Darwin and Common Descent
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Darwin and Common Descent

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Darwin and Common Descent Darwin and Common Descent Presentation Transcript

  • Some Highlights of Charles Darwin’s 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 Darwin’s 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 Galápagos 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. Galápagos 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 Darwin’s, 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”. Darwin’s “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 MESOZOIC ERA • 250 to 65 mya PALEOZOIC ERA • 600 to 250 mya PRECAMBRIAN ERA • 4,600 to 600 mya
  • • How a particular group of organisms evolved can often be determined by arranging its fossils in chronological order. o The horse (Equus) has a nearly-complete fossil record found in North American sedimentary rock dating from 55 mya to the present.
  • o From skeletons to teeth, hominid (human-like) fossils have been found of more than 6,000 individuals; some species are represented by only one or a few fossils, others are represented by thousands of fossils.
  • Transitional Fossils • Fossils that resemble intermediate species, having skeletal and other body features in common with two distinct groups of organisms, such as reptiles and mammals, or mosses and ferns, are especially important. o For example, Tiktaalik (aka the “fishibian” or the “fishapod”), a fossil found in 2004, was a large, scaled fish that shows a perfect transition between fins and feet, combining certain features of both lobe-finned fish and four-limbed vertebrates (tetrapods).
  • Tiktaalik had fish-like scales, as well as fish-like fin rays and jaw and mouth elements, but it had a shortened skull roof and mobile neck to catch prey, an ear that could hear in both land and water, and a wrist joint that is like those seen in tetrapods. [tetrapod characters=yellow, fish=blue]
  • o Archaeopteryx, a fossil found in 1861, combined certain features of both reptiles and modern birds, such as: (1) a wishbone (fully fused clavicle), which is only found in modern birds and some dinosaurs, (2) feathers on its body, as seen on many of the theropod dinosaurs from which it evolved, (3) wing-like forelimbs, (4) teeth (no birds alive today have teeth), (5) a long bony tail (tails on modern birds are entirely feathers, not bony), (6) long hind legs and toes, and (7) a specialized hand with long bony fingers (unlike modern bird wings in which the fingers are fused into a single element).
  • Artist’s restoration of Archaeopteryx.
  • Homologies • Naturalists long before Darwin recognized two major types of similarities among living organisms, but could not explain why they exist: o Analogies (analogous structures) – body parts in different species that have the same function but are different in structure. Example: bird and insect wings. o Homologies (homologous structures) – body parts in different species that have the same structure, regardless of function. Example: the forelimb bones of birds and humans.
  • • Evolutionary theory says homologies are due to inheritance from a common ancestor that also had that structure. o For example, the set of bones in the forelimbs of tetrapods (vertebrates with four limbs) are the same (humerus, radius, and ulna) because these organisms share a common ancestor which had these bones. These bones are homologous. The origin of forelimb bones in birds and bats is shown in this diagram, which is supported by a large number of other characteristics.
  • • Evolutionary theory says that analogies, on the other hand, are not due to inheritance from a common ancestor, but evolved independently more recently. o For example, bat wings consist of flaps of skin stretched between the bones of the fingers and arm, but bird wings consist of feathers extending all along the arm. These differences suggest that bird and bat wings have separate evolutionary origins, but are superficially similar because they evolved to serve the same function. The origin of wings in birds and bats is shown in this diagram, which is supported by a large number of other characteristics.
  • Although the bones of the forelimbs are homologous, the wings a bird (left) and a bat (right) are analogous.
  • • Not all anatomical homologies are obvious. If the same body parts have evolved for different functions in different organisms, they may not look very much alike. o For example, the chomping front teeth of a beaver look quite different than the tusks of an elephant, and they have different functions. o But if you examine these two structures closely, you will see that each is a modification of the basic incisor tooth. They are homologous structures, inherited from a common ancestor with incisor teeth.
  • • Likewise, leaves may have very different shapes and functions, yet all are homologous structures, derived from a common ancestor that had leaves. o o o o Pitcher plant (left) – leaves modified into pitchers to capture insects; Pea plant – leaves modified into tendrils used for physical support; Poinsettia – red leaves resemble flower petals to attract pollinators; Cactus – leaves modified into spines to reduce water loss and protect the cactus from herbivory.
  • • Naturalists long before Darwin recognized the vestigial structure, a type of homologous structure that is reduced and rudimentary compared to its counterpart in other organisms. Evolutionary theory explains the vestigial structure as no longer required for survival. The pelvic bones of a whale (Balaena) resemble those of other mammals, but are only poorly developed in the whale and have no apparent function.
