The adaptive radiation is a phenomenon involving the evolutionary diversification of a set of species that leads to the appearance, for “fast” adaptation to different ecological niches, new forms from the same ancestral species.
The concept of adaptive radiation was proposed by Charles Darwin, a 19th century English naturalist, after a trip he undertook to the Galapagos Islands, where he observed in detail several species of island finches, descendants of continental ancestors, which had different modifications in their beaks .
The finding of these finches represented for Darwin the main zoological evidence to support his theories of “descent with modification”, since the different forms of beaks that he observed, all derived from the same ancestral lineage, seemed to be adapted to exploit different resources of the same trophic niche, “partitioning” it.
It has been determined that the main causes of adaptive radiation have to do with the interruption of the gene flow between individuals of the same species (geographic isolation), with marked environmental variations and with the absence of predators or negative selective pressures.
In this sense, it is a fact of natural history that mass extinction events have led to the impressive adaptive radiation of many groups of living beings, since the absence of organisms offers opportunities for surviving species to colonize empty niches and go through processes. adaptive radiation.
Adaptive radiation process
Adaptive radiation, as explained, is the appearance of new ecologically different species from a common ancestral lineage.
These events necessarily occur through a speciation process that, according to the biological concept of the species, implies an interruption of the gene flow (reproductive isolation) between the “modified” offspring and their immediate ancestor.
Many authors are in favor of the idea that adaptive radiation is a kind of “extension” of the speciation process, but that it is driven by ecological factors and subject to quite special initial conditions.
Usually, geographic isolation is one of the main factors that influence adaptive radiation, since populations that isolate themselves are “forced” to adapt to take advantage of new ecological niches or the absence of old predators.
One of the examples that best illustrates how the adaptive radiation process occurs is the colonization of islands, the tops of some mountains and young (or virgin) lakes by species from other sources.
Geographic islands can be the product of different geological events, but they can also be considered ecological islands, since the isolation of individuals from a population due to behavioral or environmental differences also represents a barrier to gene flow, which leads to speciation.
How does adaptive radiation occur on an island?
If we consider a geographic island of recent formation (geologically speaking) we can imagine that it will initially be “naked” or devoid of any living being.
This island, then, represents an extraordinary ecological opportunity for the colonization or settlement of any species from the mainland (continental) or not, that can take advantage of the ecological resources that this island offers.
Colonizing species can be bacteria, fungi, algae, terrestrial plants, animals, etc., which, upon settling on the “new” island, may adapt or specialize some of its traits for trophic exploitation, for example, of a new niche. probably very different from the niche from which they emerged.
Sooner or later, on the evolutionary scale, the differences in traits that have been established will signify a reproductive barrier that will impede gene flow between colonizing individuals and those “ancestral individuals” from the mainland from which they diversified.
Adaptive diversification vs adaptive radiation
It is important to note that the modern definition of adaptive radiation has two main characteristics:
– Adaptive radiation is not the same as diversification by adaptation (by natural selection) within the same species
– Adaptation gives rise to adaptive radiation
What these two sentences mean is that without speciation we cannot speak of adaptive radiation, just as we cannot speak of speciation without adaptive radiation (it is not just small individual changes as an adaptive response to a new environmental condition).
According to some authors, adaptive radiation events can be “classified” into three types, this according to the stimulus that triggers the process. Thus, adaptive radiation can occur due to an environmental change, a general adaptation or the formation of an archipelago.
When it comes to adaptive radiation triggered by environmental changes, this occurs due to the selective pressure that forces species to adapt to an environment that has changed considerably, in order to survive.
Radiation occurs, then, in such a way that the new species that will form will do so to colonize new ecological niches that arise as a consequence of these environmental changes.
The adaptive radiations that arise from general adaptations do so thanks to the appearance or development of new abilities in individuals of the same species, which allows them to colonize new ecological niches.
The formation or existence of archipelagos, large mountainous elevations or oceanic islands represent one of the main causes of adaptive radiation, since these sites can be colonized by new species that must adapt quickly to these places, evolutionarily diverging from their immediate ancestors.
Examples of adaptive radiation
Darwin gave the scientific community the best example he could get to demonstrate adaptive radiation in animals: the case of finches in the Galapagos Islands.
Finches are a type of bird that feed on seeds and belong to the genus Geospiza. These birds are thought to have diverged from each other recently, where reproductive isolation took place due to ecological divergence partially associated with adaptation to different types of seeds.
In birds, differences in body shape and size, as well as some characteristics of the song that are related to morphology, can lead to pre-reproductive isolation, which gradually involves a process of speciation.
Another classic example of adaptive radiation is that of cichlids, which belong to the Cichlidae family of tropical freshwater fish.
Species in this family are descended from a species that fed on algae and soft sediments, but different species diverged at different sites from this.
In a lake known as Barombi Mbo, an adaptive radiation process originated 11 different species, among which there are predatory species of other fish and insects, species that feed on eggs, filter species and others that feed like sponges.
Adaptive radiation also occurred in another lake, Lake Malawi, perhaps the most significant among cichlids, where fish are found that feed on the ectoparasites of other fish and fish that tear off pieces of the skin of other fish. Cichlids have also been identified that feed on scales, the eggs of other fish and their larvae, etc.
The adaptive radiation of these fish was not only due to the exploitation of new trophic niches (food), but also to some behavior and habitat patterns that have been described by various authors.
An example of adaptive radiation is that of vascular plants and the colonization of terrestrial environments. Seedless vascular plants first appeared in the Silurian fossil record, but became more diversified during the Carboniferous.
The sudden adaptive radiation of angiosperms (flowering plants) occurred in the Jurassic, at which time it is thought that they arose from ferns with seeds. Its complexity increased, however, only about 100 million years ago.
During the diversification of angiosperms, that is, their adaptive radiation, there were at least three transitions: first, the appearance of the carpel, then the appearance of double fertilization and finally the appearance of the flowers as we know them in the present.
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