Ever opportunistic, living organisms have repeatedly shown an ability to move into and colonize new habitats, resources, or geographic areas. Such exploitation of a new environment gives rise to an evolutionary pattern called adaptive radiation.
Through migration, isolation, or possibly a new mutation giving rise to a brand new adaptation, a population of organisms moves into a new geographic area where that species never previously existed.
Suddenly there are unlimited resources available, few predators, and a wide variety of new territories, habitats, and environments. Variants from all phenotypic extremes of the distribution curve may be able to find conditions that favor their particular adaptations. Consequently, they spread out, discover a niche that suits them, and exploit their new capacity for growth and reproduction.
Rapid isolation and speciation lead to proliferation and new species as each new gene pool diverges and separates.
On phylogenetic diagrams, adaptive radiation resembles a fan-shaped, branching pattern because a multitude of new species arise in a short time and quickly diverge.
Eventually, however, these new species will begin to encounter one another again and start living closer and closer together. Inevitably, there will be competition for resources such as food, shelter, and nesting sites.
Natural selection will favor those variants that do not compete directly. For example, if two closely related plant species with similar growth requirements gradually invade a common territory, they will begin to feel the competition for soil space.
However, variants of one species can tolerate much dryer conditions than any other of the related plants, so these variants will flourish in those areas. If variants of the other species favor wet areas, they too will grow without much competition. Over time, these two variants will leave more offspring because they are more fit, and gradually the competition between the two species will lessen.
The strong tendency for closely related species to evolve diverging characteristics that reduce competition between them is called character displacement.
Adaptive radiation and character displacement are phylogenetic patterns that occurred on a very large scale when animals first crawled out onto dry land. They found virgin territory temporarily free of predators and filled with new resources to exploit.
These patterns can also take place on a small scale when new islands are thrust up by volcanic activity, when old ecosystems are wiped out be a catastrophe, when a new type of tooth pattern comes about, or when a new mode of life, like flying, is exploited.