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Darwin Presents His Case Darwin Presents His Case In 1859, Darwin published “On the Origin of Species” – in it he proposed a mechanism for evolution which he called natural selection In addition, he presented evidence that evolution has been taking place for millions of years—and continues in all living things Inherited Variation Members of each species vary from one another in important ways Example: Some plants in a species may produce larger fruit than others (lima beans), some cows produce more milk than others, birds have varying sized beaks Darwin learned that some of this variation is inherited → differences that are passed from parents to offspring Inherited Variation At the time, Darwin had no idea how heredity works, but we now know that heritable variation is caused by variation in an organism’s genes → many people viewed this variation as defects, but Darwin knew that this variation (which we now know as genetic variation) was significant Artificial Selection Darwin knew that plant and animal breeders took advantage of this variation to improve crops and livestock through artificial selection artificial selection – nature provides the variation and humans selected those variations that they found useful Artificial Selection Evolution by Natural Selection Next, by comparing the processes in nature to artificial selection, Darwin developed a hypothesis to explain how evolution occurs The Struggle for Existence Darwin realized that high birth rates and a shortage of life's basic needs would force organisms to compete for resources The Struggle for Existence means that members of each species compete regularly to obtain food, living space, and other resources The Struggle for Existence Examples: Predators: individuals that are fast or have a particular way to catch their prey will be more efficient at collecting food Prey: individuals that are fast, well camouflaged, or protected can avoid being eaten Darwin’s Finches: certain beak shapes/sizes are more efficient at eating different types of food → led to speciation Survival of the Fittest A key factor in the struggle for existence is how well suited an organism is to its environment fitness – the ability of an individual to survive and reproduce Darwin proposed that fitness is the result of adaptations adaptation – any inherited characteristic that increases an organism’s chance of survival Survival of the Fittest Successful adaptations enable organisms to be better suited to their environment, and thus, better able to survive and reproduce Adaptations can be structural (ex. beak shape in finches), physiological processes (how living things function; ex. photosynthesis), or they can be behavioral (choosy females, mating rituals, siblicide) Adaptations Survival of the Fittest Individuals with characteristics that are not well suited to their environment have low fitness → they either die or leave few offspring Individuals that are better suited to their environment – that is to say they have adaptations that enable fitness Survival of the Fittest Darwin called this process survival of the fittest → because of its similarity to artificial selection, Darwin referred to survival of the fittest as natural selection Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species’ fitness in its environment Descent with Modification Over long periods of time, natural selection produces organisms that have different structures and occupy different habitats → as a result species today look different from their ancestors Descent with Modification Darwin referred to this principle as descent with modification → over time, each living species has descended, with changes, from other species this implies that all living things are related to one another common descent – all species – living and extinct – were derived (came from a common ancestor