In evolutionary biology, adaptive radiation (Latin: adaptare "to adapt"; radiatus "radiant", "radiating") means the diversification (radiation) of a less specialized type into several more specialized types through the development of specific adaptations to existing environmental conditions. Associated with this is a shift in the ecological niche for those varieties or genetic variations that have new characteristics. These processes are also known as cladogenesis (branching), idioadaptation (self- adaptation) , allogenesis (change of species) or allomorphosis (change in shape).
Species formation in isolated habitats
Species formation through adaptive radiation is one of the fundamental mechanisms of evolution. As in all evolutionary processes, the driving forces of adaptive radiation are genetic variation and natural selection (selection; e.g. through intraspecific competition ) within a population . Examples of adaptive radiation arise from the settlement of previously unpopulated, isolated habitats, e.g. B. volcanic oceanic islands or newly formed freshwater lakes. These are initially only populated by a few species, which means that the immigrating species have far fewer competitors than in "old" habitats with many species and a long evolutionary history. Here one can observe that sometimes in an evolutionarily short time (that is a few thousand to ten thousand years) very many new species arise.
However, this only applies in cases in which the habitat has been newly created. If, on the other hand, a previously cohesive habitat breaks down into separate islands, one also observes here how the now isolated populations develop morphologically and form new species, but these processes take place very slowly. Accelerated new formation of species is not to be expected here, since it is crucial for the accelerated formation of species that a species has large amounts of resources (especially food resources) available without competition. Unoccupied ecological niches are beneficial for adaptive radiation in an area .
The mechanisms of accelerated speciation during the settlement of new islands (actual islands or habitat islands) represent a test case for the theory of evolution and have therefore attracted particular scientific attention.
A species that populates a newly created habitat will usually be relatively unspecialized. On the one hand, this is due to the fact that pioneer species are generally less specialized; on the other hand, a specialist usually needs special habitats and communities that may not yet exist. The newly arriving species can use the resources that occur with very little efficiency. This is not so important at first because it does not have any potentially superior competitors. After a short time, the population will have grown so much that intra-species competition occurs. From now on, a selection pressure acts on the population so that it can make better use of the available resources. Since better efficiency can be achieved in different ways, various new traits emerge in the population. When their characteristics change and there is no gene exchange with the donor population, new species will gradually develop. Thus, this peculiarity of an island species is relatively easy to explain.
It is particularly interesting that the new island species sometimes develop features that are very similar to those that occurred in the specialized species in the original habitats of the donor population (position equivalence ). This is because they occupy very similar ecological niches. Such a development is impossible in the donor population since the corresponding niche is already occupied and the more specialized species in this case is usually the more competitive.
The question remains as to which mechanisms prevent a renewed genetic mixing of the local populations that have arisen. Different researchers have developed different models of speciation in adaptive radiation. Different models may be correct in different cases. To answer the question, two sub-problems must be distinguished:
- the change in body structure and morphology
- the splitting of a species into two distinct species
Species can change morphologically significantly without splitting. On the other hand, split pairs of species can remain very similar in their way of life, sometimes even almost indistinguishable in terms of shape ( twin species ) or actually ( cryptic species ). So there are two separate problems.
The classic model
The founders of the synthetic theory of evolution, especially Ernst Mayr and Theodosius Dobzhansky , developed a model in which a separation into geographically separate sub-habitats is normally necessary to separate a species ( allopatric speciation ). If there is not an island but a group of islands (e.g. the Galapagos Islands or the Hawaiian Islands), a species can form on each island, which increases the number of species in the archipelago. Whether the separate island populations actually represent species, however, only becomes apparent when they come into contact again. If individuals from one island later colonize another that is already occupied by a related species, there are various possibilities: a) The populations mix again. b) The populations remain separate. Only in case b have two new species formed. If one of the two species does not displace the other through exclusion of competition , two species now live on one island. The same sequence can now run many times in a row, which gradually increases the number of species.
The model explains some cases of adaptive radiation very well, e.g. B. the Darwin's finches on the Galapagos Islands or the high number of species of lizards of the genus Anolis on the Caribbean islands (although the actual conditions are of course much more complicated than shown here in a simplified manner).
Other cases are more difficult to explain, e.g. B. the radiation of hundreds of cichlid species in the great East African lakes. Ernst Mayr spoke here of the "cichlid problem".
In recent years, numerous researchers have developed new models that make the emergence of new species appear plausible even when there is physical contact between the original populations. One speaks here of sympatric or parapatric speciation . The essential basis is a selection that does not work in one but in different directions. For example, a species for which medium-sized prey would be optimal can be found in a habitat with a lot of particularly small and particularly large prey. An undivided population is trapped here to a minimum, as the advantages and disadvantages of both possible specializations cancel out. When separated into two separate populations, each can specialize in a class of prey, thereby increasing overall fitness. The process is known as disruptive selection . Further assumptions are necessary so that separate species can form in the event of disruptive selection. Either the species develop spatially adjacent, but separate with a narrow hybrid zone ( parapatric , this model was already pursued by Charles Darwin as a thought model) or in the case of mating, the sexual partners specifically prefer partners with similar characteristics as their own ( assortative pairing , English . assortative mating ). In these cases one speaks of ecological speciation, because the separation of the populations is based not on geographical but on ecological factors. Perhaps this can better explain how an extremely large number of fish species can arise in a single lake almost simultaneously. Numerous other factors are likely to be involved in speciation. A phenomenon that has long been known in botany but has long been neglected in zoology is z. B. the influence of hybrids on radiation.