  • • Though many vestigial structures lack any function, this is not a requirement for vestigiality. o Darwin wrote that vestigial organs “are either quite useless, such as teeth which never cut through the gums, or almost useless, such as the wings of an ostrich, which serve merely as sails.” Impacted human third molar, or wisdom tooth (top), and running ostrich.
  • • How do scientists tell the difference between analogies and homologies? In general, two anatomical structures are likely to be homologous if: o They have the same basic structure. o They have the same relationship to other parts of the body. o They have the same development.
  • • Different species share genetic homologies as well as anatomical ones. o Roundworms, for example, share 25% of their genes with humans. These genes are slightly different in each species, but their striking similarities nevertheless reveal their common ancestry. o Evolutionary relationships determined from genetic homologies have been astoundingly similar to those determined from anatomical homologies. What percentage of your genes do they have? Another human 100%* A chimpanzee 98% A mouse 92% A fruit fly 44% Yeast 26% Thale cress (a weed) 18% *That is, everyone has genes for hair color, eye color, skin color, blood type, etc.
  • Geographical Distribution of Species • Evolutionary theory predicts that the species in any one area have evolved in place from local species from the surrounding areas. So, species should be more closely related to nearby species than to species far away, regardless of habitat.
  • • And empirical evidence supports this: o For example, native species on oceanic islands are most closely related to the species on the nearest mainland, even if the environment there is very different from the islands. Lemur (top), galago (middle), and loris (bottom).
  • o The numerous species of Galapagos finches are most closely related to the Dull-colored Grassquit (Tiaris obscurus), which lives on the nearby mainland of South America. The Dull-colored Grassquit (top) and the Large Ground Finch (bottom).
  • • But what about closely related species that are widely separated? For example, there are only a few species of lungfishes, living in freshwater habitats in Australia, South America, and Africa. Australian lungfish (top), African lungfish, and South American lungfish (bottom).
  • o Lungfishes originally evolved about 300 mya on a supercontinent we call Pangaea, which included all of Earth’s major landmasses. As it gradually broke up due to tectonic forces into the modern continents over hundreds of millions of years, the lungfishes were separated. They survived only on South America, Africa, and Australia.
  • Pangaea: 250 million years ago
  • Continental Drift in Action (2013) http://vimeo.com/diogenesii/pangaea
  • Test Your Understanding #1 • Sugar gliders and flying squirrels look amazingly similar. The facts: o Both are mammals of about the same size, with big eyes and a white belly. o Both glide from treetops using a thin piece of skin that is stretched between their legs. These animals have the same lifestyle: leaping from treetops (hence, the gliding "wings") and foraging at night (hence, the big eyes.)
  • o Sugar gliders live in Australia, and flying squirrels live in North America. o Sugar gliders are marsupial mammals (like kangaroos) because their embryos do not receive nutrients from their mothers. Flying squirrels, on the other hand, are placental mammals – their embryos receive nutrients through the placenta and are born much more fully developed.
  • o By studying their genes and many other traits, biologists have concluded that the closest relatives of sugar gliders are other marsupials (like kangaroos and opossums), not the flying squirrels. Likewise, the closest relatives of flying squirrels are other placentals (like elephants and humans), not the sugar gliders. • Considering all of the evidence, are the "wings“ of sugar gliders and flying squirrels homologous or analogous structures?
  • • Answer: analogous! o Since sugar gliders and flying squirrels are very distantly related, it seems very unlikely that their common ancestor had flaps of skin stretched between its legs, which both modern animals inherited. o Instead, the “wing” evolved for gliding and tree-living in each animal due to similar habitats – but from different ancestors.
  • Test Your Understanding #2 • Marsupial mammals are native to the Americas as well as Australia/New Guinea, as shown in brown on the map. The facts: o They do not swim across the Pacific Ocean, nor do they wander the Asian mainland. There appear to be no routes of migration between the two populations. o Fossils of marsupials have been found in Antarctica as well as in South America and Australia. • So, how could marsupials have gotten to locations half a world apart?
  • • Answer: Marsupials didn’t need a migration route from one part of the world to another – they rode the continents to their present positions! o Marsupials are descended from a common ancestor that lived over 140 mya on the southern part of Pangaea we call Gondwana, before it split into Australia, South America, and Antarctica.