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Chapter 7 Evolution The Theory and Its Supporting Evidence

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Chapter 7 Evolution The Theory and Its Supporting Evidence

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    1. Chapter 7 Evolution – The Theory and Its Supporting Evidence The present concept is the result of contributions by numerous scientists. Charles Darwin is most-remembered of these. During 1831 to 1836 voyage of HMS Beagle, he made observations of fossils and living creatures around coasts of South America and Galapagos Islands (west of Ecuador). Fossils with living relatives observed during studies: Llamas, sloths, armadillos. Living organisms on Galapagos with relatives on S. American continent – finches, giant tortoises, & iguanas.

    3. Observations convinced Darwin that organisms had descended, with changes, from earlier ancestors = central theme of theory of evolution. Evidence included “Darwin Finches” related birds with various traits and feeding habits. Evolution (progression of species) involving inheritable changes through time is fundamental to biology and paleontology. It provides a framework for the discussion of life history. Primary misconceptions of evolution: Survival of the strongest Humans evolved from apes There are no transitional forms

    4. Early 1700s were dominated by Genesis-based creation of Earth in seven 24 hour days. With growth of science during 1700s, new discoveries were difficult to reconcile with strict interpretation of Genesis. In this changing intellectual atmosphere, scientists gradually accepted the principle of uniformitarianism (James Hutton), Earth’s great age, the progression of species (William Smith),… A theoretical framework to explain the changes was lacking, though.

    5. Early attempts included Jean-Baptiste de Lamarck (1744-1829) theory of inheritance of acquired characteristics. Example – stretching of necks of giraffes to reach higher leaves resulted in longer necks in offspring. Seemed logical prior to knowledge of genes as agents of inheritance. In 1859, Charles Darwin (1809-1882) published On the Origin of Species (based on his HMS Beagle findings) and detailed his ideas on evolution and proposed a mechanism for evolution. Similar findings were independently published the same year by Alfred Russel Wallace.

    6. Mechanism is termed Natural Selection – changes in environmental conditions. During times of change, certain traits favor survival. Microevolution occuring within individual organisms (by random mutations) produces these traits. For these traits to be passed along – Genetic codes must be passed along through reproductive cells. Trait must improve survivability and/or adaptibility, e.g. long-necked giraffes have more access to food. Species must reproduce rapidly enough to sustain the population. Trait must exist before the env. change.

    7. In essence: Evolution occurs when the environment acts on genetic variants within a population to select those best able to survive and produce fertile offspring. Natural selection could not account for the origin nor number of variations. Critics argued that variations would “blend” with the population and be lost. Within a species, physical difference can be called genetic variation. Work of Gregor Mendel and later scientists showed that traits are controlled by genes, which exist in alternate forms called alleles, some of which are dominant over others. Offspring receive one allele (from each pair) from each parent.

    8. The factors (genes) controlling traits do not blend during inheritance. Traits not expressed in each generation may not be lost. Therefore, some variation in populations results from alternate expressions of genes (alleles). Variation can be maintained. Modern discussions over evolution focus on gradual or episodic changes. Evolution by natural selection works on variations in populations, by the reshuffling of alleles from generation to generation.

    9. The potential for variation is enormous with thousands of genes, each with several alleles, and with offspring receiving 1/2 of their genes from each parent New variations arise by mutations, i.e., change in the chromosomes or genes. Chromosonal mutations affect large segments of chromosones. Point mutations affect individual genes. Beneficial mutations enhance adaptability to environ-mental changes or survivability in the status quo. A mutation may broaden an individual’s range of tolerance to a particular abiotic (non-living) environmental component or condition.

    11. Mutations occur spontaneously (naturally) or by the presence of mutagens (chemical or physical agents that alter genetic material in reproductive cells. Mutagens can include: Certain chemicals UV radiation X-Rays Elevated temperatures Over time, a series of mutations can change one species into another, though because of genetic variation, it may be difficult to identify species boundaries.

    12. Species - a population of similar individuals that in nature interbreed and produce fertile offspring. Species are reproductively isolated from one another. Goats and sheep do not interbreed in nature, so they are separate species, yet in captivity they can produce fertile offspring. In some cases, reproductive barriers are incomplete. Speciation is the phenomenon of a new species (or two new species) arising from an ancestral species (common ancestor), by mutation (genetic change) over time. Changes in morphology may also occur.

