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Signing in now will connect your and Teachers' Domain accounts, so that in the future you will automatically be signed into Teachers' Domain when you come from . Molecular Clocks: Proteins That Evolve at Different Rates Resource for Grades 9-12 | Citation Media Type: Document Size: 40.4 KB View or Download SAVE TO FOLDER Loading... Save Cancel Share | Source: Adrienne Zihlman, The Human Evolution Coloring Book Resource Produced by: Collection Developed by: Collection Credits Collection Funded by: See Also: National K -12 Subject: Deep Time/History of Life Evidence for Evolution Processes of Evolution Lesson Plans Using this Resource: The Molecular Evidence for Evolution Four different proteins from humans and horses are compared in this graphic and article, and the reasons each protein evolves at its own characteristic rate are discussed. Each protein is useful for measuring evolutionary change over a different time scale. Permitted use: Download Only Accessibility Features: Long Description Background Essay The basic mutation rate is probably similar for all genes, but natural selection filters out mutations that adversely affect the functioning of the protein each gene makes. The structure of some proteins is rigidly defined by the function they perform -- any mutation that causes even a small change in the molecular structure will impair the protein's function. These proteins accumulate very few mutations and may be identical in many species, which means they have limited use in working out how closely different species are related. Other proteins can tolerate a large amount of change and still carry out their function. Such proteins accumulate many mutations, and they can be used to help work out the evolutionary relationships between even closely related species. Print Background Essay Discussion Questions When the concept of molecular clocks was first developed, some researchers thought that it might be a relatively straightforward way to place dates on major events in the history of many groups of organisms. Discuss the reasons why, as molecular biologists learned more about the structure and functions of proteins the idea of a single, universal molecular clock had to be discarded. Discuss why it has turned out to be quite useful to have molecular clocks that "tick" at different speeds. In your discussion, compare the kinds of molecular clocks you might look for if you were trying to date the divergence of the ancestors of: a. living great apes b. humans and whales c. mammals and starfish d. animals and plants Explain why mutations accumulate more slowly in the genes that code for histones than in genes that code for fibrinopeptides. 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