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Chapter 25 - CHAPTER 25 PHYLOGENY AND SYSTEMATICS...

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CHAPTER 25 - PHYLOGENY AND SYSTEMATICS Introduction Evolutionary biology is about both processes (e.g., natural selection and speciation) and history. A major goal of evolutionary biology is to reconstruct the history of life on earth. Systematics is the study of biological diversity in an evolutionary context. Part of the scope of systematics is the development of phylogeny , the evolutionary history of a species or group of related species. A. The Fossil Record and Geological Time Fossils are the preserved remnants or impressions left by organisms that lived in the past. In essence, they are the historical documents of biology. The fossil record is the ordered array in which fossils appear within sedimentary rocks. These rocks record the passing of geological time. 1. Sedimentary rocks are the richest source of fossils Sedimentary rocks form from layers of sand and silt that settle to the bottom of seas and swamps. As deposits pile up, they compress older sediments below them into rock. The bodies of dead organisms settle along with the sediments, but only a tiny fraction are preserved as fossils. Rates of sedimentation vary depending on a variety of processes, leading to the formation of sedimentary rock in strata. The organic material in a dead organism usually decays rapidly, but hard parts that are rich in minerals (such as bones, teeth, shells) may remain as fossils. Under the right conditions minerals dissolved in groundwater seep into the tissues of dead organisms, replace its organic material, and create a cast in the shape of the organism. Rarer than mineralized fossils are those that retain organic material. These are sometimes discovered as thin films between layers of sandstone or shale. As an example, plant leaves millions of years old have been discovered that are still green with chlorophyll. The most common fossilized material is pollen, which has a hard organic case that resists degradation. Trace fossils consist of footprints, burrows, or other impressions left in sediments by the activities of animals. These rocks are in essence fossilized behavior. These dinosaur tracks provide information about its gait. If an organism dies in a place where decomposition cannot occur, then the entire body, including soft parts may be preserved as a fossil. These organisms have been trapped in resin, frozen in ice, or preserved in acid bogs. 2. Paleontologists use a variety of methods to date fossils When a dead organism is trapped in sediment, this fossil is frozen in time relative to other strata in a local sample. Younger sediments are superimposed upon older ones. The strata at one location can be correlated in time to those at another through index fossils.
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