In ecological speciation, it is the diversity of the emerging species themselves that triggers species separation. If, on the other hand, two species arise through allopatric speciation according to the classical model, they will normally be somewhat different because different islands have different selection conditions, or simply by chance ( founder effect ). The sometimes impressive adaptations of the species to different habitats ( niches ) do not usually explain this alone. If previously separated populations come into contact again later, a character displacement often occurs here . Due to a shift in characteristics, species or populations that were previously similar become more dissimilar because they are also affected by a disruptive selection. Individuals with particularly similar traits who have similar diets are more competitive. By shifting characteristics, different species can divide a habitat among themselves and thus reduce inter-species ( interspecific competition ). Sometimes groups of species with exactly the same specializations arise independently of one another in neighboring lakes or islands. This was observed e.g. B. cichlid species in East African lakes, sticklebacks in lakes in North America or, particularly impressive, spider species on the Hawaiian Islands.
The actual proof of a characteristic shift (in the sense defined here) presupposes proof that the species involved compete with one another and is therefore not easy to provide. Species can naturally develop in different directions simply by chance in different habitats (e.g. plausible for some species of Darwin's finches).
Role of sexual selection
Several well-documented case studies, e.g. B. Drosophila species on the Hawaiian Islands or cichlids in the East African lakes, point to the great importance of sexual selection for the emergence of species in these cases. Species with conspicuously colored or marked males and much more similar and inconspicuous females ( sexually dimorphic species ) are typical . The species differ in the coloring of the males and in the preference of the females for these coloring ( prezygotic isolation ). Experimentally produced hybrids can be viable indefinitely, but as a rule mating no longer occurs because males that are incorrectly colored from the female perspective are unattractive. The populations that are reproductively isolated as a result can then also specialize in different ecological ways. Whether and to what extent sexual selection significantly facilitates sympatric speciation is still controversial in research. One possible mechanism for facilitating speciation in these cases was developed through the theory of “sexually antagonistic coevolution”, in which the different interests of both sexes in terms of the costs and benefits of reproduction can bring about a coevolutionary “arms race” that results in characteristics that are important for mating can change significantly in a short time.
Other cases of radiations
The adaptive radiation model presented above has been applied to other cases in evolutionary research for a long time. In contrast to the settlement of islands, it is not a question of current evolutionary processes, but rather the interpretation of old splits that are only known from fossil records. This means that they can only be proven by plausibility conclusions, but not by direct experiments. The most important cases are:
- Radiation due to mass extinction . If numerous species become extinct as a result of a catastrophic event, habitats and niches are open to the few survivors that were previously closed by superior competitors . A classic example is the radiation of mammals after the extinction of the dinosaurs (except for the birds) in the course of the mass extinction after the impact of a giant meteorite at the turn of the Middle Ages to the New Era .
- Radiation as a result of a key innovation . If a species acquires a trait as a result of slow evolution that enables it to use completely new habitats and new resources, or if innovation enables it to use a resource in a completely new way, this new key trait can lead to the success and spread of the species in a short time Accelerate time enormously, which means that radiation into vacant niches can easily follow. For the most species-rich group of insects, the holometabolic insects , key innovations were e.g. B. suggested: flight ability, wing joint to fold the wings over the abdomen, holometabolic transformation (metamorphosis) with imagines and larvae with completely different physiques and different ways of life. Each of these "inventions" could have been followed by accelerated speciation in the new group, which is indicated by the extremely different number of species in the respective groups.
In recent years, adaptive raditation processes have also been increasingly investigated experimentally on the basis of populations with rapid growth.
In the real cases examined, the various hypotheses presented proved to be of varying success. Above all, the actual importance of ecological speciation is still controversial in science. Advances in DNA sequencing ( PCR ) have made it much easier to compile family trees of groups of species that have recently exhibited adaptive radiation. This has made it easier to test hypotheses. The high speed of speciation in some cases of adaptive radiation is a problem here, however, since it is difficult to find DNA markers that react quickly. Despite these problems in detail, the processes involved in adaptive radiation appear to be easy to explain in principle.
Well-known examples include
- Darwin's finches in the Galapagos Islands
- Clothes birds in Hawaii
- Cichlids of the African Great Lakes
- Tenreks and lemurs in Madagascar
- Giant crab spiders from Asian mountains, e.g. B. Himalaya
- Anole in Jamaica
- Cone snails
- Fruit flies or fruit flies (Drosophilidae) in Hawaii
- Marsupial mammals in Australia
- Aeonium , a thick-leaf plant with around 40 species in the Mediterranean area
- Three-spined stickleback that gave birth to 10 new species within the last 10,000 to 20,000 generations
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- Jones, FC: http://www.fml.tuebingen.mpg.de/jones-group.html