"New species have come on the stage slowly and at successive intervals; and the amount of change, after intervals of time, is widely different in different groups. The extinctions of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitable follows on the principle of natural selection; for old forms will be supplanted by new and improved forms".
Voyage of the Beagle, 1835.
Different species may be defined as two individuals that may not reproduce due to barriers either prior to, (pre), or after, (post), mating. A pre mating barrier may be such as exists between an elephant and a mouse, whereas a post mating barrier may be that of infertility of the offspring, as with a mule, the product of a donkey and a horse.
Each successive arrival to the islands creates a founder effect, whereby the available selection of genetic material, or gene pool, is limited to the individual or individuals that arrived on the islands. From this original gene pool, all future genetic material must be drawn, unless there occur further arrivals of the same species. In those populations where successive arrivals take place, speciation is unlikely to ensue, as the population is not sufficiently isolated. Isolation is one of the key factors for evolution.
Having established a viable community on the Islands, each species is subjected to variations in its environment. These variations can be daily changes, for example in temperature, climatic changes between the hot rainy season and the cool dry garua season, and sporadic but drastic changes such as those brought about by an El Niño event and the ensuing years of drought.
Within any particular population, there exists an enormity of genetic variations, and with each successive population, mutations and further variations are therefore possible. The genetic material within a population is hence not constant, but ever changing.
Certain conditions may favour particular individuals within a population, or species as a whole, some individuals being those that survive by the process of natural selection. At any point in time there may be, let us say, varieties A to Z within a species. All are related genetically, but each form distinct as the population drifts genetically. However, if through the process of natural selection, the intermediate varieties, B to Y, cease to exist, it is possible that Z is now different enough from A that each may be considered a separate species. This is the process of speciation, when one form evolves from another similar form.
This transformation of the genetic composition of a population, or its evolution, is permanent and irreversible.
Whereas evolution on an island may produce a single new species, the same forces within an archipelago may result in the appearance of multiple forms and eventually species by the process of adaptive radiation. This is the divergence of a single species into many species, each adapting to a particular habitat or way of life by adjusting to environmental change.
Good examples of this process are the Darwin finches, mocking birds, tortoises, and Scalesia plant family. At the beginning, numerous habitats or "niches" would be available, but as time passes, and existing habitats are filled, these openings would become fewer and the chances for adaptive radiation of a species lower. That is unless conditions changed and nature selects an alternative form.
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