Phenotypic variation refers to the differences in characteristics between members of the same species or related species. In evolution , hereditary phenotypic variation in population is a basis for evolutionary change .
No two individuals are exactly identical in the population of a species. Some of the variations are hereditary, they are passed on to the offspring. Variation in phenotype encompasses all characteristics of anatomy, physiology, biochemistry, and behavior. Variation in the population is the phenotypic result of the interaction of the prevailing environmental factors with the genetic makeup of an organism, which determines its reaction norm. Mechanisms during embryonic development help explain how variation arises ontogenetically . Mutation can be further multiplied by sexual recombination , which rearranges the parental genes in the offspring. Variation leads to the biological diversity of a population. It provides the raw material for evolutionary change. Evolution cannot take place without variation.
Variation before and with Darwin
The variability of the organisms of a species was not first discovered by Charles Darwin . The phenomenon was described before him in France by Georges Cuvier , Étienne Geoffroy Saint-Hilaire and in Great Britain by Darwin's grandfather Erasmus Darwin and Robert Chambers . C. Darwin speaks of a principle of divergence ( principle of divergence ). By this he means that initially barely noticeable differences keep increasing and the resulting races continue to differ among themselves and from their common ancestors. Variations as individual differences are passages in the formation of geographical populations, and such populations are passages or precursors of species. Darwin thus took a clear position of the gradual, gradual transition of characteristics as new species emerge. The explanation of the variation was clearly the weakest point in Darwin's thinking. He did not yet understand where the variation came from.
When studying the inheritance of traits, Gregor Mendel devoted himself to certain clearly distinguishable variations in peas, such as the shape of the seeds (round, wrinkled), the color of the seeds (yellow, green) and five other traits that vary in inheritance (see also Mendelsche Rules ). In doing so, he selected discrete distinguishing features from which he concluded that they are also based on the inheritance of specific, discrete units. Mendel's offspring variability is not related to the emergence of new traits, rather it arises from the combination of already existing traits. His answer to the question of how traits are inherited initially seemed inconsistent with Darwin's considerations about when and how they change. Darwin's theory came to the conclusion that speciation resulted from the slow, gradual accumulation of small, if not imperceptible, variations. The incompatibility of Mendel's theory of inheritance with Darwin's theory of evolution with regard to the meaning of discontinuous variation was the subject of a number of well-known researchers, including William Bateson and Hugo de Vries, at the turn of the 20th century. These researchers promoted the importance of discontinuous variation for evolution. The direction was named as saltationism or mutationism . Only the synthetic theory of evolution was able to resolve the supposed contradiction. In doing so, Ronald Aylmer Fisher created mathematical, population-genetic models in which he showed that quantitative characteristics, i.e. continuously varying properties that can be measured in numbers, such as body size, are determined by many genetic loci . Individually, these only make a small contribution to the development of such a feature or its variation.
Darwin pointed to the geographical variability of phenotypic characteristics. However, it was left to later generations of researchers to empirically prove that the continuous variability of species is actually relevant for evolution in wild populations. For this purpose, individuals had to be compared with geographically different populations and their variation had to be proven to be inheritable and not due to environmental conditions. Such studies were first carried out by Richard Goldschmidt in 1918 with sponge spiders ( Lymantria dispar ), a butterfly that occurs in different species worldwide under different climatic conditions. According to today's terminology, variations could be traced back to different gene expressions . A study of geographic populations of the jerboa in California was also carried out in 1918 and demonstrated the heritability of subspecific phenotype features by translocating individuals from local wild populations to other regions . The individuals retained their characteristics, which spoke in favor of their inheritance. In 1936, the Russian evolutionary researcher Theodosius Dobzhansky , together with Alfred Sturtevant , finally succeeded in demonstrating their phylogenetic relationship in fruit flies ( Drosophila melanogaster ) from different geographical locations by identifying phenotypic differences on genetic maps . The persuasiveness of these studies of geographical variation cemented Darwin's theory and laid essential foundations for the synthesis based on mutation and variation in the theory of evolution.
