until the 1930s that the selection processes could be brought into agreement with the Mendelian rules of inheritance , from which the synthetic theory of evolution developed. She defined evolution as the change in relative allele frequencies ( allele frequencies ) in a population over time. Through its descriptive and causal statements, this theory became the central organizing principle of modern biology and provided a well-founded explanation for the origin of the diversity of life on earth.

Charles Darwin at the age of 51, shortly after the publication of On the Origin of Species

In 1944, the work of Oswald Avery and his colleagues provided strong evidence that deoxyribonucleic acid (DNA) is the carrier of genetic information, because until now proteins were more “suspected” of harboring such information. Along with the deciphering of the structure of DNA by Rosalind Franklin , James Watson and Francis Crick in 1953, the physical basis of inheritance was clarified. Since then, molecular genetics has also been a central part of evolutionary biology .

Inheritance

Using peas, Gregor Mendel showed that inheritance takes place in narrowly defined (discrete) units, in that traits are passed on from the parent generation to the offspring, and that these traits are discrete: if one parent had round peas and the other had wrinkled peas, then it showed Offspring not a mixture, but either round or wrinkled peas. Mendel also demonstrated that the characteristics of the parents were passed on to the offspring in a precisely defined and predictable manner, namely according to Mendel's rules . His research formed the basis for the concept of discrete, hereditary traits, the genes . Mendel's work answered the long open question why trait variants remain stable in populations. In retrospect, it must be stated that it was a great coincidence that he chose nothing but discrete characteristics, because with many other characteristics (such as the height of the plants reached) more complex genetic and environmental influences would have occurred.

Later research revealed the physical basis of genes and identified DNA as the genetic material. Genes have been redefined as specific regions of DNA. DNA is stored as chromosomes by living things . A certain location on a chromosome is called a gene locus (or locus for short ), the variant of a DNA sequence at a certain locus is called an allele . The DNA is not copied perfectly, and changes ( mutations ) in the genes produce new alleles and therefore affect the traits that these genes control. This simple relationship between a gene and a trait exists in many cases, but complex traits, such as resistance to disease, are controlled by many interacting genes (“polygenic”).

The structure of a section of a DNA double helix

Genetic variability

The genetic variability or variation results from mutations in DNA, the migration of individuals between populations (the gene flow ), and the mixing of genes during sexual reproduction ( recombination ). In some life forms, such as bacteria and plants, variability is also created by the mixing of genetic material between species through horizontal gene flow and hybridization . Despite all these variability-causing processes, most areas of the DNA of a species (the genome ) are identical in all individuals of a species. Comparatively small changes in the genotype (the trait-coding part of the genome) can, however, have considerable effects on the phenotype (the entirety of the genetically determined characteristics of an individual; in simple terms, on the genetically determined external appearance). For example, the difference in DNA sequences between chimpanzees and humans is only five percent.

The phenotype results from the interaction of its individual genetic makeup, its genotype, with the environment. The variability of the heritable traits within a population thus reflects the variability of the genome within this population. The frequency of individual trait variants can fluctuate in a population and become larger or smaller in relation to other alleles of the gene. All evolutionarily effective forces act by promoting these changes in allele frequencies in one direction or another. The variability of a trait disappears when an allele reaches a fixed frequency, i.e. when it either disappears from the population or when it has replaced all other alleles that were previously present.

The evolutionary factors

In biology , evolution factors are processes through which the gene pool (the entirety of all gene variations in a population ) is changed. This occurs mainly through changes in the allele frequencies in the population's gene pool. These processes are the central cause of evolutionary changes.

The essential evolutionary factors that change the gene pool (total of all gene variants in a population) are mutation , recombination , selection and gene drift .

mutation

Duplication of a section of a chromosome

Mutations can occur anywhere in the body outside of the germline and are called somatic mutations. In contrast to germline mutations, these have no direct influence on subsequent generations, at most indirectly, if the somatic mutation affects the fitness of the carrier organism and thus the probability of passing on one's own genes is statistically reduced. Germline mutations are mutations to offspring via the germ line inherited may be; they affect egg cells or sperm and their precursors before and during oogenesis or spermatogenesis . They usually have no influence on the carrier organism in which they take place.

