VIEWS: 59 PAGES: 2 POSTED ON: 5/1/2010
THE EVIDENCE FOR EVOLUTION: BIOGEOGRAPHY Biogeography is the science that seeks to explain the distribution of living things through space and time on Earth. Biogeography looks at current species of animals and plants as well as the evidence of past life (fossils) in order to determine how species arose. If species were created as perfectly adapted to their environment, we might assume similar environments to have similar plant and animal life. This is NOT the case. Instead, the animals and plants of a region are most closely related to those of nearby regions and reflect the history of the region. Biogeography states that the range and dispersal (basically, where an organisms lives) is dependent upon the barriers in moving from one area to another. Since the theory of evolution states that life forms of the present evolved from common ancestors from the past, we could predict that the geography of Earth and changes in the geography of Earth can account for the diversity of life on Earth. In essence, physical, climactic and other barriers caused by the geography of earth are one of the causes for evolution. Islands or larger land masses (including continents) that have been isolated for long periods of time have groups of animals that have adapted to different local environments, yet are clearly closely related to each other. In cases with long histories, the distributions of fossils will correspond with the distributions of living organisms. Continental Drift and Isolation: For example, Australia has been an isolated land mass for over 30 million years. (Before that, it was connected only to Antarctica for an additional 70 or so million years.) The mammals of Australia include creatures as diverse as kangaroos, wolf-like carnivores, opossums, woodchuck-like wombats and honey gliders. Despite this incredible variety, all of these animals belong to a single group of mammals: the marsupials. These marsupials are not closely related to animals such as mice, moles and woodchucks that are found elsewhere. In addition, almost all fossil mammals from Australia are also marsupials. The Arctic and Antarctic have very similar environmental conditions. However, the Arctic supports polar bears (closely related to northern hemisphere grizzly and brown bear) and walrus while the Antarctic supports penguins (which are found only in the southern hemisphere). Of course, seals live in both places, but they live in warmer waters too. South America was also isolated as an island continent for many millions of years. During this time, a whole menagerie of mammals called xenarthrans diversified there. Xenarthrans include our modern sloths, armadillos and anteaters, as well as extinct giant ground sloths and tank-like glyptodonts. When the Central American land bridge (removal of a barrier) arose about 3 million years ago and connected South and North America, many xenarthrans migrated north and many northern mammals (such as the ancestors of lamas, monkeys, jaguars and otters) went south. Island Geography: The development of ecosystems on islands is somewhat unpredictable. However, pioneer organisms do arrive and gradually alter the environment until a stable community is established on an island. What is unpredictable is which plant and animal species will colonize these new environments. Much of this is left to climate, proximity to other land masses, and of course, chance. The Galapagos Islands west of South America and the Cape Verde Islands off west Africa have similar tropical environments. But, their native plants and animals are not closely related; instead, they closely resemble the flora and fauna of their respective nearby large land masses. The Canary Islands form an archipelago of seven volcanic islands just west of the African continent. Amongst this grouping of islands there are three different species of a lizard. The arrival of the Gallotia lizards to these volcanic islands was probably by rafting. Rafts of natural vegetation are often washed out to sea when high river levels cause river banks to collapse. They carry away both plants and clinging animals alike. Colonization by airborne organisms, such as insects and birds, usually occurs during storms. In any case, there are some general principles of island colonization: 1) The closer the island to another land mass, the higher the probability of colonization. 2) The older the island, the more likely it will be colonized. 3) The larger the island, the more species are likely to be established. 4) Geographic isolation reduces gene flow between populations. 5) Over time, colonial populations become genetically divergent from their parent population due to natural selection, mutation, and/or genetic drift. Species in the Making New species often evolve when populations of one species become separated, and the separate populations find themselves in different environmental conditions (so natural selection can drive them in different evolutionary directions). Many examples exist right now where we can catch species in the process of evolving. For example, in species with wide distributions, members of a local population may be able to breed with individuals from nearby populations. But, more distant populations differ more and more until they are unable to interbreed and effectively constitute distinct species. A classic example concerns sea gulls in the far northern hemisphere. Over northern Europe, the herring gull (Larus argentatus) and the lesser black-backed gull (Larus fuscus) are distinct species that do not interbreed. But, over much of Canada, only the herring gull exists. Over Russia, you only find the black-backed gull. If you follow these populations west to Alaska and east to Siberia, they eventually meet as one species. So, you have gulls that represent a single species in part of their range, and two species where the extreme ends of the populations meet. If, say, the Alaskan populations were to become extinct, these two gulls might continue to diverge and become completely distinct species. If, on the other hand, enough interbreeding took place between, say, Alaskan and Siberian populations, genes might spread eastward through Canada and westward through Russia until the herring and black-backed ends could interbreed, and we'd be back to a single species. Questions: Answer these questions on a separate piece of paper in complete sentences: 1. Describe why biogeography gives evidence in support of the theory of evolution. 2. Why would areas that have very similar climates (like the Arctic and Antarctica) contain very different species? 3. Which islands in the Canary archipelago should contain lizard species that are the most different from the species found on the coast of western Africa? 4. How does the sea gull example show that a species can change due to biogeographical barriers? 5. THINK – Biogeography is based on barriers (physical, climactic etc) to the movement of organisms. What are some additional types of barriers that can cause a population to evolve into separate species? (List and explain at least 2).
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