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Annotated Bibliography Topic: Adaptive radiation as a characteristic form of cladogenesis

Gavrilets, S., & Losos, J. B. (2009). Adaptive radiation: contrasting theory with data. Science, 323(5915), 732-737.

This article discusses adaptive radiation as the major cause of diversity on Earth. As examples of adaptive radiation, the author cites Darwin's finches (Galápagos islands), Anolis lizards (Caribbean islands), Hawaiian silverswords, and cichlids (East African Great Lakes.) The author also discussed the two components of adaptive radiation, the approaches to studying adaptive radiation, and the 10 patterns of adaptive radiation

Petren, K., Grant, P. R., Grant, B. R., & Keller, L. F. (2005). Comparative landscape genetics and the adaptive radiation of Darwin's finches: the role of peripheral isolation. Molecular Ecology, 14(10), 2943-2957.

This article talks about how the fragmented and varied landscape/geography of the Galapagos Islands caused the adaptive radiation of the finches. This adaptive radiation is said to have occurred in the past 3 million years. Dramatic phenotypic evolution with little genetic divergence is characteristic of adaptive radiation and Darwin’s finches definitely exemplify this.

Rainey, P. B., & Travisano, M. (1998). Adaptive radiation in a heterogeneous environment. Nature, 394(6688), 69-72.

This article discusses the importance of adaptive radiation to evolutionary history and the exact causes of adaptive radiation. The author examines how an aerobic bacterium diversifies in a new environment and concludes that the selection in this new environment can cause rapid speciation. In several different environments, the same species can evolve in different ways which drives adaptive radiation.

Schluter, D. (1995). Adaptive radiation in sticklebacks: trade-offs in feeding performance and growth. Ecology, 82-90.

This article studies the effects of resources on the adaptive radiation of sticklebacks. The author claims that the ability of these fish to obtain necessary resources in different environments would be the primary cause of Adaptive radiation: the fish that had traits suitable for that environment would have a higher growth rate. In a separate environment, however, a fish with different traits may be favored and therefore the fish evolve separately.

Seehausen, O. (2004). Hybridization and adaptive radiation. Trends in ecology & evolution, 19(4), 198-207.

This article discusses adaptive radiation and some of its causes. Adaptive radiation is often said to occur in isolated, often underpopulated or underutilized niches where a species can diversify from its ancestral species. Some examples of good niches include the Galapagos Islands and the Hawaiian archipelago. Some facilitators of adaptive radiation include release from competition in an underutilized environment, the ability to utilize resources that were there but could not be utilized before, and mating systems that lead to rapid divergence. The author argues that hybridization could also be a major facilitator.

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Cases of Adaptive Radiation Throughout Evolutionary History

Speciation is the evolutionary manner in which new species arise. It has played a tremendous role throughout evolutionary history and appears in many forms. Two main varieties of speciation include cladogenesis and anagenesis. One characteristic form of cladogenesis that has been the source of many studies is adaptive radiation, the rapid diversification of a species into many new forms. These new forms frequently arise due to differing environments and are therefore evolving to better suit these environments. Adaptive radiation has been very important when it comes to introducing diversity within a population. It has affected many different species in many different environments. A few of the most famous cases which have been studied extensively include Darwin’s finches, cichlid fishes, the Hawaiian silverswords, and the Caribbean Anolis lizards.

To understand how these species diversified, one must first understand the mechanisms behind their speciation. Adaptive radiation often occurs under the process of mutations and selection (Rainey & Travisano, 1998). Through adaptive radiation, a great deal of diversity arises within species that may not have existed before, indicating beneficial mutations must have arisen which were then fixed within a species. Adaptive radiation consists of two main components, the formation of new species (speciation) and the adaptation of these species to a collection of niches or environments with different traits (Gavrilets & Losos, 2009). Adaptive radiation not only gives rise to many new, diverse species, but it does so within a very short period of time. It is in fact sometimes considered one of the most important methods for diversifying life on Earth (Gavrilets & Losos, 2009). It occurs to serve several different purposes. It brings about the evolution of key adaptations. These key adaptations often allow organisms to occupy certain niches they initially didn’t have the ability to occupy. It may also occur in order to fill vacated niches. Oftentimes, if there is a decline or release of competition, adaptive radiation will occur because there is nothing to keep species from spreading and diversifying throughout the vacant niches. A third purpose is specialization within the environment. A species may diversify and split, allowing organisms with certain traits to obtain some of the resources of the area while organisms with different traits can obtain others. Because each new species requires different resources, there is less competition and the environment can sustain more organisms.