The formation of new types of enzymes can be observed in microorganisms in fast motion due to the shorter generation time. Examples of newly created enzymes are nylonases .

Recombination

Recombinations can occur in the context of the presumably phylogenetically older parasexual recombination (in prokaryotes and some fungi), as well as in the context of sexual reproduction. In the case of the latter, which is typical for almost all plants and animals, a distinction is made between intrachromosomal recombination by recombining alleles within chromosomes (as a result of the crossing-over on the occasion of the 1st meiosis ) and interchromosomal recombination by recombining entire chromosomes in the chromosome set .

selection

Selection occurs when individuals with traits beneficial for survival and reproduction can produce more offspring than individuals without these traits. In this way, overall better adaptations to the environmental conditions can arise for the population over the course of the generations. Under such feature changes a kind of can under speciation split into new species. The "normal" selection takes place between individuals of different species and is also known as natural selection . A special case is the "artificial selection" or breeding used by humans , which is responsible for the numerous dog breeds, for example. As soon as domestic dogs have to survive in the wild, which is a common phenomenon in many countries around the world, only certain genotypes will soon prevail. The dogs become relatively uniform in size, in color characteristics and in behavior, since the “more extreme” cultivated characteristics in natural surroundings prove to be disadvantageous and the corresponding dogs have less reproductive success.

A male peacock's tail is a classic example of a sexually selected trait.

A special case of selection or natural selection is sexual selection , which has an intraspecific effect (i.e. within a species): The selection for characteristics whose presence is directly correlated with the success of copulation through preferred partner choice. Characteristics evolved through sexual selection are particularly common in the males of animals. Although these characteristics can reduce the probability of survival of individual males (e.g. through obstructing antlers, mating calls or bright colors), the reproductive success of such males is normally higher.

Helper systems and eusociality represent further special cases: With more than 200 bird species and around 120 mammalian species, there are social structures in which some of the individuals at least temporarily renounce their own reproduction and instead support conspecifics in their reproduction. This appears to contradict Darwin's theses. Investigations of these helper systems have shown, however, that the more closely the helpers are related to the offspring to be raised, the stronger the help. Since part of the genome of the helper and the offspring raised by others is identical, the helper is able to pass on part of his genome despite not having to reproduce himself. Since selection no longer starts at the phenotype level, but rather at the genotype level, Richard Dawkins coined the term “egoistic gene” for these and similar cases. In the case of eusocial insects such as ants and social wasps , the majority of the females refrain from reproducing themselves for life. Eusociality is not associated with reduced evolutionary fitness for these females, as they are more closely related to their sisters than to their own potential offspring due to a genetic peculiarity ( haplodiploidy ). When raising sisters, they pass on a larger part of their genome than when raising their own daughters.

Genetic drift

Genetic drift is the change in allele frequencies from one generation to the next, which happens because, statistically speaking, the alleles of a generation of offspring represent a random sample of the alleles of the parent generation and their selection is therefore also subject to a random error. Even if there is no selection, allele frequencies tend to increase or decrease over time until they finally reach 0% or 100% (“fixation” of the allele). Fluctuations in the allele frequencies in successive generations can therefore, by pure chance, lead to individual alleles disappearing from the population. Two separate populations with initially the same allele frequencies can therefore drift apart due to random fluctuations into two different populations with a different set of alleles.

Whether natural selection or genetic drift have the greater influence on the fate of new mutations depends on the size of the population and the strength of the selection. Natural selection dominates in large populations, genetic drift in small ones. Ultimately, the time that an allele needs to reach a fixed frequency in a population due to genetic drift (until 0% or 100% of the individuals in the population carry the allele) depends on the population size; this happens faster with smaller populations.

The size of a population (more precisely the effective population size ) therefore has a great influence on the course of evolution. For example, when a population goes through a genetic bottleneck (a temporarily very small population size), it also loses a large part of its genetic variability. The population becomes more homogeneous overall and loses many rare variants. Such “bottlenecks” can be caused by catastrophic events, climate fluctuations, migration or the division of populations and of course by anthropogenic pressure.

