Interest in determining "racial purity" was not, and is not, a problem found exclusively in any society. as we have seen, numerous methods were devised in an attempt to ensure that one's racial makeup was exact and clear. the "one-drop" rule was one method, arising out of miscegenation laws forbidding interracial marriage. the fundamental flaw with the "one-drop" rule is that
a. the one-drop rule depended on a nonexistent ability or test to detect, biologically, the presence of racial impurities.
b. the one-drop rule required blood testing verification of race on birth certificates, which were later used to reinforce antimiscegenation laws.
c. enforcement of the one-drop rule meant that, if found out, the individuals would need to get a divorce.
d. the one-drop rule was not as accurate a determination as other tests

A. the one-drop rule depended on a nonexistent ability or test to detect, biologically, the presence of racial impurities.

Explanation:

The one-drop rule is a system of classifying an individual to a particular race just because one of his/her ancestor is of Sub-saharan origin despite the physical appearance (phenotype) of the individual. The short coming of this rule is that there is no scientific test carried out to determine the individuals genetic make up.

1) gene flow

- it is described as the transfer of genes from one population to another population so that frequency of gene is increased.

-a zebra migrates to join a different herd of zebras.

2) natural selection

- a mechanism of evolutionary change in which individuals with advantageous traits survive and reproduce whereas the others ( those that can not adapt) eliminate from the population.

-competition for sunlight leads to taller trees.

3) mutation

- permanent change in the nucleotide sequence of dna ( genetic material) that could have harmful or useful consequences.

-the dna of a snake changes to make its venom stronger.

4) genetic drift - it is a random phenomenon ( that occurs by chance such as fire) that results in change in the gene frequency in a small population.    

-a grassfire randomly sweeps through a population of buffalo and kills most of the animals.

explanation:

when it is at an intermediate population density