I just need a little help answering this on my lab.
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That would seem intuitive. After all, if there is strong, directional selection, then changes will occur and will be most rapid. However, strong selection can also prevent change. For example, the structure of flight feathers have not changed much over 150 million years. The flight feather of Archaeopteryx, for example, is asymmetrical and it is a composite structure, make up with smaller elements that are hooked together. That means Archaeopteryx's feathers are already so well adapted to their purpose that the chance of a mutation that can further improve it is extremely small. Most mutations would in fact be deleterious, and would be strongly selected against because birds with feathers that may them unable to fly or even if it makes them worse flyers would be quickly eliminated from the gene pool. Strong selection, therefore, can result in stasis, or a lack of change.
OTOH, some structures that have become useless can quickly degenerate. Take, for example, they eyes of cave animals such as the olm, a blind salamander found in European caves. Normally, eyes are important as they allow animals to navigate, find food and find shelter. In the dark cave environment, however, eyes are useless. There is therefore no longer any selection to keep the eye functional, even if there is strong selection against eyes that do not function in salamanders that live on the ground. In fact, the olm is not the only blind cave salamander in existence, there are blind cave salamanders of other species living in the United States. That shows the same lack of selection can result in the loss of visual function in different species due to random genetic drift.
Of course, strong selection does not always result in lack of change. For example, humans last shared an ancestor with the chimp about 5 million years ago. Yet fossils that are between 3.5-4.5 million years old (Ardipithecus and Australopithecus) are already bipedal. In contrast, their brains that no larger than those of the chimpanzee. It shows that strong selection for bipedalism indeed brought about rather rapid evolutionary changes in our locomotion.
OTOH, some structures that have become useless can quickly degenerate. Take, for example, they eyes of cave animals such as the olm, a blind salamander found in European caves. Normally, eyes are important as they allow animals to navigate, find food and find shelter. In the dark cave environment, however, eyes are useless. There is therefore no longer any selection to keep the eye functional, even if there is strong selection against eyes that do not function in salamanders that live on the ground. In fact, the olm is not the only blind cave salamander in existence, there are blind cave salamanders of other species living in the United States. That shows the same lack of selection can result in the loss of visual function in different species due to random genetic drift.
Of course, strong selection does not always result in lack of change. For example, humans last shared an ancestor with the chimp about 5 million years ago. Yet fossils that are between 3.5-4.5 million years old (Ardipithecus and Australopithecus) are already bipedal. In contrast, their brains that no larger than those of the chimpanzee. It shows that strong selection for bipedalism indeed brought about rather rapid evolutionary changes in our locomotion.