Remember the five conditions for Hardy-Weinberg equilibrium: no mutation, no gene flow, random mating, no natural selection, and large population size. Any deviation from these conditions will disrupt the equilibrium. Random mating is a condition that, when met, ensures the equilibrium is maintained.
Step 1: Understand the Hardy-Weinberg Principle and its conditions.
The Hardy-Weinberg Principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. For a population to be in Hardy-Weinberg equilibrium, five specific conditions must be met:
(A) No mutation: There are no new alleles introduced or changes in existing alleles.
(B) No gene flow (no migration): No individuals or their genetic material enter or leave the population.
(C) Random mating: Individuals in the population mate randomly with respect to their genotype.
(D) No natural selection: All genotypes have equal chances of survival and reproduction.
(E) Large population size: The population is large enough to prevent random fluctuations in allele frequencies due to chance (genetic drift).
Factors that \textit{affect} or \textit{disturb} the Hardy-Weinberg equilibrium are those that violate one or more of these conditions, leading to changes in allele and genotype frequencies over time (i.e., evolution).
Step 2: Evaluate each option against these conditions. (A) Natural selection: Natural selection is a major evolutionary force. It favors certain genotypes over others, leading to differential survival and reproduction, thereby changing allele frequencies. Thus, natural selection affects Hardy-Weinberg equilibrium.
(B) Random mating: Random mating is one of the \textit{conditions} required for maintaining Hardy-Weinberg equilibrium. If mating is random, it ensures that allele frequencies themselves do not change from generation to generation due to mating patterns. Therefore, random mating itself does not affect (i.e., does not disturb) Hardy-Weinberg equilibrium; rather, its \textit{absence} (non-random mating) would disturb it.
(C) Crossing over: Crossing over is a genetic recombination event that occurs during meiosis. It shuffles alleles between homologous chromosomes, creating new combinations of alleles on a chromatid. However, crossing over does \textit{not} change the overall allele frequencies (the proportion of 'A' alleles versus 'a' alleles) in the gene pool of the population. It rearranges existing genetic variation but does not introduce new alleles or change their overall proportions. Therefore, crossing over does not affect Hardy-Weinberg equilibrium.
(D) Mutation: Mutation is a process that introduces new alleles into a population or changes existing ones. This direct alteration of the genetic makeup of the gene pool causes a change in allele frequencies. Thus, mutation affects Hardy-Weinberg equilibrium.
Step 3: Conclude the factor that does not affect equilibrium.
Both "Random mating" and "Crossing over" do not affect the Hardy-Weinberg equilibrium because they do not change allele frequencies in the gene pool. However, "Random mating" is a direct and explicit assumption for the maintenance of the equilibrium. Its presence ensures the stability of allele frequencies. While crossing over also does not change allele frequencies, random mating is a more fundamental and frequently stated condition when discussing the principle's assumptions. In the context of typical questions about Hardy-Weinberg conditions, "(B) Random mating" is the most commonly expected answer for a factor that does \textit{not} disturb the equilibrium, as it is a condition for the equilibrium's existence.
