WEIT 2012 Chap4
THE GEOGRAPHY OF LIFE
What is an endemic species ?
An endemic species is a species that is unique to a defined geographic location. An organism that is native of a particular place is not endemic if it can be found somewhere else. There is two different subcategories of endemism: paleoendemism and neoendemism. A paleoendemism is an organism which was at the origin widespread but that is now restricted to a smaller area whereas neoendemism would be a species that is "recent", like a species that has diverged and become reproductively isolated, like on an island, or a species that has formed following hybridization and that is now classified as a separated species.--TheoL 16 mars 2012 à 16:04 (CET)
Juan Fernandez Islands
Where are these islands located ?
The islands of Juan Fernandez are situated in the South Pacific ocean, off the coast of Chile. There are three of them: Robinson Crusoe (also Isla Más a Tierra) , Alejandro Selkrik Island (alsoIsla Más Afuera) and Santa Clara Island (Isla Santa Clara). They are three volcanic islands and only one of them is inhabited by humans, the Robinson Crusoe island (around 633 people).
Which are the endemic species found on the islands ?
We can count some 5 species of birds present on the different islands. Among these five species of birds is an endangered hummingbird, the Juan Fernandez Firecrown. This hummingbird is endangered because its population is in general decline since years, the male and the female of the species are very different, so much that until the 19th century people thought that they weren’t part of the same species. Two other species endemic to the Juan Fernandez Islands are the Juan Fernández Tit-tyrant, endangered because of the loss of habitat and the Masafuera Rayadito. This last species is mainly threatened by the introduced species which degrade its habitat. The Juan Fernandez Firecrown is mostly found on the Robinson Crusoe Island whereas the Masafuera Rayadito lives on the Alejandro Selkirk Island. 126 endemic species of plants can also be found on the diverse Juan Fernandez islands, within them, we can found many strange members of the sunflower family, also called Asteraceae. This family is one of the two largest flowering plant families with more than 22,750 currently accepted species and it's largely distributed throughout the world. The members of this family are mostly herbaceous plants but we can also find some shrubs, trees and climbers as Asteraceae. Their differentiation seems to have taken place some 42-36 million years ago but the reason why they developed so successfully remains unknown.Some fur seal are also present on the islands, these "Juan Fernandez fur seal" have a particular history because they were discovered in the 16th century and their members were so rare that they were considered as extinct in the mid-20th century until we found some 200 fur seals which were protected so that the population grows up. Now we think that they are at least 10'000 animals spread on and around the Juan Fernandez Islands. And of course a handful of insects can also be found on these three islands.
Why are these islands notable ?
In fact, the islands miss some groups very common on other areas such as the amphibians, the reptile and the mammals. And we can't count any native species from these groups on the islands. This lack of major groups is even more impressive knowing that they are groups widely spread around the world and that are completely absent of these islands which are on the other side the habitat of lots of endemic species. These two characteristics of the Juan Fernandez Islands seem to be repeated on oceanic islands in general, which gives us many informations about evolution.
Biogeography
What is biogeography ?
The biogeography is the study of the distribution of species throughout the world. This tends obviously to also study the evolutionary explanation of the geography of life because we cannot deny that the distribution of species is closely related to their evolution.However, the first theory about biogeography is ironically based on religion. It was trying to connect the Noah'Ark in the Bible with the distribution of species. But the explanation had too obvious problems, that's why an other theory took place. This theory pretended that the distribution of species had been done through multiple creations distributed around the world with the quotation of Louis Agassiz (renowned Swiss zoologist): "not only were species immutable and static but so were their distributes, with each remaining at or near their site of creation. This theory was also proved to be false (by fossils between others) and finally, Darwin's theory was formulated, saying that "the distributions of species were explained not by creation, but by evolution." Even if part of this theory isn't true, the major part was an important discover for all the study of distribution.
What is continental drift ?
The continental drift is the fact that the Earth's continents move. The first hypothesis about continental drift was expressed in 1596 by Abraham Ortelius and Alfred Wegener developped it in more details in 1912. Some time later, the idea of tectonic plates came out and the theory that the Earth is broken into around ten of these tectonic plates containing the continents which move with the sea floor. Continental drift has allowed the diversity of species to grow and develop.
- see Appendix 1
What is molecular taxonomy ?
The molecular is the classification of organisms with regard to the distribution and composition of chemical substances in them. This new way of classify the organisms is due to the information taken from DNA and protein sequences that help us understand not only the evolutionary relationships between species but also approximately when they separated from common ancestors.
What is molecular clock ?
The molecular clock is a way to deduce the time in geologic history when there is a divergence between two species or other taxa thanks to the use of fossil constraints and rates of molecular change. And with this method of the molecular clock, we can match the movements of continents that we know but also the movements of glaciers and the formation of genuine and bridges with the evolutionary relationships between species.
