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Adaptive radiation is the emergence of many new species from a single strain. Adapative radiations occur when the species is introduced into different ecological niches. Factors such as geographical isolation and the absence of natural enemies favor the process of Adaptive Radiation.
The adaptive radiation based on the Darwin's finches
The Darwinfinken are an absolute paradigm when it comes to the explanation of an adaptive radiation. Altogether there are 14 closely related species, all of them descended from a common ancestor. Striking are above all the different beaks of the Darwinfinken, which indicate different dietary habits. The main food source of Geospiza magnirostris (1) are seeds, while the Certhidea olivacea (4) is an insectivore. This principle of competition avoidance by adapting to different ecological niches will be explained in more detail later.
The Galapagos Islands are located about 1000 km west of South America and are therefore geographically isolated from the mainland. As an island of volcanic origin, the Darwin finches can not have developed on the island, but must have their origin from the mainland. By chance, for example by a storm or by driftwood, at least two finches (male and female) or a fertilized female must have arrived on the island and thus formed a founding population. First of all, the songbird species increased very strongly, because in addition to the over-abundant food supply, there were no predators anywhere on the island. At some point, however, the pressure of intraspecific competition on the finches increases, because habitat and food are limited.
Geographic separation now provides for the evolution of the strain in different directions. From the island of origin, individual individuals arrive again by chance to another island. The process of rapid multiplication is repeated, but under different conditions, because not only the selection pressure is different on this other island (eg other food supply), but also the gene pool differs now, albeit only slightly, from that of the original population the island of origin (bottleneck effect). Furthermore, different mutations and recombinations, a changed direction of selection, as well as gene drift lead to a non-convergent development, as compared to the strain species on the island of origin.
Since there is no more gene flow between the islands, the songbirds can no longer exchange any alleles with each other and the two populations separate. If the two populations are separated long enough, reproductive isolation can occur during this process of separation, so that the two populations can no longer reproduce among themselves. Now, if individuals of the newly emerged species return to the island of origin, this can lead to their being adapted to the same ecological niche and thus to competing with the other species for this ecological niche. According to the competitive exclusion principle, only one species can occupy an ecological niche and either extinction of one species occurs, or one species is able to evade another ecological niche and co-exist alongside the other species. However, it may also be that the newly developed species has developed on the other island in such a way that it immediately occupies a free ecological niche on the island of origin and does not compete with the strain species.
This process of geographic isolation and immersion repeated itself several times, resulting in 14 species, all of which originated from one species of origin.