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1 Review What happens in the process of natural selection Explain Why do organisms with greater fitness generally leave more offspring than organisms that are less fit Compare and Contrast How are natural selection and artificial selection similar and how are they different 2 Review Why were Hutton’s and Lyell’s ideas important to Darwin Visual Thinking Think about the teeth in a lions mouth. How is the structure of the lion’s teeth an adaptation CH 16 DARWIN’S THEORY OF EVOLUTION 16.3 Darwin Presents His Case Darwin wrote up a complete draft of his ideas about natural selection, but he put the work aside and didn’t publish it for another 20 years His theory was very radical, so he wanted to gather as much evidence to support his ideas before he made them public He wrote on barnacles during that time. 1858, Darwin reviewed an essay containing similar ideas about evolution by Alfred Russel Wallace Darwin decided to move forward with his own work Wallace’s essay was presented together with some of Darwin’s observations at a scientific meeting in 1858. “On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life” was published in November of 1859 First printing was 1250 copies, 15 schillings each sold out on the first day Never been out of print since. Natural Selection 1. 2. 3. Occurs when: More individuals are born than can survive (the struggle for existence) There is natural heritable variation (variation and adaptation) There is variable fitness among individuals (survival of the fittest). The Struggle for Existence If more individuals are produced than can survive, members of a population must compete to obtain food, living space, and other limited necessities of life Came from reading Malthus. Variation and Adaptation Individuals have natural variations among their heritable traits Some of those variants are better suited to life in their environment than others Adaptation Heritable characteristic that increases an organism’s ability to survive and reproduce in its environment. Adaptations Survival of the Fittest Fitness How well an organism can survive and reproduce in its environment Individuals with adaptations that are well-suited to their environment can survive and reproduce Individuals with characteristics that are not wellsuited to their environment either die without reproducing or leave few offspring. Natural Selection Process by which organisms with variations most suited to their local environment survive and leave more offspring Darwin’s mechanism for evolution The environment - not a farmer or animal breeder - influences fitness. Steps of Natural Selection 1. There is a population with variation 2. not only cup lip, but body size, eye color, etc… Some variations prove favorable. Steps of Natural Selection 3. Those with favorable variation produce more offspring (why), and their offspring share their favorable trait (why) 4. Over time, each new generation has a greater percentage of individuals with the favorable trait until the whole population has it. From generation to generation, populations continue to change as they become better adapted, or as their environment changes Natural selection acts only on inherited traits because those are the only characteristics that parents can pass on to their offspring. Grasshoppers can lay more than 200 eggs at a time, but only a small fraction of these offspring survive to reproduce. This population includes yellow and green body color. Green is camouflaged, green grasshoppers have higher fitness and so survive and reproduce more often than yellow grasshoppers do. Green grasshoppers become more common than yellow grasshoppers in this population over time because more grasshoppers are born than can survive. Natural selection does not make organisms “better” Adaptations don’t have to be perfect Natural selection also doesn’t move in a fixed direction. If local environmental conditions change, some traits that were once adaptive may no longer be useful, and different traits may become adaptive If environmental conditions change faster than a species can adapt to those changes, the species may become extinct. Common Descent All species - living and extinct are descended from ancient common ancestors Every organism alive today is descended from parents who survived and reproduced.