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Chapter 25 - Chapter 25 Phylogeny and Systematics Chapter...

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Chapter 25 - Phylogeny and Systematics Chapter 25 Phylogeny and Systematics Lecture Outline Overview: Investigating the Tree of Life Evolutionary biology is about both process and history. o The processes of evolution are natural selection and other mechanisms that change the genetic composition of populations and can lead to the evolution of new species. o A major goal of evolutionary biology is to reconstruct the history of life on earth. In this chapter, we will consider how scientists trace phylogeny, the evolutionary history of a group of organisms. To reconstruct phylogeny, scientists use systematics, an analytical approach to understanding the diversity and relationships of living and extinct organisms. o Evidence used to reconstruct phylogenies can be obtained from the fossil record and from morphological and biochemical similarities between organisms. o In recent decades, systematists have gained a powerful new tool in molecular systematics, which uses comparisons of nucleotide sequences in DNA and RNA to help identify evolutionary relationships between individual genes or even entire genomes. Scientists are working to construct a universal tree of life, which will be refined as the database of DNA and RNA sequences grows. Concept 25.1 Phylogenies are based on common ancestries inferred from fossil, morphological, and molecular evidence Sedimentary rocks are the richest source of fossils. Fossils are the preserved remnants or impressions left by organisms that lived in the past. In essence, they are the historical documents of biology. Sedimentary rocks form from layers of sand and silt that are carried by rivers to seas and swamps, where the minerals settle to the bottom along with the remains of organisms. o As deposits pile up, they compress older sediments below them into layers called strata. o The fossil record is the ordered array in which fossils appear within sedimentary rock strata. These rocks record the passing of geological time. o Fossils can be used to construct phylogenies only if we can determine their ages. o The fossil record is a substantial, but incomplete, chronicle of evolutionary change. o The majority of living things were not captured as fossils upon their death. Of those that formed fossils, later geological processes destroyed many. Only a fraction of existing fossils have been discovered.
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o The fossil record is biased in favor of species that existed for a long time, were abundant and widespread, and had hard shells or skeletons that fossilized readily. Morphological and molecular similarities may provide clues to phylogeny. Similarities due to shared ancestry are called homologies. Organisms that share similar morphologies or DNA sequences are likely to be more closely related than organisms without such similarities.
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