Adaptive radiation
In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic interactions or opens new environmental niches. Starting with a single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits. The prototypical example of adaptive radiation is finch speciation on the Galapagos, but examples are known from around the world.
Characteristics
Four features can be used to identify an adaptive radiation:- A common ancestry of component species: specifically a recent ancestry. Note that this is not the same as a monophyly in which all descendants of a common ancestor are included.
- A phenotype-environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments.
- Trait utility: the performance or fitness advantages of trait values in their corresponding environments.
- Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.
Conditions
Occupying a new environment might take place under the following conditions:
- A new habitat has opened up: a volcano, for example, can create new ground in the middle of the ocean. This is the case in places like Hawaii and the Galapagos. For aquatic species, the formation of a large new lake habitat could serve the same purpose; the tectonic movement that formed the East African Rift, ultimately leading to the creation of the Rift Valley Lakes, is an example of this. An extinction event could effectively achieve this same result, opening up niches that were previously occupied by species that no longer exist.
- This new habitat is relatively isolated. When a volcano erupts on the mainland and destroys an adjacent forest, it is likely that the terrestrial plant and animal species that used to live in the destroyed region will recolonize without evolving greatly. However, if a newly formed habitat is isolated, the species that colonize it will likely be somewhat random and uncommon arrivals.
- The new habitat has a wide availability of niche space. The rare colonist can only adaptively radiate into as many forms as there are niches.
Relationship between mass-extinctions and mass adaptive radiations
Examples
Darwin's finches
Darwin's finches on the Galapagos Islands are a model system for the study of adaptive radiation. Today represented by approximately 15 species, Darwin's finches are Galapagos endemics famously adapted for a specialized feeding behavior. Darwin's finches are not actually finches in the true sense, but are members of the tanager family Thraupidae, and are derived from a single ancestor that arrived in the Galapagos from mainland South America perhaps just 3 million years ago. Excluding the Cocos finch, each species of Darwin's finch is generally widely distributed in the Galapagos and fills the same niche on each island. For the ground finches, this niche is a diet of seeds, and they have thick bills to facilitate the consumption of these hard materials. The ground finches are further specialized to eat seeds of a particular size: the large ground finch is the largest species of Darwin's finch and has the thickest beak for breaking open the toughest seeds, the small ground finch has a smaller beak for eating smaller seeds, and the medium ground finch has a beak of intermediate size for optimal consumption of intermediately sized seeds. There is some overlap: for example, the most robust medium ground finches could have beaks larger than those of the smallest large ground finches. Because of this overlap, it can be difficult to tell the species apart by eye, though their songs differ. These three species often occur sympatrically, and during the rainy season in the Galapagos when food is plentiful, they specialize little and eat the same, easily accessible foods. It was not well-understood why their beaks were so adapted until Peter and Rosemary Grant studied their feeding behavior in the long dry season, and discovered that when food is scarce, the ground finches use their specialized beaks to eat the seeds that they are best suited to eat and thus avoid starvation.The other finches in the Galapagos are similarly uniquely adapted for their particular niche. The cactus finches have somewhat longer beaks than the ground finches that serve the dual purpose of allowing them to feed on Opuntia cactus nectar and pollen while these plants are flowering, but on seeds during the rest of the year. The warbler-finches have short, pointed beaks for eating insects. The woodpecker finch has a slender beak which it uses to pick at wood in search of insects; it also uses small sticks to reach insect prey inside the wood, making it one of the few animals that use tools.
The mechanism by which the finches initially diversified is still an area of active research. One proposition is that the finches were able to have a non-adaptive, allopatric speciation event on separate islands in the archipelago, such that when they reconverged on some islands, they were able to maintain reproductive isolation. Once they occurred in sympatry, niche specialization was favored so that the different species competed less directly for resources. This second, sympatric event was adaptive radiation.