Variation in Classical and Molecular Genetics
Classical, pre-molecular genetics in the first half of the 20th century initially had to do with two phenomena that could not be easily reconciled. On the one hand, one saw the genetic constitution of organisms in which genotypic deviations or genetic mutation occur, which is also called genetic variation. The first geneticists saw the genotype as something fairly uniform within a species. On the other hand, one had to deal with the phenotypic, fluctuating variation that results from the interaction of the genetic makeup with the respective environmental situation. Here, too, the synthesis was able to bring about consensus, mainly through researchers such as Ernst Mayr and others who focused on phenotypic variation. With the advancement of molecular genetics it gradually became clearer that the supposed uniformity of the genome of a species concealed considerable fluctuations. In 1927 the American Hermann J. Muller was able to induce mutations in fruit flies for the first time with the help of X-rays . Specific forms of mutation were later discovered: the mutation that triggers a variation can take place within a coding gene, in a transcription factor for a coding gene or in a non-coding cis element and can be inherited. Also linked to this include mutations (Example trisomy ). There are limits to the variants to be expected; Variability always occurs in a specific context of DNA . Furthermore, the variability or fluctuation in individual individual cases cannot be predicted, but exists statistically. For most traits, a statistical standard deviation from the mean can be observed within a population. It is generally 5–10%. This applies to traits in the expression of which many genes are each involved to a small extent, called "quantitative" traits by geneticists. Natural selection is involved in the statistical distribution of the variation .
The neutrality of alleles with regard to phenotypic change and thus the fitness of the phenotype was recognized. In the case of silent mutations, there is no difference in phenotype. In this case, natural selection does not have a point of attack. Only (random) genetic drift can affect the mutations in the population. Polymorphism , the presence of varying characteristics within a species, such as different eye or hair color, could be explained genetically.
A modification is a change in the phenotype, the appearance of a living being, caused by environmental factors.  The genes are not changed, which means that a modification - unlike a change through mutation - cannot be inherited, but an epigenetic transmission of this change cannot be ruled out.
Phenotypic variation in evolutionary developmental biology
The evolutionary developmental biology is based in part on that of C. H. Waddington first described in 1942 buffering the genotype. According to this, many genes are combinatorially involved in a phenotypic result during a development process. Mutations often remain buffered and have no effect on the phenotype. The development is channeled . Only a permanent environmental stressor can overcome the buffering or channeling, so that a (discontinuous) variation occurs as a result, which is genetically assimilated afterwards . The buffering of the genotype or the channeling of the development is nothing other than the constancy (robustness) of the wild type of species in their natural environment. Because of their greater genetic diversity, species there are known to be more immune to phenotypic variation than is the case with breeding animals.
In addition to this genetic issue, EvoDevo increasingly deals with individual development processes and mechanisms at higher organizational levels, such as cell-cell communication, threshold value effects , pattern formation and others. The special peculiarities of phenotypic or organismic variation are therefore increasingly seen in the context of processes in which, in addition to the genetic initiation factors, the complex structures of the developmental systems come into consideration, i.e. those systems that control the development of a fertilized egg cell to a full-grown organism .
The increasingly complex view of the origin of variation in the organism and the self-organization skills of development lead evolutionists to increasingly demand that a theory of variation is necessary in addition to the theories of heredity and selection. It has to help overcome the restricted, classical view of random mutation and explain the principles and development mechanisms with which the organism generates phenotypic variation. Concepts for this can be found in the theory of facilitated variation by Kirschner and Gerhardt and in various findings from EvoDevo . The ideas lead to efforts for an extended synthesis in the theory of evolution .
A genetic mutation does not exactly determine a phenotypic variation. The phenotype is neither clearly readable nor predictable from the genome, nor can the genotype be unequivocally inferred from the phenotype. The problem is dealt with in science under the term genotype-phenotype ratio or genotype-phenotype mapping .
Continuous and discontinuous variation
Darwin's theory of evolution, and subsequently the synthetic theory of evolution, assume marginal changes in the phenotype, which accumulate in the course of evolution, including major changes in characteristics and even species change. The evolution of the vertebrate eye can be seen as an example of this . For this, 364,000 variations and around 450,000 years have been calculated as necessary, in fact it took more than a hundred times longer. In other cases, a change in phenotype caused by a (point) mutation remains within a species for a long time, with a possible geographically limited selection advantage. Both variations then remain in the population. One speaks of a single nucleotide polymorphism . Typical examples are lactose persistence , a mutation that a few thousand years ago gave people in Northern Europe the ability to metabolize animal milk, or sickle cell anemia , a sickle-shaped deformation of red blood cells with circulatory disorders that also causes resistance to malaria .
