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Science 20 – Biology Ch. 2 Populations Lesson 18
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Changes in Population Very gradual changes within a population are called gradualism –e.g. changes to the shape of a shell for a snail population over time Punctuated equilibrium – theory that states changes to organisms in a population can occur in rapid spurts followed by long periods of little change
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Case study: Punctuated Eq. e.g. Alberta’s Bighorn Sheep population Hunters are only allowed to shoot rams if their horns have curved to almost make a circle (trophy sheep) The largest & strongest male sheep are the ones with fully curved horns
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Since they are being removed by hunters, the male sheep left to breed are smaller & weaker (they never mature to trophy status) The male sheep in the next generations tend to have decreased body mass & shorter horns Hunting is inadvertently encouraging these characteristics in a short time
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A change in instructions from a gene is a mutation this is another reason why variation occurs in a species (it can be harmful, beneficial, or neutral) Mutations can sometimes be beneficial the development of a new characteristic or trait might increase an organism’s chances for survival
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Variation a difference in the frequency of genes & traits among individual organisms within a population e.g. the ability to roll your tongue, hitchhikers thumb, or widows peak.
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Adaptation Any structural OR behavioral trait that improves an organism’s success at surviving & reproducing in a particular environment
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Galapagos Finches Charles Darwin observed a chain of volcanic islands off the Pacific coast of South America called the Galapagos Islands There were a variety of different climatic regions, each with their own unique ecosystem along the islands with a variety of food sources Darwin observed that the same animals had adapted differently to take advantage of the local conditions
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E.g. Several varieties of finches had different beak types that were adapted for a diet specific to their regions Darwin suggested that all varieties of finches came from a common seed-eating finch ancestor
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Darwin’s Theory of Evolution Published “On the Origin of Species by Means of Natural Selection” to describe his theory of evolution – theory stating that the nature of population gradually changes form over time
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Darwin also described his theory of natural selection based on the following observations: Organisms usually produce more offspring than can survive There is a variation among individuals with respect to any trait in a population Organisms within a population compete for limited resources
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The best adapted individuals in the population are considered to have greater Darwinian fitness because they’ll be able to survive, breed, & pass on traits Reproductive success of an organism
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More Evidence The fossil record indicates that organisms did not remain static
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Many organisms have vestigial structures – structures that serve no function. (i.e. appendix in humans, undeveloped hind legs in a whale.)
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Homework Read p.494-495 first Practice Question #29-32
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Homework Case Study in readings: James Lamark (Incorrect Hypothesis) and The Peppered Moth Assignment: p.500-503 #36-40a, 42-46
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