Reconstruction of the evolutionary process

The reconstructable processes of earthly evolution - the directions it took and the temporal classifications - can be reconstructed on the basis of the fossil record and the analysis of recent patterns and processes. The processes that took place in the past with regard to direction, temporal classification and the speed of evolution can be reconstructed more reliably, the more independent indicators can contribute to illuminating historical events. Basically the will actualism geological for the process and biological processes assumed the past, that is, on the assumption that the biological, ecological and geological processes have proceeded in the past for the same or similar principles as they can be today observed and measured. For this purpose, experiments can also be carried out, which to a certain extent can be projected onto processes in the past.

In the case of forms that have left no or almost no fossils, including almost all prokaryotes and the majority of eukaryotic unicellular organisms, as well as all other skeletal organisms such as worms, jellyfish, nudibranchs, etc., mostly only comparisons from the recent fauna or -Flora are employed with a simultaneous critical plausibility check of the conclusions drawn from it. Here, the reconstructions of the previous processes are carried out mainly on the basis of the molecular similarity using the analysis method of the phylogenetic relationship. The principle of the molecular clock can help to roughly date the junction points of the various kinship lines. Processes of coevolution, for example the former uptake of bacteria in archaeal cells, which there (perhaps two billion years ago, estimation difficult) became the mitochondria and the chloroplasts as endosymbionts of a eukaryotic cell, can practically only be derived from recent data on Molecular genetic similarities as well as similarities in the metabolism and structure of the endosymbionts are revealed.

Most often, skeletal marine forms are found as fossils, and species from the mainland in inland waters (rivers, lakes) are much rarer. Very few fossils are usually found from mountain regions, from bog and spring waters as well as generally from former dry zones of the earth, since embedding and preservation in such places is generally rather unlikely.

Fossil lore

Archeopteryx fossil

The fossils , which can be morphologically examined, give clues to the temporal framework of the evolutionary processes, but from which often also biological-ecological properties, such as the populated habitats, modes of movement or sometimes even social behavior (e.g. when they are in packs or swarms occur) can be read. Corresponding to the vertical succession of fossil-bearing rock layers , communities of fossil living beings can be brought into a chronological order according to the (principle of stratigraphy ). While this initially only provides information about the “relative age” (which fossils were earlier, which later?), Absolute dating can be carried out in the rocks and / or the fossils contained therein with the help of suitable radiometric methods ( geochronology ).

The age of the fossils provides information about when certain innovations and splitting events (adaptive radiations) occurred in the course of the tribal history of individual groups and living beings as a whole . From fossil finds that can be dated, it is known, for example, that (with a few exceptions such as Cloudina ) all groups of skeletal animals first appeared in a narrow time window in the early to middle Cambrian , around 540 to 500 million years ago. To what extent this was caused by biological innovations or to what extent the environmental conditions changed in such a way that skeleton formations and deposits were now chemically and physically possible, is still a question that has not been finally clarified.

Fossil transitional forms ( mosaic forms ) ( missing links ) are a proof of the relationship between systematic large groups that is independent of studies on recent living beings. Famous examples of this are " feathered dinosaurs ", Archeopteryx and the birds of the Jehol group as transitional forms from the non-flying amniotes (colloquially known as "reptiles" or "dinosaurs", both paraphyletic units) and modern birds as well as Panderichthys , Tiktaalik and Ichthyostega as transitional forms between bony fish and terrestrial vertebrates . In the best-case scenario, the transition between the original group and the group derived from it is evidenced by a sequence of fossil finds that, with decreasing age, become more and more morphologically similar to the new group.

Evolutionary trends are in many cases well documented within systematic groups, e.g. B. in the equine mammals: From many-toed, fox-sized, leaf-eating forms in the Old Tertiary , today's horses emerged over several intermediate stages .

