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Adaptive radiation can be defined as a type of evolutionary process in which organisms show diversification in physical and anatomical structure from their ancestral species for better adaptations to the changing environment (Schluter 2000).
According to Charles Darwin’s book Origin of Species, various processes have played an important role in the evolution of spectacular biological diversity on Earth. Among all, adaptive radiation has been considered as the most crucial factor for maintaining this diversity. It is comprised of two main features: speciation (formation of new species driven by certain ecological conditions) and phenotypic adaptation resulting in the emergence of an array of new species showing distinct morphological and physiological characters. This phenomenon was first put forward by Darwin during his voyage to the Galápagos Islands (Grant 1999) and relies on the notion of ecological opportunity that is described as an abundance of evolutionarily accessible resources underutilized by competing taxa (Schluter 2000).
Ecological Opportunity: A Prerequisite for Adaptive Radiation
Appearance of new resources: Ecological opportunity can prompt adaptive radiation if new resources appear in the given area. In other words, evolutionary diversification of a clade may prove beneficial for other species. For example, the evolution of grasses presented an opportunity for the primitive species of horses to exploit and evolve under changing environmental conditions.
Geographic colonization: Geographic colonization has undoubtedly accounted for the majority of adaptive radiations on remote islands like the Hawaiian archipelago, Galápagos Islands, Caribbean islands, etc. One of the main reasons behind this is the absence of species competing for the available resources on islands which were otherwise present on the mainland. Furthermore, the absence of predators may also permit the use of habitats which were earlier inaccessible, thereby facilitating adaptive diversification.
Mass extinctions: Mass extinction events leading to the removal of complete or large proportions of ecologically dominant species support the hypothesis of ecological opportunity. After such events, surviving species are released from competitive and predatory pressure, and are left with plenty of resources that pave the path for their evolutionary diversification. For example, many clades of birds and placental mammals showed explosive radiation after cretaceous-paleogene mass extinction of the dinosaurs (Erwin 2015).
Key innovations: Key innovations can be basically defined as evolution of novel features or traits that provides the capability for species to take advantage of available resources which they could not utilize previously and thus promotes adaptive radiation (Rabosky 2014). Examples of key innovations include the evolution of adhesive toepads in anoles and geckos, evolution of wings in birds and bats, and the evolution of pharyngeal jaws in labrid fishes. All these cases of key innovations have opened new opportunities for evolutionary radiation within the existing environmental milieu (Stroud and Losos 2016).
Classical Examples of Adaptive Radiation
Darwin’s finches: Darwin’s finches are members of tanager bird family Thraupidae and represent the most symbolic example of adaptive radiation inhabiting the patchy and diverse landscape of Galápagos Island. To date, approximately 15 species of Darwin’s finches have been identified. These species arose from a single ancestor that probably migrated to Galápagos from the mainland of South America 3 billion years ago. All such species differ from each other in several aspects of ecology, morphology, and songs (Grant 1999). One of the major difference occurred in the shape and size of finches’ beak which are specialized according to the primary source of nutrition available to them. For example, the largest ground finch (Geospiza magnirostris) has developed the thickest beak, which allows it to break open the hardest seeds. Likewise, the smallest ground finch (Geospiza fuliginosa) has evolved a smaller beak, which allows it to specialize on small seeds and the medium ground finch (Geospiza fortis) has a medium-sized beak, which facilitates consumption of intermediary sized seeds (Weiner 1994).
Caribbean Anolis lizards: Anolis lizards are another fascinating example of adaptive radiation. There are about 400 species of Anolis which are widely distributed in the West Indies as well as mainland Central and South America. These Anolis lizards provide an insight to determine how adaptive radiation varies in mainland and island condition. Divergence of Anolis on mainland has essentially been attributed to the process of speciation and is not adaptive to any substantial extent. However, Anolis on the islands like Cuba, Jamaica, and Puerto Rico have evolved in such a convergent way that each species can be grouped in one of six “ecomorphs” named as trunk-ground, trunk-crown, grass-bush, crown-giant, twig, and trunk depending upon the microhabitat they colonize (Irschick et al. 1997).
African cichlids: Cichlids fishes of east African great lakes also show adaptive radiation. There are around 2,000 species of cichlids inhabiting different lakes such as Lake Tanganyika, Lake Malawi, and Lake Victoria. Each species has distinct morphology in terms of habitat, diet, sexually selected traits, and perform various ecological roles including those of scavengers, predators, and herbivores, etc. For example, Lethrinops furcifer commonly resides in sandy area nearby beaches whereas Hemitilapia oxyrhynchus lives in shallow vegetated environment in Lake Malawi (Konings 2016).
Hawaiian honeycreepers: Hawaiian honeycreepers belonging to the family Fringillidae have specific beaks which are well adapted according to their dietary requirements. For example, lower mandible of Hemignathus wilsoni is short and sharp for scraping tree bark while its upper mandible is long and curvy for searching out insects underneath the wood. However, Telespiza cantans has evolved a thick beak for eating tough seeds. Presently, 17 species are known to exist in the Hawaiian Islands (Olson 2004).
Adaptive radiation is a remarkable and complex route for evolutionary divergence and is governed by several different parameters such as ecological, genetical, developmental, geographical, etc. When coupled with ecological opportunity, adaptive radiation resulted in the burst of new species from preexisting species through a process known as rapid speciation. Ever since the publication of the Darwin’s work, this process has managed to amaze and motivate ecologists as well as the public. However, the exact mechanism by which radiation takes place under diverse environmental backgrounds and why it varies among different taxa are still an area of active research. In this regard, more information related to genes and genomes along with detailed knowledge of natural selection will enhance our understanding of adaptive diversification.
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