The population of America is ethnically heterogeneous. What does this mean?

Any sound above what db can cause hearing loss in human beings

Which measure of an earthquake depends on how close you are to the focus?

Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.