Darwin and Common Descent

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  • Some Highlights of Charles Darwins Life (in his own words) He was a lazy teenager. To my deep mortification my father once said to me, "You care for nothing but shooting, dogs, and rat catching, and you will be a disgrace to yourself and all your family." Charles Darwin, age 6, with his sister Catherine. Chalk drawing, 1816, by Ellen Sharples (1760-1849).
  • His father wanted him to be a doctor, but he dropped out of medical school in his second year. After having spent two sessions in Edinburgh, my father perceived, or he heard from my sisters, that I did not like the thought of being a physician, so he proposed that I should become a clergyman. He was very properly vehement against my turning into an idle sporting man, which then seemed my probable destination. Darwin's rooms in Christ's College, Cambridge, where his father sent him to become a clergyman.
  • 1831-1836 HMS Beagle voyage around the world. Considering how fiercely I have been attacked by the orthodox, it seems ludicrous that I once intended to be a clergyman. Nor was this intention and my father's wish ever formerly given up, but died a natural death when, on leaving Cambridge, I joined the Beagle as naturalist. . . . The voyage of the Beagle has been by far the most important event in my life, and has determined my whole career. . . I have always felt that I owe to the voyage the first real training or education of my mind. . .
  • Charles Darwins route around the world in HMS Beagle. The ship explored and charted the South American coastline, and returned via New Zealand and Australia.
  • Provocative observations. Darwin observed unexpected patterns on the Beagle voyage that got him thinking o Finch variation on the Galapagos Islands. The Galpagos Islands have species found in no other part of the world, though similar ones exist on the west coast of South America. Darwin was also struck by the fact that the birds were slightly different from one island to another. Galpagos finches showing different beak shapes.
  • o Similarity between fossils of extinct species and the living species in an area. In South America, Darwin found fossilized fragments of armor which he thought were like giant versions of the scales on the modern armadillos (top) living nearby, but the anatomist Richard Owen showed him that the fragments were in fact from gigantic extinct glyptodons (bottom), related to the armadillos.
  • Years of thinking. In October 1838, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long- continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work. At last gleams of light have come, and I am almost convinced (quite contrary to the opinion I started with) that species are not (It is like confessing to murder) Immutable.
  • Almost scooped by Alfred Russel Wallace. early in the summer of 1858 Mr. Wallace, who was then in the Malay archipelago, sent me an essay "On the Tendency of Varieties to depart indefinitely from the Original Type;" and this essay contained exactly the same theory as mine. Mr. Wallace expressed the wish that if I thought well of his essay, I should send it to Lyell for perusal. Charles Darwin (left) and Alfred Russel Wallace around the time of the Linnean Society announcement of their discovery of natural selection.
  • The Origin of Species. It is no doubt the chief work of my life. It was from the first highly successful. The first small edition of 1250 copies was sold on the day of publication, and a second edition of 3000 copies soon afterwards. Sixteen thousand copies have now (1876) been sold in England; and considering how stiff a book it is, this is a large sale. It has been translated into almost every European tongue, even into such languages as Spanish, Bohemian, Polish, and Russian.
  • Parts of last paragraph of Charles Darwins, Origin of Species by Means of Natural Selection or the Preservation of Favored Races in the Struggle for Life Sixth Edition, January 1872: It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. . . . There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.
  • Common Descent Charles Darwin proposed the idea of universal common descent through an evolutionary process in The Origin of Species. o He twice stated the hypothesis that there was only one ancestor for all currently living organisms on Earth, ending his book with There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one. Darwins tree of life from page 36 of his Transmutation Notebook B.
  • Common descent is easy to understand in terms of a family tree: o You and your cousins share common descent because you are both descended from a common grandmother (and grandfather), who is therefore your common ancestor. o However, you are more closely related to your siblings than to your cousins because both you and your siblings share a more recent common ancestor, your mother (and father).
  • The Evidence for Common Descent Because it is so well supported empirically, common descent among living organisms is considered to be a scientific fact. The three main kinds of evidence for common descent are: o The fossil record; o Contemporary homologies; and o Geographical distribution of species. An Evolutionary Tree.
  • The Fossil Record Fossils are the physical evidence of former life from a period of time prior to recorded human history. o This includes the preserved remains or traces, such as burrows and footprints, of once-living organisms. o All of the fossils that exist, whether discovered or not, provide us with a record of the history of life on Earth. This is called the fossil record. A fossilized trilobite, an extinct marine arthropod.
  • When an animal or plant dies, usually the remains are eaten or decompose. On rare occasion, if it dies in water, or near enough to fall in shortly after death, the following may occur: 1) The body is quickly buried in a layer of mud or sediments settling at the bottom of a lake or ocean. 2) Water and minerals dissolve the bone or original material, replacing it, and then it hardens over time. 3) At the same time, the surrounding layers of sediment turn into rock. 4) Geological uplift raises the layers of sedimentary rock, and the fossil is now buried on dry land. 5) Erosion by wind or water exposes the fossil to human passers-by.
  • Layers of sedimentary rock exposed by the Colorado River.
  • Geologists long before Darwin noticed patterns in the distribution of fossils in sedimentary rock: o Older rocks usually lie below younger rocks. o Each layer contains a distinctive group of fossils, called index fossils. Each rock layer, identifiable by its index fossils, was named for a division of geological time.
  • o The sequence of rock layers is the same all around the world. o Beginning in the 1830s, geologists noticed how fossils tend to gradually change through time. Trilobites lived before ammonites and belemnites. Finding a trilobite fossil in a rock tells you the rock was formed in the Paleozoic era.
  • Scientists use two kinds of dating techniques to work out the age of rocks and/or fossils: o Relative dating (age in geologic time) by observing which index fossils are found in the same layer with the sample. o Absolute dating (numerical age in years) by analyzing the amount of radioactive decay in the minerals of the rocks. The time at which an event occurred (its age), may be expressed in two ways.
  • As a result of absolute dating, geologists now know the number of years ago that each division of geologic time corresponds to. For example, the four major divisions of geologic time and their dates are (mya = million years ago): CENOZOIC ERA 65 mya to present


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