Several human populations spread throughout the Sahara desert have multiple mutations in Gene R that confers heat tolerance. Independently, a small population of humans in the Mojave desert of California have a unique mutation in Gene Z that also confers heat tolerance. The evolution of adaptation of heat tolerance across all of these populations would be considered an example of

Variation

Explanation:

Migration into a population from other populations with varying gene frequencies, evolution, or adaptation of heat tolerance is a cause of variation. The allele frequency of the resulting hybrid population would be right in the middle of its original value and the donor population's frequency. In tropical and subtropical ecosystems (Sahara deserts), high temperatures are a significant restriction to human population productivity; they induce morphological and physiological modifications at all times, resulting in significant yield losses but the ability and evolution of human species to cope in this region is as a result of variation.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.

you know they are heterozygous for a trait because they have it as jj or jj.