18 - Evidence For Evolution

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1840s

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Darwin composed long essay on his theory for evolution by natural selection but did not publish

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1858

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Alfred Russel Wallace conceived a similar hypothesis to Darwin

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why did Darwin release essay on same year?

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afraid that Wallace's work be published first

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The book title of Darwin

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On the origin of species by means of natural selection

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Year published

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1859

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Evidences for evolution

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fossils transitional forms, homologies

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existence of fossils of transitional forms can link

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very different groups of organisms

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Cetaceans believed to evolve from

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four-legged animals

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Cetaceans' transitional forms are

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reduced hind limb and pelvic bones

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1960s, ancestors of whales were

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hoofed, wolf-like carnivores

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Late 1970s,. transitional fossils unearthed in

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Pakistan and Egypt which supported hypothesis

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But upon using DNA analysis they found a close relationship between

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whales and hippopotamus (herbivorous, cloven-hoofed mammals)

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Hypothesised that whales and hippos are descendants of

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cloven-hoofed ancestor

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What fossils discovered to resolve issue

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Packicetus and rodhocetus

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Both had

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distinctive ankle bone of a cloven-hoofed mammal

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In homologies, what gives evidence for evolution?

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Analysing similarities between different organisms

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Evolution is a process of

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Descent with modification

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Define descent with modification

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Characteristics present in an ancetral organism are altered over time by natural selection as it's descendants face different environmental conditions

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related species can have characteristics that have underlying similarity yet

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function differently

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Homology

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similarity resulting from common ancestry

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An example of homology

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Vertebrae forelimbs

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Advances in molecular biology, scientists have deeper understanding of homologies than

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Darwin

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Darwin's boldest hypothesis

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all life forms are related

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Evidence for this hypothesis

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All forms of life use same genetic language (DNA and RNA), genetic code (how RNA triplets are translated into amino acids)

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vestigial structures

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remnants of features that served important functions in the organisms ancestors (no more importance to present organism)

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evolutionary trees

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patterns of descent, history of life (turned sideways nowadays)

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branch point

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common ancestor (common to right of it)

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hatch mark

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homologous character (common to right of it)

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Steps in biological evolution

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genetic variability through mutations and natural selection

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Natural Selection

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environmental conditions favour some individuals over others

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adaptation

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heritable trait that improves ability of an organism to survive and reproduce at higher rate than other individuals in a population under prevailing environmental conditions

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Two limitations on adaptation through natural selection

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(1) change in environmental conditions can lead to adaptation only for genetic traits present in populations gene pool or for traits resulting from mutations

(2) even if heritable trait is present, population's ability to adapt maybe limited by it's reproductive capacity

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Incorrect ideas about natural selection

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survival of the fittest and strongest, organisms develop certain traits because of need, evolution involves grand plan of nature

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fitness is measured by

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reproductive success not strength