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S-B-9-1_Principles of Natural Selection-Teacher Version Principles of Natural Selection Darwin’s theory was based on several observations: Individual organisms within a species differ from each other - there is variation. Offspring resemble their parents - characteristics are inherited. Organisms produce more offspring than can survive, and many that do survive do not reproduce. Organisms compete for limited resources in the environment. Individuals that are best suited to their environment are more likely to survive and reproduce (a.k.a. “natural selection”). They pass on their traits to their offspring. This process of natural selection causes species to change over generations. *Darwin wasn’t the first to suggest evolution of species. He was the first to suggest natural selection as a mechanism for evolution, and to provide many kinds of evidence for it. *The alleles of a species that are favored in the environment will become more frequent in the gene pool of the species, due to the organisms’ higher likeliness of surviving as part of the species at large. *“Descent with modification:” Species today are descended with modifications from ancestral species. Species can diverge from common ancestors. Question: Why do giraffes have long necks? Lamarck’s Explanation (Theory of Acquired Characteristics): When a giraffe stretches its neck to reach high branches, it can grow longer. Then, it can pass on the long-neck trait to its offspring. Over time, this can lead to a change in the species. Darwin’s Explanation (Theory of Evolution by Natural Selection): In a population of giraffes there is random genetic variation in neck length. In an environment where there are trees and bushes, the longer-necked animals are better adapted and so compete well compared to their shorter-necked relatives. These animals live longer, through more breeding seasons, and so they can have more offspring. In the next generation, there are more long-neck genes than short-neck genes in the population. If this continued over very many generations, then in time the average neck length would increase. Modern-Day: Lamarck’s hypothesis has been proven to be incorrect. Only genetic traits can be passed on to offspring. Scientists have collected many more kinds of evidence to support Darwin’s theory. Today, scientists think it is more likely that giraffe natural selection was for long legs to run away from predators faster, and if you have long legs you need a long neck to drink. Either way, the natural selection process works the same.