As homology ( ancient Greek ὁμολογεῖν homologein "match") is known in the biological systematics and the comparative anatomy the fundamental harmonization of organs , organ systems , body structures, physiological processes or behaviors of two taxa because of their common evolutionary origin. So this article deals with homology in terms of the phenotype .
Homologous traits go back to traits of the common ancestor, so they are equivalent in terms of their ancestral origin. The original features can then have developed in different directions and be used in different functions.
"A characteristic of two or more taxa is homologous if it is derived from the same (or a corresponding) characteristic of their closest common ancestor."
Homologous characteristics play an important role in the creation of family trees in classical biological systematics.
Definition of terms: homology in genetics and cytogenetics
When looking at the chromosomes of a cell , the term homology is used in a different context. Here those two chromosomes are called homologous, which contain the same genes . If it is an organism with sexual reproduction, one of the two homologous chromosomes was inherited from the father and one from the mother.
Chromosomes that can be traced back to a common ancestor can also be homologated when comparing two closely related species. In this broader sense, two chromosomes of different species are homologous if they have very similar contents.
In genetics and evolutionary biology , the term homology is also used at the level of genes whose sequence can be traced back to a common ancestor.
In the pre-Darwinian era, the term homology was used to refer to the similarity of characteristics in different species, regardless of their form and function, but depending on their location or their development in the organism. Work on this can be found by the French doctor Félix Vicq d'Azyr (around 1770) and a little later by Goethe (1790). The founder of paleontology, Georges Cuvier , successfully classified fossils in the family tree (from around 1803 onwards) based on the anatomical comparison of homologous structures with recent organisms. In 1843 Richard Owen (1804-1892) introduced the term " homology " into literature. With the recognition of the mutability of species, published by Darwin in 1859, it became necessary to correct Owen's original "explanation" of homologies. Instead of structuring according to location or development, homologies were more and more defined as a function of ancestry. The large number of homologies found promoted Darwin's evolutionary theory right from the start. In ignorance of the inheritance rules, a concept of abstract "construction plans" was initially postulated until the occurrence of homologies was finally clarified by the knowledge of geneticists.
In order to establish homology between anatomical features , the zoologist Adolf Remane (1898–1976) proposed the three main criteria "location", "structure" and "continuity" in 1952 . Of these three, at least one must be true for one to speak of a "homology". Based on the similarities, the relationship of the organisms and thus a homology can be inferred.
1. Criterion of location
Structures are homologous if they always have the same positional relationship in a comparable microstructure, despite their different characteristics in terms of shape and number.
- Within the class of insects, the differently shaped insect legs meet the criterion of location, because they are all attached to the animal's thorax.
- Within the class of vertebrates, the same applies to the digestive organs, which can be divided into mouth, esophagus, gastrointestinal and anus.
- Likewise, the structure of the heart in mammals (left ventricle, right ventricle, left and right atrium and their connection to the blood vessels).
- Structure of the forelimbs of terrestrial vertebrates . The basic construction plan consists of bones that are laid out in the sequence of a humerus, two forearm bones, several carpal bones, five metacarpal bones and five fingers, the thumb has two finger bones and the remaining four are each made up of three finger bones.
- Despite the identical internal structure, the appearance and function of the forelimbs in different vertebrate classes can be very different because they are used for running, digging, flying, grasping or swimming. In the course of the development of the phylogenetic history, specializations of the front extremity have emerged. Only the same basic construction plan points to common ancestors from which the development to today's forms took place.
2. Criterion of specific quality and structure
Similar structures can also be homologated regardless of the same position if they match in numerous features. The security increases with the degree of complexity of the structure being compared. The “internal structure” of an organ or structure is therefore decisive for the homology criterion of specific quality.
- According to the homology criterion of specific quality, the human tooth and the skin flakes of a shark are homologous because the outermost layer consists of tooth enamel and the underlying layer of dentin . Since teeth, like placoid scales, can be understood as specifications of a primal bony exoskeleton, both structures are actually homologous.
3. Criterion of continuity (continuity)
Organs are homologous if they can be connected through intermediate forms that can be homologated despite their different location.
- Certain parts of the blood vessel system ( carotid artery ) of mammals can be homologated with the gill arch arteries of the fish on the basis of embryonic stages (“cleft gill”) .
- the swim bladder of bony fish and the lungs of terrestrial vertebrates are homologous, since the development from the swim bladder to the lungs can be traced with the help of amphibians and sauropsids .
Recent intermediate forms
Example: The homology of the articular and square of amphibians , reptiles and birds with anvil and hammer in the middle ear of mammals , as well as the hyomandibular columella of birds, reptiles and amphibians with the stirrup of mammals, was already established in 1837 by Carl Bogislaus Reichert (1811–1883) discovered.
Rudiments and atavisms
Another anatomical evidence for evolution are rudiments . This is understood to mean regressed remains of organs that have lost their function and have thus become superfluous. For example, the pelvic girdle of the baleen whale is a rudiment. If these organs do not acquire new functions over time, they can be completely lost: The slow worm is not a snake, but a lizard whose extremities have receded completely. In rare cases, regressed organs can reappear, such as the tailbone extension in humans. In these cases one speaks of atavisms .
