How does a stream typically change as it flows from its headwaters to its mouth?

Which of the four major uses is predicted to change the least by 2020?

The gene that causes sickle-cell disease is present in a higher percentage of residents of sub-saharan africa than among those of african descent living in the united states. even though this gene causes sickle-cell disease, it also provides some protection from malaria, a serious disease that is widespread in sub-saharan africa but absent in the united states. discuss an evolutionary process that could account for the different percentages of the sickle-cell gene among residents of the two regions.

Blood has traveled from the heart to the toes. which describes the next step of the circulation process?