Finally, the fossil record documents decreases and increases in the diversity of systematic groups. Faunenschnitte are mass extinctions, in which the number of fossil taxa has been greatly reduced in geologically speaking short periods of time and some large groups either completely disappeared or greatly decreased in their diversity. The best known, albeit not the largest, of the fauna is the extensive extinction of the “ dinosaurs ” (with the exception of the group of birds) and other large groups at the end of the Cretaceous Period . The ecological niches that became free could be occupied by the mammals and modern birds as part of radiation .

An overview of the occurrence of the animal and plant phyla is presented under evolutionary history .

Phylogenetic systematics

The comparison of characteristics of the organisms in the context of biological systematics has shown for a long time that the characteristics do not appear in any combination, but in a system of graded similarities. Characteristic groups can be distinguished from one another, on the basis of which the recent organisms can be summarized (classified) and hierarchically arranged in groups ( taxa , singular taxon ).

Skeleton of a baleen whale . The c marks the rudiments of the hind legs; a trait that proves the descent from land mammals.

That this is possible was seen by Darwin as a strong indication of a common descent of all living beings. So that these groupings (systematizations) reflect the actual relationships ( phylogenetic systematics ), the homology of characteristics was introduced as a central criterion, i.e. that the similarity is only meaningful if the corresponding characteristics or organs can be traced back to the same original characteristics . The cacti of America and milkweed plants of Africa, some of which look similar due to their succulence, show analogous formations. Their often similar appearance is no indication of a closer relationship and descent from a common ancestor.

A special case of homologous features are morphological features or even behaviors that no longer serve a recognizable purpose for their present-day carriers ( rudiments ), for example the remains of the hind leg skeleton of giant snakes and whales . In both cases, these rudiments point to the descent from four-legged animals ( lizards or even ungulates ).

Remnants of earlier functioning genes, which are obviously functionless today, can be found in the genome of some pseudogenes, for example .

Comparative biogeography

The spread of taxa provides clues for evolutionary developments in many cases. Many taxa have geographical distribution that cannot be explained solely by today's local ecological conditions or adaptations. This is especially true for endemics . A well-known example of relic endings are the lemurs , a taxon within the wet-nosed monkey . Lemurs were widespread in Tertiary North America and Eurasia, but are now endemic to Madagascar . They were displaced everywhere by the later evolved and obviously more competitive dry-nosed monkeys (Haplorhini) and could only survive on Madagascar, which could not be colonized by dry-nosed monkeys because of the geographic isolation that had taken place in the meantime.

Different Darwin's finches

Equally important for evolutionary biology are Entstehungsendemiten . The most famous example here are the Darwin's finches (family Emberizidae) on the Galapagos Islands, 965 km off the coast of Ecuador . On the archipelago of volcanic origin, the oldest island of which was formed five to ten million years ago, 14 different species in three genera developed from a predecessor species that had drifted from the mainland through adaptive radiation . The main changes were the shape of the beak, the plumage and the size of the birds. Charles Darwin collected the different species on his world tour in 1836.

Comparative embryonic development

Karl Ernst von Baer was the first to recognize that the early stages of development of related organisms are more similar to one another than adult individuals. On the basis of these findings, Ernst Haeckel formulated that ontogenesis recapitulates the evolution ( phylogenesis ) of a living being ( basic biogenetic rule ). Today, however, this rule is viewed as over-simplifying. It is not the adult stages that are repeated, but the early embryonic stages of the respective preforms of a species. This can be explained by the fact that evolutionary new developments are always based on existing species and thus on existing organ systems. Therefore, the development (ontogenesis) of the individual of a species also includes the steps that were already in place before.

Examples of such repetition are the appearance of a gill gut and cleft gill in all vertebrate embryos . This proves that all vertebrates today are descended from fish. Baleen whale embryos form tooth systems while the adult individuals no longer have teeth, which indicates that the baleen whales evolved from toothed whales .

