Biogeography

By Branden Holmes (Surroundx) on November 28, 2010

The very specific distribution of plants and animals across the face of the globe is arguably the best evidence for evolution. As with many things pertaining to evolution, Darwin inaugurated the field in 1859. He started off chapter XII (Geographical Distribution) with these words: “In considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be wholly accounted for by climatal and other physical conditions.” (Charles Darwin, On the Origin of Species, pg. 482) In a typically brilliant experiment he floated 87 different species of seeds in sea-water for 28 days before sowing them. He noted that 64 of the species thus germinated. He then performed a similar experiment but this time used branches with ripe fruit on them. This time he obtained results of 18 out of 94 kinds. He thus reasoned that the seeds and/or fruits of 14/100 species from any country in the world could survive a 28-day ocean crossing. Averaging figures of the speeds of various of the Atlantic currents he returned a figure of 33 miles per day, and thus reasoned that a seed would on average travel 924 miles in that time-span (if they had not encountered land during that time). He additionally noted that several of the species could survive immersion for a much greater period (at least 137 days in the case of some of the seeds) and could thus colonize islands much more distant from the original dispersal point.

He also perceptively noted the near-total absence of Batrachians (what we would call amphibians) and mammals from oceanic islands. He correctly reasoned that most frogs cannot stand sea-water, except for one Indian species (today we know of a number of others). He additionally noted that there were no known large mammals inhabiting oceanic islands, which were situated more than 300 miles from the nearest mainland, because a whole group of them could not survive such a long and perilous journey. (paragraph) But it was Alfred Russell Wallace who really set the foundations of biogeography. During a period of prolonged financial hardship Wallace had decided to put down on paper everything he had learned about biogeography through his voyage to South-east Asia. The result was Island Life: The Geographical Distribution of Animals. Upon his visit to the island of Lombok many years prior, for collecting specimens of plants and animals, he realised their similarity to the plants and animals of Australia and yet the sheer dissimilarity to that of Bali, only 15-miles westward: “It hadn’t taken Wallace long to realise the implications of Lombok's distinctive birds. Boundaries were his specialty; as a surveyor, he'd accrued years of experience in framing maps and demarcating limits. In faunal terms, Asia and Australia faced each other across a fifteen-mile moat.” (Iain McCalman, Darwin’s Armada, pg. 271) ISLAND GENETICS: There are two types of islands: continental (coastal) and oceanic; each of which is split into various sub-types (but that dissection need not concern us here). The great majority of both are caused by either volcanic or tectonic plate activity. But some are caused by a sudden decline in current strength, which then deposits large amounts of sediment within a small area until it builds up and penetrates the surface of the water. Coastal islands are islands that on the continental shelf of a ocean-bordered country and are usually a headland originally attached to the mainland and having been separated by rising sea-levels over the last 13,000 years (since the last ice age). Oceanic islands are formed either by underwater volcanoes depositing so much lava during an eruption that some of it penetrates the water surface after it has cooled. Or alternatively, and underwater version of the Himalayas (when the Indian sub-continent and Asia collided). (paragraph). Island genetics is a field in its own right. Most insular species are endemic; migratory birds making up a large proportion of the rest. Islands provide unique conditions under which animals can undergo radical changes. islands are completely surrounded by the ocean (or freshwater in rare cases) and as such are at the mercy of the weather infinitely more so than continental species. And because of this harsh weather the vegetation that all animals rely upon for one reason or another, usually consists of a disproportionate number of gymnosperms (pine trees and palms) which can survive in such a saline environment. This unique vegetation breeds unique animals. (paragraph). If the island is big enough it may contain a tropical rainforest in its centre. But the main reason that islands have such unique flora and fauna is that each species’ population size is limited: species cannot migrate to new territory (with the exception of bats and birds). They cannot escape a changing environment and migrate to a place where their phenotype better suits them. They must adapt to the ever-changing conditions of the island. But because of the small gene pool, genetic change can be effected much quicker than under normal conditions. (paragraph). But islands are not confined to plots of land in the middle of the ocean. Any hospitable environment surrounded by a ring of inhospitable land, qualifies. The accidental voyage of a few vagrants from one ‘island’ to another, has the same isolating effects as geographical islands. So long as the groups remain isolated again, genetic drift will occur. This topic is more fully dealt with under, Speciation. (paragraph). Many species of lizards, snakes, amphibians, turtles, crocodiles, insects, birds, bats, rodents and other small invertebrates and such, can quite easily travel from one island to another either under their own power, or inadvertently being swept out to sea clinging to a piece of driftwood or vegetation raft. But other larger non-aquatic mammals cannot ‘island hop’ and their existence on isolated islands indicate that the sea level must have been considerably lower in the past, to facilitate land bridges which they can cross. Very few large vertebrates are found on islands, and unless the sea-level falls dramatically in the future due to another ice age (which seems unlikely given our apparently warming planet) when they do go extinct, the islands won’t be repopulated by the former. (paragraph). PLATE TECTONICS: The geographical distribution of plants and animals does not make sense except in the light of plate tectonics. That fossils of a single species can be found on Australia, India, Africa and South America, is a striking fact, but was downright disheartening until we had a plausible mechanism to account for such anomalies. Alfred Wegener proposed the theory of continental drift in 1912 (1915?), noticing that the coasts of some of the continents complimented each other as if they had once been joined together. Although it would have to wait until the 1950's to be bestowed with the prefix 'theory'. And Wegener thought that the continents basically ploughed through the ocean. It wasn't until the 1960's that it was shown that what actually happens is that the seafloor "grows" as a result of volcanic activity depositing molten larva at the surface of the crust, which subsequently, as a result of the movement of the tectonic plates, gets pushed outwards allowing more new crust to be formed, and so on.

Related articles:
biogeography, Charles Darwin, Evolution, natural selection

About Branden Holmes
I am an amateur evolutionist interested in the theoretical side of the subject.

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