Embryonic intermediate forms
During the embryonic development process, homologies to specific species become visible, which, however, are only of ephemeral (temporary) duration and can no longer be determined in the finally developed organism. These intermediate forms led Ernst Haeckel to the assumption that every mammal undergoes complete phylogenesis (complete stem development of its kind) in a short time during its embryonic development . He formulated this in the basic biogenetic rule , the modern version of which is formulated in such a way that profoundly transforming changes of evolutionary importance can occur at every stage of ontogenesis.
- At a certain stage of development, the human embryo has a cleft gill (gill ridge) and thus an unmistakable resemblance to the fish, which later disappears.
- The bird wing can be homologated with the human hand, since the bird embryo still has a hand with five fingers, which are remodeled in the course of its development through fusion and reduction.
Fossil intermediate forms (also fossil bridging animals )
The step-by-step development of evolution can be documented on the basis of found and restored fossils . This is done either through progression series or regression series , i.e. a traceable development from less complex structures to very complex structures or vice versa.
Similar behaviors can also be homologous, as classical ethological research has specifically worked out. Examples: the (innate) courtship behavior within related bird groups, for example of different types of ducks or chickens .
The similarity of a trait is not always due to a common ancestor. If different species were exposed to similar selection pressure, they can develop similar characteristics independently of one another. If such analog features are present, one speaks of convergent development or, for short, of convergence or analogy. Analogies perform similar tasks, but they cannot be traced back to a common ancestor. They are therefore not based on common hereditary traits and are not suitable for establishing a close evolutionary relationship. Put simply, homologous traits are based on common ancestors and analogous traits on a common environment.
An example of analogy is the wings of birds and the wings of bats with respect to the wing. Feathers and flight skin did not emerge from a common flying ancestor, but emerged twice independently of each other. Further examples of similar characteristics are:
- Leglessness in snakes and lizards
- the front extremities of mole (mammal) and mole cricket (insect) suitable for digging
- the stinging-sucking proboscis of bed bugs (Hemiptera) and mosquitoes (Diptera)
- Succulence in cacti of the New World and in some Euphorbiaceen of the Old World
- White or uncolored eggs in birds that breed in burrows
The distinction between analog and homologous features is not always possible without any doubt, it is scale-dependent, i.e. H. the same characteristic must be assessed differently at different time and observation levels. For example, the front extremities of birds and bats used for flying are anatomically homologous. But if you consider instead of z. B. the bone structure the characteristic "flight ability" is an analogy (since the last common ancestor had an anatomically corresponding front extremity with the same bones, but was not able to fly). In kinship analysis, professionals do not use characteristics that are “known to be prone to convergence”.
Conversely, homologous features are observed in closely related species, which appear very differently due to divergent developments, genetic drift or bottleneck effect; sometimes homologies are no longer recognizable at first glance, for example if a feature is completely reduced in one species. Without fossil finds, embryonic stages, genetic examinations or knowledge of intermediate stages in recent species, it is then difficult to decide how the absence of this characteristic is to be interpreted. For example, despite the lack of a shell , nudibranchs are closely related to the shell -bearing snails , because their common ancestors had a shell that the slugs then lost.
Instead of the term analogy, the more comprehensive term homoplasia is preferred in the more recent specialist literature as the opposite of homology. In addition to analogous development, homoplasia also includes parallel evolution (e.g. parallel development of a stripe pattern with a different molecular basis from an unstriped ancestor in two fish species) and evolutionary regression, in which a lost trait reappears in a developmental line through mutation . While anatomical regression is very rare (one example would be the re-development of functional wings within flightless ghosts ), it plays a very important role when comparing homologous DNA sequences, as only four permutations are possible for each position due to the four base pairs.
Understanding of the phylogenetic system
Traits that can be traced back to a common origin in phylogeny are homologous. Therefore, homologies were among the first criteria used to establish relationships between the taxa.
In 1950, Willi Hennig supplements or specifies the term homology with the terms phylogenetic systematics (cladism):
- Apomorphy is a comparatively derived feature (acquired later in tribal history).
- Plesiomorphism is a comparatively original (phylogenetically acquired earlier) characteristic.
- Autapomorphy is the new acquisition of characteristics in a parent species (example: the feather in the parent species of all birds).
- Synapomorphism is the appearance of derived features of a parent species in its two daughter species and their descendants (example: echolocation in toothed whales and baleen whales).
- Symplesiomorphism is the appearance of common features in different species, which is not due to their last common parent species, but to an even older parent species, which is also the ancestor of other species (example: hair in chimpanzees and humans).
The evaluation of the characteristics depends on the reading direction (original vs. derived) and homology (tracing back to a common basic plan): The characteristic "neurocranium" is an autapomorphy of the cranial animals , a synapomorphy of all groups belonging to the cranial animals and a plesiomorphy for individual ones groups belonging to the skull animals. The apomorphic feature state is linked to the basic plan in which it occurs first; in basic plans that emerge from the further splitting of the resulting group, the same feature is then in the plesiomorphic state.
The term building plan is replaced by the term basic plan or basic pattern . The latter is defined as the sum of all plesiomorphies and autapomorphies of the last common parent species of a monophyletic group.
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- Illustration from: Gegenbaur 1870
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