    13. Allopatric speciation – occurs when a small part of a population becomes isolated from its parent population.

    14. A population of “sand dollars”, the larvae of which can migrate “back and forth” through the seaway, maintain the population in both the Caribbean and the Pacific. When individual minor mutations occur, they can be spread by interbreeding through the entire population.

    18. Bats and birds fly, suggest a closer relationship than to dogs.

    19. Cladistics help paleontologies see relationships between different fossil lineages, though convergent evolution or the lack of data may confuse the issue. Evolutionary Trends – A series of adaptations to changing environ-mental conditions; or Responses to colonization of new habitats. Mosaic evolution – some features change while others are retained. The evolution history of a group of organisms is termed its Phylogeny.

    22. Evolution by natural selection occurs by: 1) The presence or occurrence of a variation within a population. 2) The selection of that variation during changing conditions. Natural selection yields organisms adapted to a specific set of circumstances at a particular time. It’s about adaptability, not being biggest or strongest. Within each organism there is a range of tolerance for each abiotic environmental component and each component must remain in a “livable” range for the organism to survive, i.e., if one component exceeds the tolerances of the organism, it cannot survive. If this intolerable change occurs over the entire population, extinction occurs.

    23. Niche – organism’s role within its ecosystem. Habitat – organism’s living space within its ecosystem. Background extinctions – involve individual species or related species over time. Opens individual niches and habitats. Mass extinctions – involve many different species within a geologically short period of time. Opens many niches and habitats. Adapative radiation – re-occupation of niches and habitats by other organisms. After the extinction of dinosaurs (end of Cretaceous Period), mammals occupied niches and habitats previously occupied by dinosaurs.

    24. The most successful, adaptive organisms, that have broad ranges of tolerance and flexible behavior patterns are termed Generalist species. They have advantages during times of environmental change (stress). Less adaptive organisms with narrow ranges of tolerance and more fixed behavior patterns are termed Specialist species. They have advantages during times of environmental stability. Specialized diets may = less competition. Many endangered species are specialist species.

    25. Since Darwin’s time, additional studies in biochemistry and molecular biology, and a more complete and better understood fossil record, offer evidence supporting the central claim of the theory, though details are still under debate. A truly scientific theory must contain testable predictive statements. Evidence supports, but proof is unavailable, in most cases. Theories may be questions, revised, and occasionally replaced. Evolution “predictions”: If evolution has taken place, the oldest fossil-bearing rocks should have very different fossils than organisms of today.

    26. Evolution “predictions” continued: More recent rocks should have more fossils similar to today’s organisms. Closely related species should have similarities in a whole range of areas, not just anatomy. Classification of organisms should show a nested pattern of similarities. Neighboring plants and animals should be more similar to each other than to ones farther away. A mechanism should exist that allows the evolution of one species to another & fossils should appear in the fossil record in order of the organisms’ evolution.

    27. Testability Suppose, contrary to evolutionary prediction wolves and coyotes were not similar (bio-chemically, genetically, and in embryonic development) – theory would have to be modified. If a mammal fossil was found before the first fish, theory would have to be abandoned and a better explanation found. Since the theory of evolution is testable, it is truly scientific. Classification of organisms uses a nested pattern of similarities.

    28. Carolus Linneaus (1707-1778) proposed a clas-sification scheme with a two-part name (genus and species). Example: Coyote is Canis latrans. Linnaean hierarchy is an ordered list of cate-gories that becomes more inclusive as one proceeds up the hierarchy.

    29. Coyote, Canis latrans

    30. Small-scale (micro-)evolution examples adaptations of some plants to contaminated soils, insects and rodents developing resistance to new insecticides and pesticides development of antibiotic-resistant strains of bacteria. Variations in these populations allowed some variant types to live and reproduce, bringing about a genetic change. The fossil record consists of first appearances of various organisms over time. One-celled organisms appeared before multicelled ones, plants appeared before animals, invertebrates before vertebrates. Fish appeared first followed by amphibians, reptiles, mammals, and birds.

    31. Times when major groups of verte-brates appeared in the fossil record Thickness of spindles shows relative abundance

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