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Name_____________________________ Date________________ Adaptations: Investigation 2 Natural Selection In 1858, Darwin and another British biologist, Alfred Russel Wallace, proposed an explanation for how evolution occurs. ____________________________________________________ ____________________________________________________. In his book, Darwin explained that evolution occurs by means of natural selection. ___________________________________________________? Natural Selection continued ____________________________________________________ _____________________________________________________ ____________________________________________________. Darwin identified a number of factors that affect the process of natural selection: overproduction, _______________, and variations. Overproduction ____________________________________________________ In many species, so many offspring are produced that there are not enough resources—food, water, and living space—for all of them. For example, each year a female sea turtle may lay more than 100 eggs. ____________________________________________________. Darwin knew that this doesn’t happen. Why not? Competition Since food and other resources are limited, the offspring must compete with each other to survive. ____________________________________________________ ____________________________________________________. Instead, competition is usually indirect. For example, some turtles may fail to find enough to eat. ____________________________________________________. Only a few turtles will survive long enough to reproduce. Variations ____________________________________________________ ____________________________________________________. Traits are physical characteristics. ____________________________________________________ ____________________________________________________. For example, some newly hatched turtles are able to swim faster than others. Selection ____________________________________________________ ____________________________________________________. Those individuals are more likely to survive and reproduce. When those individuals reproduce, their offspring may inherit the allele for this helpful trait. ____________________________________________________ ____________________________________________________. Selection continued ____________________________________________________ ____________________________________________________. In effect, the environment has “selected” organisms with helpful traits to be the parents of the next generation—hence the term “natural selection”. ____________________________________________________. Helpful variations gradually accumulate in a species, while unfavorable ones disappear. Selection continued For example, suppose a new fast-swimming predator moves into the turtles’ habitat. ____________________________________________________ ____________________________________________________. The faster turtles would thus be more likely to survive and reproduce. ____________________________________________________ ____________________________________________________. The Role of Genes in Evolution Without variations, all the members of the species would have the same traits. ____________________________________________________ ____________________________________________________. But where do variations come from? How are they passed on from parents to offspring? ____________________________________________________. The Role of Genes in Evolution continued Darwin did not know anything about genes or mutations. It is not surprising that he could not explain what caused variations or how they were passed on. ____________________________________________________ ____________________________________________________. Only genes are passed from parents to their offspring. ____________________________________________________ ____________________________________________________. Evolution in Action ____________________________________________________ ____________________________________________________. In a 1977 study of the finches on Daphne Major, one of the Galapagos Islands, scientists observed that beak size could change very quickly by natural selection. Evolution in Action continued In 1977, very little rain fell on the island—only 25 millimeters instead of the usual 130 millimeters or so. ____________________________________________________. Fewer of the seeds that the finches usually ate were available. ____________________________________________________ ____________________________________________________. Evolution in Action continued ____________________________________________________ ____________________________________________________. Many of the finches with smaller beaks did not survive the drought. The next year, more finches on the island had larger and stronger beaks. ____________________________________________________. How Do New Species Form? ____________________________________________________ ____________________________________________________. But how does an entirely new species evolve? Since Darwin’s time, scientists have come to understand that geographic isolation is one of the main ways that new species form. ____________________________________________________ ____________________________________________________. How Do New Species Form? continued Sometimes a group is separated from the rest of its species by a river, volcano, or mountain range. ___________________________________________________ ___________________________________________________ ___________________________________________________ __________________________________________________. This may have happened on the Galapagos Islands. ___________________________________________________ __________________________________________________. How Do New Species Form? continued A new species can form when a group of individuals remains separated from the rest of its species long enough to evolve different traits. ____________________________________________________ ____________________________________________________. How Do New Species Form? continued For example, the Abert’s squirrel and the Kaibab squirrel live in forests in the Southwest. ____________________________________________________ ____________________________________________________. About that time, however, a small group of squirrels became isolated in a forest on the north side of the Grand Canyon in Arizona. ____________________________________________________ ____________________________________________________. Scientists are not sure whether the Kaibab squirrel has become different enough from the Abert’s squirrel to be considered a separate species. Continental Drift ___________________________________________________. For example, hundreds of millions of years ago all of Earth’s landmasses were connected as one landmass. It formed a supercontinent called ____________________. Organisms could migrate form one part of the supercontinent to another. ____________________________________________________ ____________________________________________________. Continental Drift continued ____________________________________________________ ____________________________________________________. Perhaps the most striking example of how continental drift affected the evolution of a species is on the continent of Australia. ____________________________________________________ ____________________________________________________. Because of this, unique organisms have evolved in Australia. Example For example, most mammals in Australia belong to the group known as marsupials. ____________________________________________________ ____________________________________________________. In contrast, few species of marsupials exist on other continents. Review What is evolution? What did Darwin observe on the Galapagos Islands that he thought was the result of evolution? _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ Explain why variations are needed for natural selection to occur. _____________________________________________________ _____________________________________________________ _____________________________________________________ Describe how geographic isolation can result in a new species. _____________________________________________________ _____________________________________________________ _____________________________________________________