The idea of ​​evolution in population, politics and religion

The fact that an evolution occurred in the sense of an evolutionary change of species and in connection with regular extinction events has always been denied by individuals or by a more or less large part of the population, although scientific arguments were only involved in the early phase. Mostly it is about basic attitudes based on a completely different conception of the world, which often also viewed planet earth itself as only a few thousand years old. Sometimes individual aspects, such as a single or multiple mass extinction , which is then brought into line with the religiously inspired flood reports, were accepted, but other components and consequences were not.

Wherever the principle of evolution has been recognized, aspects of it have been interpreted and demanded as a mandate and maxim for political action in human society in the last 150 years or so, detached from ethical principles. The focus was particularly on the concept of a universal "struggle for survival" that also encompasses human societies and " races ", which was derived from the idea of ​​selection and led to the attitude in social Darwinism . The idea of eugenics , which became popular in the Anglo-Saxon area from the last quarter of the 19th century and there, for example, helped determine immigration policy, also acquired special significance . After adopting the underlying theories and views in other states and in their political leaderships, the most radical and momentous effects of the time of National Socialism (1933–1945) occurred, especially in Germany . Due to the mass murder associated with it, evolution and the theory related to it was largely ignored and little worked on, especially in the German-speaking area. For decades, most of the modern research developments took place only in the Anglo-Saxon area.

A fundamental opposition to the principle of an evolution of earthly life, especially insofar as the species or genus humans themselves, including their properties, such as consciousness and intellectual creativity, are included in the consideration, has been represented for a long time and has been increasingly represented by science-skeptical population groups for several decades . They consider a development to a being like humans subject to the laws of nature to be either fundamentally incompatible with their worldview or so improbable that they postulate a superordinate guiding authority, a god , who either created the species or at least initiated the process has steered. This fundamental controversy has become known in Europe primarily through the effects of the dispute over the teaching of the theory of evolution in school biology lessons in some US states . The corresponding representatives often use the exact wording of the history of creation as formulated in the Old Testament and follow it word for word . This skepticism about the results of the theory of evolution is known as creationism . The American intelligent design idea is a special form of creationism .

The Catholic Church has repeatedly expressed itself on the theory of evolution, according to Pope Pius XII. in his encyclical Humani generis , Pope John Paul II at the plenary session of the Pontifical Academy of Sciences and Pope Benedict XVI. in his inauguration sermon. The theory of evolution is now called "compatible with the Christian faith" by the Vatican , see also Theistic Evolution .

In the early Islamic scientific heyday, partly based on Greek models, including Aristotle , animal descriptions and trains of thought were formulated that are reminiscent of the evolutionary biological discussions of the 19th century, including "Lamarckist" environmental influences on evolution, but also about one “Struggle for existence” has been speculated, reminiscent of Charles Darwin. A prominent representative was Al-Jahiz from Basra in what is now southern Iraq in the 9th century . In current Islam, which is characterized by strongly diverging currents, there is no uniform opinion as to whether and to what extent evolution is compatible with religion; An evolutionary skeptical attitude is relatively widespread: in a number of Islamic countries, a large majority of people are also convinced that today's species Homo sapiens did not develop evolutionarily, but is static and was created by God ( Allah ).

See also

Selected literature

Web links

Commons : Evolution  - collection of pictures, videos and audio files
Wiktionary: Evolution  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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  2. ^ A. Stoltzfus: Mutationism and the dual causation of evolutionary change. In: Evol. Dev. Volume 8, No. 3, 2006, pp. 304-317.
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    MA Norell, X. Xu: Feathered dinosaurs. In: Annual Review of Earth and Planetary Sciences. Volume 33, 2005, pp. 277-299.
    F. Zhang, Z. Zhou, G. Dyke: Feathers and 'feather-like' integumentary structures in liaoning birds and dinosaurs. In: Geological Journal. Volume 41, No. 3-4, 2006, pp. 395-404.
    Z. Zhou: The origin and early evolution of birds: Discoveries, disputes, and perspectives from fossil evidence. In: Natural Sciences. Volume 91, No. 10, 2004, pp. 455-471.
    Z. Zhou, F. Zhang: Mesozoic birds of china - A synoptic review. In: Frontiers of Biology in China. Volume 2, No. 1, 2007, pp. 1-14.
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