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16.3 – Darwin Presents His Case Chapter 16: Darwin’s Theory of Evolution 16.3 – Darwin Presents His Case THINK ABOUT IT Darwin wanted to gather as much evidence as he could to support his ideas before he made them public. His work contradicted the accepted beliefs of the time. In 1859 Darwin published his first complete work on evolution: On the Origin of Species 16.3 – Darwin Presents His Case Evolution by Natural Selection The Mechanism of Evolution = Natural Selection Natural Selection – Process by which organisms that are most suited to their environment survive and reproduce successfully. Well-adapted individuals survive and reproduce. Acts only on inherited traits because they are passed to offspring 16.3 – Darwin Presents His Case Evolution by Natural Selection Driving Forces for Natural Selection The Struggle for Existence Variation and Adaptation Survival of the Fittest 16.3 – Darwin Presents His Case The Struggle for Existence When more individuals are produced than can survive – all of the members of the population must compete for resources. Darwin called this the struggle for existence. Example: Classroom full of 12 students. The room has 12 desk and enough food for 12 people. Suddenly 2 individuals are added to the classroom. With only the same resources available all of the members must compete for a desk “living space” and food. 16.3 – Darwin Presents His Case Variation and Adaptation Variation among individuals of a population occurs naturally. Differences between characteristics provide some individuals with a greater chance of survival. Adaptation - Any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment. 16.3 – Darwin Presents His Case Variation and Adaptation Adaptations: include body parts or structures, colors for camouflage or mimicry, physiological functions, and behaviors. Example: The scarlet king snake (harmless) exhibits mimicry— It copies the coloration of the eastern coral snake (poisonous), so predators avoid it, too. 16.3 – Darwin Presents His Case Survival of the Fittest Fitness describes how well an organism can survive and reproduce in its environment. Individuals with characteristics that are well-suited to their environment – a have high fitness. Individuals with characteristics that are not well-suited to their environment – a have low fitness. Survival of the fittest!!!!! In evolutionary terms, survival means reproducing and passing adaptations on to the next generation. Briefly discuss the diagram below: 16.3 – Darwin Presents His Case • What is occurring in this diagram? • Natural Selection 16.3 –Darwin’s Darwin Presents His Case Finches: (All 13 different species of finches have different beaks) 16.3 – Darwin Presents His Case Natural Selection This hypothetical population of grasshoppers changes over time as a result of natural selection. Grasshoppers can lay more than 200 eggs at a time, but only a small fraction of these offspring survive to reproduce. 16.3 – Darwin Presents His Case Natural Selection Certain variations, called adaptations, increase an individual’s chances of surviving and reproducing. In this population of grasshoppers, heritable variation includes yellow and green body color. Green color is an adaptation: The green grasshoppers blend into their environment and so are less visible to predators. 16.3 – Darwin Presents His Case Natural Selection Because their color serves as a camouflage adaptation, green grasshoppers have higher fitness and so survive and reproduce more often than yellow grasshoppers do. 16.3 – Darwin Presents His Case Natural Selection Green grasshoppers become more common than yellow grasshoppers in this population over time because more grasshoppers are born than can survive, individuals vary in color and color is a heritable trait, and green grasshoppers have higher fitness in this particular environment 16.3 – Darwin Presents His Case Natural Selection Natural selection: Does not make organisms “better.” Allows an organism to pass its genes to the next generation. Doesn’t move in a fixed direction. If the environment changes, traits that were once useful may no longer be. If the environment changes too quickly the population may go extinct. 16.3 – Darwin Presents His Case Natural Selection Peppered Moth Evolution Majority of the population 200 years ago was light colored The industrial evolution in England caused wide spread pollution and killed the lichens the moths were camouflaged for and left the tree bark covered in dark soot. Birds easily found the lighter individuals and preyed upon them. Only the darker individuals survived to reproduce and the population became dominated by darker moths 16.3 – Darwin Presents His Case Descent with Modification Descent with modification - Descendants of the earliest organisms spread into different habitats. Over time the descendants adapted to their environment. (With Modification) Example: Snowshoe Hare and Jackrabbit 16.3 – Darwin Presents His Case Common Descent 16.3 – Darwin Presents His Case Common Descent Picture from Darwin’s Notebook shows one of the first evolutionary trees A single “tree of life” links all living things. 16.3 – Darwin Presents His Case