The evolutionary developmental biology but (EvoDevo) knows even complex, spontaneous, discontinuous variations in a generation, for example preaxial polydactyly , extra fingers or toes, induced by a point mutation in a non-coding regulatory element of the gene Sonic hedgehog (Shh). In the case of an extensive phenotypic variation such as this with the creation of one or more completely new fingers and / or toes including all blood vessels, nerves, muscles, tendons and their full functionality, the genetic mutation cannot alone explain the extensive phenotypic result. Rather, it just says how the variation is triggered. As a result of the mutation in a morphogen , there are tens of thousands of events directly or indirectly induced by this gene at different organizational levels, including changes in the expression of other genes, ectopic expression of the Shh gene, cell signal exchange, cell differentiation through random cell switching, and cell and tissue growth. The total changes on all these levels are then the material or form the process steps for the creation of the variation, in this case for the creation of one or more new fingers.
Variations within a species and between different species
All species have variations. In individuals of a species, variations are not the exception, but the rule. The variation of a trait within a species does not have to be spatially evenly distributed within the species' populations. The different populations can have genetic variability, but need not have the same genetic pattern. A species can have individuals of different body sizes, but not all populations within the species then have individuals of all these body sizes. Some populations may consist of smaller individuals than others. There is also variation within a population of a species, such as the analysis of the beak sizes of a particular finch population on the Galapagos Islands. The variation between the populations of a species proves the evolutionary processes among them. The gene flow between these populations is only present to a reduced extent. The populations are genetically different, a result for which, in addition to variability, natural selection is also responsible for evolution.
The best known examples of intraspecific variation include the differences in eye and hair color. A more recent example of mutation in the same gene with very different intraspecific variation are the Devon Rex and Sphinx cat breeds . Both have a mutation in the gene Keratin71 ( keratin ) (KRT71). In Devon Rex the mutation leads to an extremely shortened exon 7 compared to the wild type . Phenotypically, the species has relatively large ears and a flat skull. The fur is wavy to curly and relatively short. In Sphinx, on the other hand, there are genetically 43 base pairs more between exon 4 and 5 than in the wild type. There is a stop codon underneath . Therefore only exon 4 is expressed, not exon 5. The variation form is completely different from Devon Rex. The cat is naked. Both cats have alternative splicing , different transcriptions of the same DNA template. Although two different gene names were assigned here, re for the gene with the mutation for Devon Rex and hr for the gene with the mutation for Sphinx, DNA analyzes in 2010 confirmed that the same gene is present here. Both alleles are recessive , so both mutants are homozygous . Because of the frequency of occurrence of both alleles, a polymorphism is used here to distinguish it from mutation . Here, too, analyzes of the developmental pathways are necessary in both cases in order to be able to understand the origin of the phenotypic variations.
An example of a variation with a small genetic mutation, but the more pronounced phenotype difference, are the stripe differences in zebra species . The variation of the stripes is very different. The Burchell's zebra ( Equus burchelli ) has about 25 stripes, the mountain zebra ( Equus zebra ) about 40 stripes and the Grevy's zebra ( Equus grevyi ) about 80. The variability of the stripes of the different zebra species is based on the time at which the pattern is formed in the embryo. If this is initiated later, the embryo is already larger and more strips of the same width have space on its surface.
To clarify differences in variation between two species, that is, their relationships, z. For example, mitochondrial DNA is being examined in an archaeogenetic study to find out how closely humans and great apes , such as the chimpanzee, are related to one another. On this basis, a family tree can be developed (parallel to morphological comparisons). After a study of the DNA of the cell nucleus in 2002, z. B. the genetic makeup of humans and chimpanzees are 98.7% identical. Investigations of this kind on genetic variability also serve to elucidate the more precise relationships between man today and earlier ancestors or other close relatives of man, cf. genetic relationship of humans . As a rule of thumb, the smaller the genetic differences between two individuals, the closer they are to each other.
The close genetic relationship between chimpanzees and humans is nevertheless expressed in great phenotypic differences. The reason for this is seen in the possible diverse genetic combinations for creating variation. In the process of creating variation and its diversity, all evolutionary factors ultimately play a role in their interdependent, constantly repeating interaction: genetic variability, competition ( natural selection ), persistence of molecular bonds, reinforcement (e.g. genetic drift ), cooperation and the embryonic development ( EvoDevo ). Coen calls this repetitive principle of interaction to create variation and evolution recurrence .
