Natural selection operates on the variation that exists in populations. Several important aspects of variation are apparent in family photos. 1) Members of a population vary in almost every trait. Although photos only illustrate morphological variation, variation also exists in physiological and behavioral traits. 2) Much of the variation has a genetic basis, as can be seen in the resemblance among family members. Some of the variation is the result of the alleles at a single gene locus, such as ear lobe attachment. Most of the variation is the result of alleles at a number of gene loci (polygenic traits), such as height, IQ and skin color.