2) Biogeography is one source of evidence that evolution accounts for the diversity of life. Biogeography is the study of the geographic distribution of species and has contributed evidence for descent from common ancestors, which was hypothesized by Charles Darwin. Darwin and Alfred Wallace were both very interested in biogeography, which provided Darwin with evidence for evolution. Species distribution can be accounted for by ecological factors or by historical factors. The three major historical factors affecting geographic distribution are dispersal, extinction and vicariance.
Island biogeography has been extensively studied to show the evolution of species due to geographic barriers. Biogeography along with the history of the earth lends supporting evidence to evolution and the diversity of life on earth. Darwin showed that biogeographic facts make sense if a species has a definite site of origin, achieves a broader distribution by dispersal, and becomes modified giving rise to descendent species in the regions in which it migrates. He noticed that unrelated organisms inhabit parts of the world with similar climates and habitats, such as Old and New World organisms.
Darwin also found that organisms of various regions may be different due to barriers or obstacles that may limit migration. He also stated that there is a relation between inhabitants of the same continent or sea but that species differ from place to place. An example of this is aquatic rodents of South America are related to mountainous and grassland rodents of South America, but not to aquatic rodents of North America. Alfred Wallace noticed that several higher taxa had similar distributions and that the composition of biota is more uniform within certain regions then between them.
He had collected specimens in the Malay Archipelago and had thought of natural selection. He did extensive field work in the Amazon and noticed that geographic barriers, such as the Amazon River itself, separated the ranges of closely related species. These observations led him to designate several biogeographic realms. He observed that the fauna of Australia and Asia were different and the break between them is known as the Wallace Line. Historical factors affecting geographic distributions of species include extinction, dispersal, and vicariance. Extinction of certain opulations reduces the distribution of species but it also allows for diversification. Extinctions are selective in that some species are more likely to survive then others. For example, gastropods with wide geographic and ecological distributions and those with many species survived the end-Permian extinction. Extinction can lead to diversification in that it resets the stage for evolutionary radiations, perhaps by permitting the appearance of new community structures. Futuyma suggests that the extinction of one group permits the efflorescence of others, which is also shown in the fossil record.
Dispersal and vicariance are the major hypotheses attributing to a taxon’s distribution. An example of vicarance is taxa that have members on different land masses in the Southern Hemisphere, which is hypothesized to be due to the breakup of Gondwanaland isolating descendents of common ancestors. America, Africa, Madagascar, and India are all home to the freshwater fishes, cichlids. Molecular phylogenetic analyses has shown that two sister clades of cichlids have been found, one consisting of Madagascan and Indian species, and the other of two monophyletic groups, one in Africa and one in South America.
However, the splits between the clades are more recent than the breakup of Gondwanaland which suggests that perhaps the cichlids achieved their distribution by dispersal. Both vicariance and dispersal could be the likely cause of the geographic distribution of cichlids. Species expand their ranges by dispersal which is a critical process for geographic isolation in evolution and the current geographic distributions. Most species are restricted to certain biogeographic realms by their dispersal ability but many species have expanded their range due to human transplant.
For example, the European starling has expanded in North America following its introduction into New York City in 1896 (Futuyma, 2005). Transplanted species may disrupt the ecosystem at its new location by evolving and adapting to the new environment, possibly causing the extinction of native species. Adaptation to the environment as a primary product of evolution was suggested by Jean Baptiste Lamarck who believed that evolution is the best explanation of the diversity of life. Vicariance is the separation of populations of a widespread species by barriers arising from changes in climate, geology, or habitat.
Vicariance can lead to speciation if populations are separated by a geographic barrier and evolve genetic reproductive isolation such that if the barrier disappears, the species can no longer interbreed. Natural selection is a powerful evolutionary force and therefore the genetic changes that result in reproductive isolation in vicariant speciation are likely due to adaptive evolution. Several geological processes can fragment a population into two, such as a mountain range emerging, rivers, lakes, or land bridges.
Island biogeography is a very good example of evidence for evolution. Islands are generally where endemic species of plants and animals are found but Darwin observed that most island species are closely related to species from the nearest mainland or neighbouring island. Two islands that have similar environments in different parts of the world are populated by species that are taxonomically affiliated with the plants and animals of the nearest mainland which generally has a different environment.
Speciation may occur on islands if a species that disperses from a mainland to an island succeeds in its new environment and gives rise to several new species as populations spread to other islands. Once isolated, geographically separated populations become genetically differentiated as a result of mutation and other processes such as natural selection. Environmental factors are likely to be different from one place to another so natural selection can contribute to geographic variation, differences in the gene pool between populations.
Speciation is often a gradual process as the reproductive barriers between the groups is only partial in the beginning but leads to complete reproduction separation (Campbell and Reece, 2002). An example of vicariance and island biogeography is the finches on the Galapagos Islands. Darwin noticed that the finches he collected from the islands were very similar, but that they were in fact different species. Some were unique to individual islands, while other species were distributed on two or more islands that were close together.
New finch species had arisen from an ancestral form by the gradual accumulation of adaptations to a different environment. For example, the different beak sizes of the finches are adapted to the specific food available to them on their home island. This is an example of species adapting and evolving to suit their new environment. The Hawaiian Islands are another example of the world’s showcase of evolution and island biogeography. Each island started bare but was gradually populated by species that either rode ocean currents or blew over in the wind, either from distant islands or continents.
The physical diversity of each island provides many different environmental opportunities for evolutionary divergence by natural selection. Many of the plants and animals that are currently found on the islands are found nowhere else in the world, they are endemic to the Hawaiian archipelago (Campbell and Reece, 2002). The history of earth also helps to explain the current geographic distribution of species. For example, the emergence of volcanic islands such as the Galapagos opens new environments for species to inhabit and adaptive radiation fills many of the available niches with new species.
On a global scale, continental drift is a major factor correlated with the spatial distribution of life and with such evolutionary episodes as mass extinctions followed by increases in biological diversity. The continents drift about earth’s surface on plates of crust floating on the hot mantle and their positions can therefore change relative to one another. At the end of the Paleozoic era, plate movements brought all the landmasses together into a super continent named Pangaea. Species that had been evolving in isolation were brought together at this point and forced to compete.
The formation of Pangea reduced shoreline, drained shallow coastal areas, changed the climate and increased the area inland destroying a considerable amount of habitat and reshaping biodiversity. During the Mesozoic, Pangaea broke apart creating new continents that became separate evolutionary areas allowing flora and fauna to diverge. The diversity of life on earth is due to millions of years of evolution. Darwin and Wallace were both important figures in the field of biogeography as their interest and research led to ideas that are still apparent today.
Historical geographic factors and the history of the earth are a few aspects that have led to current biodiversity. Biogeography shows compelling evidence that species evolve through natural selection by adapting to new environments. Speciation will continue to occur as the environment changes and as the continents continue to drift. References: Campbell, N. A, and Reece, J. B. 2002. Biology; Sixth Edition. Benjamin Cummings, San Francisco. Futuyma, D. J. 2005. Evolution. Sinauer Associates Inc. , Massachusetts.
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