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sequences but with only four lettersthe four bases that pair up to connect the two phosphate sugar backbones of the double- helix adenine cytosine guanine and thymine. ACGT. Four letters that demonstrate the common origin of life on Earth. The Tree of Life Non-scientists have often observed that it is easier to see how natural selection can explain microevolution like resistance to antibiotics than to see how it could lead to the emergence of an entirely new species. One of the key concepts in evolution through natural selection is the idea of splitting where one species becomes two. How does that happen One way is through geographic isolation. The rheas in South America are a good example. When he was in Patagonia Darwin found two species of rhea one large one small with different ranges the larger north of the Rio Negro the smaller one south of the river. Originally there was only one rhea. When a small group became separated from the main population it evolved into a slightly different species. In all likelihood a small group got separated by the Rio Negro River which now forms a boundary between the two populations. The water level probably dropped during a dry spell allowing a small population to cross over. When the water level returned to normal the two populations found themselves isolated rheas are flightless birds. Because the environments are slightly different on the two sides of the river the populations slowly diverged and became different species. Perhaps the original large rheas got cut off south of the river and slowly decreased in size their beaks also changingadaptations to different food sources andor predators. Evolution Today Darwins theory of natural selection has withstood the ultimate testthe test of time. Today 150 years after he first formulated it it remains the unifying concept in the biological sciences. Advances in molecular biology population statistics paleontology and other fields have all confirmed the basic tenets of the theory. Scientists no longer debate whether or not evolution has occurred and is occurring but instead focus on the details of the process and try to unravel its complex history. In retrospect it is remarkable Darwin was able to formulate the theory of natural selection given how little he knew. With no understanding of how traits are passed on from one generation to the next genes and no explanation for variation through genetic mutation and the variation inherent in sexual reproduction through genetic recombination he looked at the world around him and figured it out with only a handful of clues the inevitable struggle for survival the logic of differential success and the evidence of an ever-changing environment. From there he put the pieces together without understanding the finer details. One hundred years later an important piece of information came to light. In 1953 Francis Crick and James Watson elucidated the now famous double-helix structure of DNA along with Maurice Wilkins they won the Nobel Prize in 1962. Here was the structure that explained how information could be replicated and passed on biochemically from generation to generation. Here was the mechanism through which variation could arise. Most importantly here was the common denominator of all living things. The language of life written in staggeringly long 41