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bild 3 lecture 2

bild 3 lecture 2 - Here are two hypotheses about how the...

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Here are two hypotheses about how the flightless cormorants might have evolved. 1. They lost their ability to fly because the birds that did fly were more likely to be blown out to sea while those that could not fly were more likely to survive. We observe many species of bird on the islands that can fly well — so why should flightless cormorants be at an advantage when they must compete with these birds for fish? 2. They lost their ability to fly because it was energetically favorable to do so. The film clips (you can see them on the class web site) show how flightless cormorants and Galapagos penguins swim in different ways, the cormorants using their feet and the penguins using their wings. The pectoral muscles of the
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flightless cormorants are reduced in size compared with flying birds. These muscles demand a great deal of energy. Flightless birds that do not use their wings to swim or fly have much lower metabolic rates than those that do use their wings. Thus, the flightless cormorants need less food to survive than the penguins, giving them an advantage as long as there are plentiful fish for them to catch near the shore. However, when there is a shortage of food, the cormorants readily abandon their offspring to starve, increasing their own chances that they will survive until their food source increases again. Can you think of ways to test these hypotheses, and can you think of others to test? What are the genetic reasons why the cormorants’ wings and pectoral muscles are reduced in size? When did these genetic differences appear in the ancestry of the flightless cormorants? We do not yet know the answers to these specific questions, but we can begin to answer similar questions about other animals and plants, as we will see. Darwin’s Theory On his return to England, Darwin began to pull together a great deal of information from the voyage and from other sources, notably from his reading of Thomas Robert Malthus’ book On Population . He was able to arrive at a mechanism that accounted for all the geographic patterns of animals and plants that he had observed. He proposed that evolution has taken place through natural selection. Summary of Darwin’s theory: 1. Species are mutable. Darwin’s observations on the Galapagos made it clear to him that species can alter over time, and can give rise to new species. This was part of Lamarck’s theory too, but Darwin realized as Lamarck did not that it is the environment in which organisms find themselves that determines what kinds of organisms will survive there.
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