What is the relationship between the species and their habitat ?
We can fist see that having the same climate or similar terrain in two different places doesn't mean that you'll find the same species on the two areas. These two species, found in two different places but with an apparently similar habitat look even often the same. This could be explained by the fact that even if their habitat seems to be the same, there are little differences that can explain these differences between the species living there but in fact, this is not a good explanation because some species introduced in an habitat where they don't occur naturally can very well develop, just like cacti, introduced into Old Wolrd deserts.
What is convergent evolution ?
The convergent evolution is the description of the acquisition of an identical biological trait in unrelated lineages. These traits are called analogous structures in opposition with the homologous structures whose origin is common. The divergent evolution, opposite to the convergent evolution is the fact that related species develop different traits. As an example of convergent evolution, we can take the marsupial and placental mammals. The first ones are mainly found in Australia whereas the second ones are rather located elsewhere in the world and even if they show important anatmocal differneces, they have astonishing similarities in other characteristics. We have for example the sugar glider (marsupial) and the flying squirrel (placental) which both glide in the same way from tree to tree even if they live respectively in Australia and Americas. A second example could be the banded anteater (marsupial)living in Australia and behaving just like the anteater (placental) living in Americas. Finally we can talk about the marsupial mole (marsupial) and the mole (placental) which look and act the same. The theory of creationism cannot explain this particular similarities but the evolution can, by the principle of convergent evolution which defines that species living in similar habitats have to face similar selection pressures from their environment, which leads to similar adaptations, even if the two species aren't directly related. But these species will still show key differences that allow us to deduce their distant ancestry.
- see Appendix 2
What is the difference between continental and oceanic islands ?
What is a continental island ?
Continental islands are islands that were once connected to a continent and then separated from it either by the rise of the sea levels that made former land bridges "disappear" under water or by the movement of continental plates. Among these islands we can find the British Isles, Japan, Sri Lanka, Tasmania, Madagascar and many others. The period at which they separated from the continent with which they were connected is not the same for all. For example, Madagascar parted from Africa about 160 million years ago whereas Great Britain separated from Europe around 300'000 years ago.
What is an oceanic island ?
Oceanic islands are on the contrary islands that were never connected to a continent. The majority of oceanic islands were at first volcanoes and those which weren't volcanic in the origin, have been "created" through the movement of tectonic plates, where two different tectonic plates have lifted up the deep ocean floor to above the surface. The examples that we can give are the Hawaiian Islands, the Galapagos archipelago, St. Helena and the Juan Fernandez among others.
- see Appendix 3
What is the difference between continental and oceanic island in the process of Evolution ?
The main difference is the fact that oceanis islands are missing some groups of native species that we can easily find on continental islands and continents. The most impressive is that, in fact the pattern of missing groups are more or less the same on all the different oceanic islands. This is mainly due, according to Descartes, to the fact that humans introduced these species on oceanic islands and the suited so well that they often took over native species. For example, on the Galápagos Islands, there isn't any native species, but an frog, introduced by humans in 1998 can be found now on three of these islands. This fact has so much developed that oceanic islands are now somehow unsuitable for mammals, amphibians, reptiles and fish. The second fact caracterising oceanic island in the process of evolution is the fact that the types that we can find on these islands are often present ther in a great number. In general, we have particularly one species that is overrepresented like for example on the Galápagos islands with the Galápagos finches. This is also true for plants and insects and it's called "adaptive radiation". But the reason why some groups are missing and some others are overrepresented can be explained by the way they scatter. The missing species are all species that can scatter through long-distance dispersal. We can give the example of birds that can fly and go far relatively quickly or trees which, with seeds can also scatter relatively quickly. On the contrary, the examples of reptiles and mammals show that, because of their weight, between others, it's hard for them to cross expanses of sea because it's hard for them to swim very far. Hence, they can't be found on oceanic islands. FInally, the species that we can find on oceanic islands are most similar to those found on the nearest mainland despite their habitats are not the same. For continental islands, it's much simpler, as they develop, in general, the same species than the continents to which they were once joined.
What is Darwin's argument against Creationism, using biogeography?
"Why would a creator put plants that are fundamentally different, but look so similar, in diversse areas of the world that seem ecologically identical? Wouldn't it make more sense to put the same species of plants in areas with the same climate?". With this sentence , Jerry Coyne is directly attacking (by laying heavely on Dawrwin's work) Creationism, highligting the fact that there is so much odd and unbalanced floras and faunas compared to the continental assemblages, and wondering that if the species were crated, then why did the creator stock distant areas having similar terrain and climate, like the desert of Africa and of the Americas, with species that were superficially similar in form but showed other, more fundamental differences.
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