Adaptive Radiation

Adaptive radiation

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, as well as 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 point out different food 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 geographically isolated from the mainland. As an island of volcanic origin, the Darwinfinken can not have developed on the island, but must have their origin from the mainland. By chance, such as a storm or driftwood, at least two finches (male and female) or a fertilized female must have arrived on the island, forming a founding population. First, the songbird species increased very much, because in addition to the excessive food supply and no predators were available 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 ensures the development of the strain in different directions. From the island of origin, individuals return 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 supplies), 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, and gene drift lead to a non-convergent development as compared to the strain 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 it comes to the separation of the two populations. If the two populations are separated long enough, there may be reproductive isolation in 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 them being adapted to the same ecological niche and thus 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 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 evolved 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 interweaving was repeated several times, resulting in 14 species, all of which originated from one species of origin.

Summary

Few founder individuals formed a founding population on one of the Galapagos Islands.

By chance, individuals of this strain came to another island and were temporarily geographically isolated.

Over time, the two populations developed so widely that they were isolated from each other reproductively.

By accident, the second species returned to the island of origin, where it either competed with the original species for an ecological niche or occupied another ecological niche.

This process of adaptive radiation (fanning of a species) has been repeated several times.