the answer is stabilizing selection. this type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. this type of natural selection is the most widely seen in which there are no drastic changes in traits of the population in long times. extreme traits of the population are weeded out hence the subsequent generations of the population have less diversity than the initial. the illustration below shows this model in relation to the others.
all of the liquid molecules in cell membranes amphipathic (or amphiphilic) -that is, they have a hydrophilic ("water loving") or polar end and a hydrophobic ("water fearing") or nonpolar end. the most abundant membrane lipids are the phospholipids. these have a polar head group and two hydrophobic hydrocarbon tails.
hope that : )