One famous example where adaptive radiation is seen is with Darwin’s finches. It has been observed by many evolutionary biologists that fragmented landscapes oftentimes are a prime location for adaptive radiation to occur. The differences in geography throughout disjointed landscapes such as islands are believed to promote such diversification. Darwin’s finches occupy the fragmented landscape of the Galapagos Islands and are diversified into many different species which differ in ecology, song, and morphology, specifically the size and shapes of their beaks. The first obvious explanation for these differences is allopatric speciation, speciation that occurs when populations of the same species become isolated geographically and evolve separately. Because the finches are divided amongst the islands, the birds have been evolving separately for several million years. However, this does not account for the fact that many of the species occur in sympatry, with seven or more species inhabiting the same island (Petren, Grant, Grant, & Keller 2005). This raises the question as to why these species split when living in the same environment with all the same resources. Petren, Grant, Grant, and Keller proposed that the speciation of the finches occurred in two parts: an initial, easily observable allopatric event followed by a less clear sympatric event. This sympatric event which occurred second was adaptive radiation (Petren et al., 2005). This occurred largely to promote specialization upon each island. One major morphological difference among species sharing one island is beak size and shape. Adaptive radiation led to the evolution of different beaks which could access different food and resources. Those with short beaks are better adapted to eating seeds on the ground, those with thin, sharp beaks eat insects, and those with long beaks use their beaks to probe for food inside cacti. With these specializations, seven or more species of finches are able to inhabit the same environments without competition or lack of resources killing several off. In other words, these morphological differences in beak size and shape brought about by adaptive radiation allow the island diversification to persist.

Another famous example is the cichlid fishes of the Great Rift Valley in East Africa. The lakes in that area are believed to support and sustain about 2,000 different species of these fish, each with different ecological and morphological characteristics such as body size (Losos, 2010). Like the Galapagos Islands, these lakes form a fragmented landscape that isolates the cichlid fish from one another, allowing them, and many of the organisms they live with, to evolve separately. The diversity of the lakes is in fact quite extraordinary because the adaptive radiation here is so young and so new. One thing that has interested scientists about the cichlid fish case is the possibility of convergent evolution. Some believe that, though these species are isolated, some may have developed analogous structures that were not present in the last common ancestor simply by living in similar environments. Studies on the convergent evolution of the fish are limited, but Losos claims that while some convergence may have occurred, the adaptive radiation in the area has successfully created many species which are ecomorphologically diverse and specialized as seen partially with their differing body sizes. Rather, the convergence is mainly observed in pictures of the similar-looking species of cichlid fish. These pictures, however, do not prove that they are similar in any way besides appearance. It would seem that the adaptive radiation thoroughly diversified the species so that none are similar anymore.

Though the most famously recognized cases of adaptive radiation have occurred in animals such as Darwin’s finches or the cichlid fish, adaptive radiation certainly occurs in plant species as well. The most famous example of adaptive radiation in plants is quite possibly the Hawaiian silverswords. The Hawaiian silversword alliance consists of twenty-eight species of Hawaiian plants which range from trees to shrubs to vines. This is exceptional diversification as can be seen through the significant morphological differences between each species of the Hawaiian silverswords (Baldwin & Sanderson, 1998). With some species, it’s virtually impossible to distinguish visually that they were ever part of one species to begin with. These radiations occurred millions of years ago, but through studies over the past few decades, it has been suggested that the rate of speciation and diversification was extremely high. These high rates, as well as the fragmented landscape of the Hawaiian Islands, are key characteristics which point directly to adaptive radiation.

