Chapter 25 Test Already Passed

The figure shows eyes found among living molluscs, ranging from a patch of pigmented cells in a limpet to a complex, image-forming eye in a squid. Is it possible that a structure as complex as an image-forming eye evolved by natural selection? {Ans: Yes, if the photoreceptor cells and simple eyes that preceded it were useful to the animals in which they arose. (In fact, such photoreceptors and simple eyes can be seen in many living animals. Read about the evolution of a complex eye.)}What did researchers discover about the genetic mutation causing the loss of pelvic spines? {Ans: 1. It occurred in a similar DNA region in freshwater stickleback populations all over the world. 2. It is found in a regulatory region (a "switch") upstream of the coding region of the Pitx1 gene. (Many mutations of evolutionary importance are found in regulatory regions. The gene remains intact, but the location of its expression changes—conveying a new phenotype without losing existing capabilities.)}Evolution works by {Ans: "tinkering" with existing structures (Evolution, and in particular natural selection, can only select for the best available traits. Modifications to those traits are usually made in small, incremental steps, and new inventions