What is the main idea behind the model of a molecular clock?
a. that neutral mutations accumulate at a steady rate
b. that certain traits are under the pressure of natural selection
c. that segments of DNA can be compared with segments of RNA
d. that phenotypes, not genotypes, are affected by natural selection

Define the apical impulse and describe its normal location, size, and duration. which abnormal conditions may affect the location of the apical impulse? explain the mechanism producing normal first and second heart sounds. describe the effect of respiration on the heart sounds. describe the characteristics of the first heart sound and its intensity at the apex of the heart and at the base. describe the characteristics of the second heart sound and its intensity at the apex of the heart and at the base.