Transcript Slide 1
HMS Beagle • Born 12 Feb 1809 • Medicine (Edinburgh University), Theology (Cambridge) • 1859, publication of The Origin of Species http://en.wikipedia.org/wiki/Evolution http://www.picturehistory.com/product/id/15157 • The second voyage of HMS Beagle from 27 December 1831 to 2 October 1836 was the second survey expedition of HMS Beagle • Captain Robert FitzRoy accompanied by a student clergyman Charles Darwin • Darwin made his name as a naturalist and became a renowned author with the publication of his journal which became known as The Voyage of the Beagle. • The Beagle sailed across the Atlantic Ocean then carried out detailed hydrographic surveys around the coasts of the southern part of South America, returning via Tahiti and Australia having circumnavigated the Earth. While the expedition was originally planned to last two years, it lasted almost five. http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_11 Aristotle’s Chain of Being The modern view. We focus on the lower realm of being! Observations 1. If all the individuals of a species reproduced successfully, the population of that species would increase uncontrollably. 2. Populations tend to remain about the same size from year to year. 3. Environmental resources are limited. 4. No two individuals in a given species are exactly alike. 5. Much of this variation in a population can be passed on to offspring. Darwin realized that the unequal ability of individuals to survive and reproduce could cause gradual changes in the population. Traits that help an organism survive and reproduce would accumulate over generations. On the other hand, traits that hinder survival and reproduction would disappear. Darwin used the term natural selection to describe this process http://en.wikipedia.org/wiki/Introduction_to_evolution Evolution Natural Selection Genetics, Speciation Evidence from biology, fossils, homologous structures, DNA http://en.wikipedia.org/wiki James Watson and Francis Crick G. Mendel Life began 3.8 billion years ago, and insects diversified 290 million years ago, but the human and chimpanzee lineages diverged only five million years ago. How have scientists figured out the dates of long past evolutionary events? 1. Radiometric dating relies on half-life decay of radioactive elements to allow scientists to date rocks and materials directly. 2. Stratigraphy provides a sequence of events from which relative dates can be extrapolated. We’ll focus on the first two techniques in this lecture 3. Molecular clocks allow scientists to use the amount of genetic divergence between organisms to extrapolate backwards to estimate dates. http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_12 How do we determine the age of a rock? Henri Becquerel 1. Relative dating - Steno's Laws, etc. "A is older than B" 2. Absolute dating Quantify the date in years. Radiometric Dating • Naturally-occurring radioactive materials break down into http://en.wikipedia.org/wiki/Henri_Becquerel other materials at known rates. This is known as radioactive decay. • Radioactive parent elements decay to stable daughter elements. • Radioactivity was discovered in 1896 by Henri Becquerel. In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium. In 1907, Boltwood dated a sample of urnanite based on uranium/lead ratios. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately. • The invention of the MASS SPECTROMETER after World War I (post-1918) led to the discovery of more than 200 isotopes. http://www.gpc.edu/~pgore/geology/geo102/radio.htm Document created by: Pamela J. W. Gore, Georgia Perimeter College, Clarkston, GA, Document created February 4, 1996, Modified April 4, 1997, Last modified February 3, 1999 Radioactive parent isotopes and their stable daughter products Radioactive Parent Stable Daughter Potassium 40 Argon 40 Rubidium 87 Strontium 87 Thorium 232 Lead 208 Uranium 235 Lead 207 Uranium 238 Lead 206 Carbon 14 Nitrogen 14 In the table to the left, note that the number is the mass number (the total number of protons plus neutrons). Note that the mass number may vary for an element, because of a differing number of neutrons. Elements with various numbers of neutrons are called isotopes of that element. Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product. Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). This is a random process on the atomic level, in that it is impossible to predict when a particular atom will decay, but given a large number of similar atoms, the decay rate, on average, is predictable. http://en.wikipedia.org/wiki/Radioactivity http://www.gpc.edu/~pgore/geology/geo102/radio.htm Half Lives for Radioactive Elements Radioactive Parent Stable Daughter Half life Potassium 40 Argon 40 1.25 billion yrs Rubidium 87 Strontium 87 48.8 billion yrs Thorium 232 Lead 208 14 billion years Uranium 235 Lead 207 704 million years Uranium 238 Lead 206 4.47 billion years Carbon 14 Nitrogen 14 5730 years Radioactive decay occurrs at a constant exponential or geometric rate. The rate of decay is proportional to the number of parent atoms present. http://www.gpc.edu/~pgore/geology/geo102/radio.htm The proportion of parent to daughter tells us the number of half-lives, which we can use to find the age in years. For example, if there are