Directed variation in phenotypic characteristics
Directed development describes how the direction of evolutionary change is influenced by the non-random structure of variation. There are numerous examples of directed variation. For example, a group of millipedes with more than 1000 species shows only odd numbers of pairs of legs. The fact that these animals do not have an even number of pairs of legs is due to the mechanism of segmentation during embryonic development, which does not allow this. Skinks , a species-rich family of lizards, come in very different sizes. They have very short to no extremities. The toe reduction with increasing body size of different species takes place in exactly the opposite order as the formation of the toes in the embryonic development. The toe that is developed first in the embryo is also the first to disappear in the case of evolutionary toe reduction; the one that is developed last, the last. This is an example of a non-random, directed variation.
In the polydactyly form of the Hemingway mutant in Maine Coon cats, there are variable additional toe numbers. The variation is plastic . According to a current study of the polydactyl toe numbers of 375 Hemingway mutants, there is a directed developmental variation in the sense that the number of additional toes follows a discontinuous statistical distribution and is not randomly evenly distributed, as would be expected with the identical point mutation. The directionality is not a result of natural selection, since the phenotypes are considered at birth and natural selection has no point of attack at this point. Such a directionality in embryonic development is alien to the synthetic theory of evolution. At most, natural selection can bring about direction there.
The variation is a polyphenism . In the Hemingway mutant of the Maine Coon (wild type: 18 toes), polydactyly occurs in some cases with 18 toes by extending the first toe into a three-jointed thumb; Much more often, however, there are 20 toes and, with decreasing frequency, 22, 24 or 26 toes (Fig. 6), more rarely also odd toe combinations on the feet. The directionality of the toe numbers is the result of development mechanisms for the formation of the toes. While the underlying genetic mutation itself can be random, the phenotypic result, i.e. the statistical number of toes, is not random but directed (see Fig. 6). Another directionality is the difference in the number of toes on the front and rear feet. A slight left-right asymmetry in the number of toes can also be observed.
Differentiation between variation and innovation
A distinction must be made between phenotypic innovation and phenotypic variation . Examples of evolutionary innovation are the feather , the mammary gland , the turtle shell , the insect wing , the exoskeleton or the luminous organ of fireflies or fish. The synthetic theory of evolution does not explain how novelties in evolution arise differently than variations. Variation is mainly associated with natural selection and adaptation by Darwin and the synthetic theory of evolution . The synthetic theory analyzes statistical changes in the gene frequency in populations on the basis of population-genetic considerations and, in its classic form, is not interested in questions of the ontogenetic development of specific characteristics of the organism. Phenotypic variation is taken for granted in standard theory. In contrast, EvoDevo in particular has been investigating since the beginning of the 1990s how embryonic development helps to explain the emergence of innovative features under ecological conditions and to understand their realization in the organism and their permanent anchoring (genetic / epigenetic integration).
Innovation is defined as "a construction element in a blueprint that has neither a homologous counterpart in the previous species nor in the same organism". This definition excludes purely quantitative changes in existing characteristics. It allows a view of features that are either completely new or result from new combinations or subdivisions of already existing structures (e.g. the panda's thumb ). Furthermore, the Müller / Wagner definition allows it to be applied to clearly delimited, unambiguous cases not only in terms of morphology , but also in terms of physiology or behavior .
Variations in humans
As a biological species, humans show many examples of phenotypic variations. Although the genes of all people - regardless of their origin - are about 99.9 percent identical, we have the impression of an unusually large phenotypic diversity of our species (skin, hair and eye color, body size, nose and lip shapes, etc.). When assessing conspecifics, however, our cognition plays an important role, as facial recognition shows: We are able to perceive the smallest differences in faces, so that they appear larger than they actually are. Phenotypically, too, all humans have far more in common than differences. If people who come from widely separated populations are placed side by side, however, the impression of clearly delimited geographical variations is created. Since the creation of categories is also a fundamental process of our psyche, the idea of different races of people (and the resulting racism ) is almost “preprogrammed” when viewed without reflection . On closer inspection, however, the differences between the alleged “races” merge into one another. Almost no group of people was geographically isolated long enough to develop features large enough to be able to speak of different subspecies of humans or races in a biological sense .
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