The final species to be addressed are the Caribbean Anolis lizards. Anolis lizards have been radiating widely in many different environments, including Central and South America, as well as the West Indies and experience great diversity of species just as the finches, cichlid fish, and silverswords. Studies have been done to determine whether radiations occur similarly for these lizards on the mainland as they do on the Caribbean islands or if differences can be observed in how they speciated. It has been observed that in fact, the radiations are very different, and ecological and morphological characteristics that these lizards developed as part of their speciation on the islands and on the mainland are very unique (Irschick, 1997). They have clearly evolved differently to the environments they inhabit. The environmental pressures on the Anolis lizards are not the same on the mainland as they are on the islands. There is a significantly larger amount of predators preying on the Anolis lizards on the mainland. This is but one environmental difference. Other factors play a role in what sort of adaptive radiation will develop. Among the Caribbean islands, a larger perch diameter correlates with longer forelimbs, larger body mass, longer tails, and longer hind limbs. However, on the mainland, a larger perch diameter correlates with shorter tails. This shows that these lizards adapted differently to their environment dep3nding on whether they were located on the mainland or the islands. These differing characteristics reconfirm that most of the adaptive radiation between the mainland and the islands occurred independently. On the islands specifically, species have adapted to certain “microhabitats” in which they require different morphological traits to survive. Irschick (1997) divides these microhabitats into six groups: “trunk–ground, trunk–crown, grass–bush, crown–giant, twig, and trunk.” Different groups of lizards would acquire traits for one of these particular areas that made them more specialized for survival in this microhabitat and not so much in others. Adaptive radiation allows species to acquire the traits they need to survive in these microhabitats and reduce competition to allow the survival of a greater number of organisms as seen in many of the examples before.

Adaptive radiation is a very important process in evolutionary history. Though it is most frequently observed in fragmented landscapes, adaptive radiation can occur almost anywhere. It plays a major role in diversifying populations and allowing organisms such as Darwin’s finches, cichlid fishes, the Hawaiian silverswords, and the Caribbean Anolis lizards to live together in the same environment but occupy different niches. Overall, adaptive radiation frequently has very positive results. Though a decent amount of research has been done on the mechanism itself, there are plenty of areas among specific case studies in need of developing. By further understanding adaptive radiation, one can more fully understand the evolutionary history of species.

References

Baldwin, Bruce G., and Michael J. Sanderson. "Age and rate of diversification of the Hawaiian silversword alliance (Compositae)." Proceedings of the National Academy of Sciences 95.16 (1998): 9402-9406.

Gavrilets, S., & Losos, J. B. (2009). Adaptive radiation: contrasting theory with data. Science, 323(5915), 732-737.

Irschick, Duncan J., et al. "A comparison of evolutionary radiations in mainland and Caribbean Anolis lizards." Ecology 78.7 (1997): 2191-2203.

Losos, Jonathan B. "Adaptive Radiation, Ecological Opportunity, and Evolutionary Determinism." The American Naturalist 175.6 (2010): 623-39. Web. 28 Oct. 2014.

Petren, K., Grant, P. R., Grant, B. R., & Keller, L. F. (2005). Comparative landscape genetics and the adaptive radiation of Darwin's finches: the role of peripheral isolation. Molecular Ecology, 14(10), 2943-2957.

Pinto, Gabriel, Luke Mahler, Luke J. Harmon, and Jonathan B. Losos. "Testing the Island Effect in Adaptive Radiation: Rates and Patterns of Morphological Diversification in Caribbean and Mainland Anolis Lizards." NCBI (2008): n. pag. Web. 28 Oct. 2014.

Rainey, P. B., & Travisano, M. (1998). Adaptive radiation in a heterogeneous environment. Nature, 394(6688), 69-72.

Schluter, D. (1995). Adaptive radiation in sticklebacks: trade-offs in feeding performance and growth. Ecology, 82-90.

Schluter, Dolph. The ecology of adaptive radiation. Oxford University Press, 2000.

Seehausen, O. (2004). Hybridization and adaptive radiation. Trends in ecology & evolution, 19(4), 198-207