equal amounts of parent and daughter, then one half-life has passed. If there is three times as much daughter as parent, then two half-lives have passed. Radioactive decay occurs by releasing particles and energy. Uranium decays producing subatomic particles, energy, and lead. http://www.gpc.edu/~pgore/geology/geo102/radio.htma How does Carbon-14 dating work? • Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide. • Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle. • All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion). • At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14. • The change in the Carbon 14 to Carbon 12 ratio is the basis for dating. • The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages). • Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years). Sample Feed A very low concentration of sample molecules is allowed to leak into the ionization chamber (which is under a very high vacuum) where they are bombarded by a high-energy electron beam. The molecules fragment and the positive ions produced are accelerated through a charged array into an analyzing tube. The path of the charged molecules is bent by an applied magnetic field. Ions having low mass (low momentum) will be deflected most by this field and will collide with the walls of the analyzer. Likewise, high momentum ions will not be deflected enough and will also collide with the analyzer wall. Ions having the proper mass-to-charge ratio, however, will follow the path of the analyzer, exit through the slit and collide with the Collector. This generates an electric current, which is then amplified and detected. By varying the strength of the magnetic field, the mass-tocharge ratio which is analyzed can be continuously varied. http://www.chem.uic.edu/web1/ocol/spec/MS1.htm http://www.chem.ucalgary.ca/courses/351/Carey/Ch13/ch13-ms.html The mass spectrum of toluene (methyl benzene) is shown. The spectrum displays a strong molecular ion at m/e = 92, small m+1 and m+2 peaks, a base peak at m/e = 91 and an assortment of minor peaks m/e = 65 and below. The molecular ion, again, represents loss of an electron and the peaks above the molecular ion are due to isotopic abundance. The base peak in toluene is due to loss of a hydrogen atom to form the relatively stable benzyl cation. This is thought to undergo rearrangement to form the very stable tropylium cation, and this strong peak at m/e = 91 is a hallmark of compounds containing a benzyl unit. The minor peak at m/e = 65 represents loss of neutral acetylene from the tropylium ion and the minor peaks below this arise from more complex fragmentation. http://www.chem.uic.edu/web1/ocol/spec/MS1.htm K-T Boundary, 65 M y.o. Stratigraphy is the study of strata, or layers. Specifically, stratigraphy refers to the application of the Law of Superposition to soil and geological strata containing archaeological materials in order to determine the relative ages of layers. In addition, stratigraphy can tell us much about the processes affecting the deposition of soils, and the condition of sites and artifacts. These are called postdepositional processes, and their study is part of Middle Range Theory. http://en.wikipedia.org/wiki/Stratigraphy http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/04_Stratigraphy.html GISP2 ice core at 1837 meters depth with clearly visible annual layers http://en.wikipedia.org/wiki/Ice_core http://web.utk.edu/~grissino/software.htm Ice core dating is analogous to dating using tree rings. Heavier water molecules (H2 180) precipitate more readily than lighter (H2 160) molecules, and the process is temperature dependent, and varies with the seasons. In the winter the precipitation at the poles is more enriched in H2 160 water molecules, resulting in a 'ring type pattern'. Reading these rings at greater depths is more difficult as the weight of the ice sheets compresses the rings progressively more the deeper one drills, and so companion techniques are employed to supplement the standard methods. http://www.awitness.org/bible_commentary/genesis /ice_core_ring_dating.html Within the framework of the joint European Greenland Ice Core Project (GRIP) a 3029 m long ice core was drilled in Central Greenland from 1989 to 1992 at 72o 35' N, 37o 38' W. Polar ice cores contain a record of the past atmosphere - temperature, precipitation, gas content, chemical composition, and other properties. The objective of the GRIP effort was to reveal the broad spectrum of information on past environmental, and particularly climatic, changes that are stored in the ice. This information will help investigators understand the major mechanisms of the earth and man's potential impact. The Greenland Ice Core Project (GRIP) http://www.ncdc.noaa.gov/paleo/icecore/greenland/s ummit/document/ K. Makinson Studies of isotopes and various atmospheric constituents in the core have revealed a detailed record of climatic variations reaching more than 100,000 years back in time. The results indicate that Holocene climate has been remarkably stable and have confirmed the occurrence of rapid climatic variation during the last ice age (the Wisconsin). Climatic instability observed in the core part believed to date from the Eemian interglacial has not been confirmed by other climate records