Three Types of Selection Are "Directional Selection," "Balancing Selection," and "Selection

Essay

Three types of selection are "directional selection," "balancing selection," and "selection against heterozygotes."
a)Shown below are three graphs (A-C).Each graph represents a different type of selection.In each case,certain individuals are being selected against (represented by the striped regions).For each graph,write the name of the type of selection that is depicted.
A)

B)

C)


b)Described below are three examples of selection.For each example,write the name of the type of selection being described.

i)A maize breeder selects for increased ear size in maize in order to increase yield.

ii)Susceptibility to malaria and sickle-cell anemia is controlled by the same gene.The Hb^A allele provides resistance to sickle-cell anemia but not to malaria (thus,individuals who have the genotype Hb^AHb^A die from malaria),and the Hb^S allele provides resistance to malaria but not to sickle-cell anemia (thus,individuals with the genotype Hb^SHb^S die from sickle-cell anemia).Hb^AHb^S individuals are resistant to both diseases.

iii)Lower-yielding beans are resistant to anthracnose,whereas higher-yielding varieties are susceptible to the disease.Anthracnose is present only in certain areas of the region.So,breeders want to select higher- and lower-yielding plants to fulfill different breeding goals.