Final answer:
The form of natural selection that increases the frequency of alleles beneficial to an individual's relatives, sometimes at their own expense, is known as kin selection.
Step-by-step explanation:
The form of natural selection that leads to an increase in the frequency of alleles which promote the fitness of genetic relatives is called kin selection. This type of selection is an example of adaptive evolution where certain alleles become more common in a population because they contribute to the evolutionary fitness of an organism's relatives, even at a cost to the organism itself. The concept of kin selection also embodies ideas of allele frequency, which relates to the rate at which certain alleles appear within a population, and evolutionary fitness, where individuals with beneficial heritable traits tend to survive and reproduce, therefore passing those alleles on to subsequent generations.