Final answer:
The phenotypic distribution for a trait not under selective pressure will generally form a bell-shaped curve, as described by the Hardy-Weinberg equilibrium, assuming allele frequencies remain stable over time. To test for stability, genetic sampling is required to compare observed allele frequencies against equilibrium predictions.
Step-by-step explanation:
If a certain trait in a population is not under any selective pressure, the likely phenotypic distribution for this trait would resemble a bell-shaped curve. This is indicative of a polygenic trait where most individuals in the population have a phenotype that is average, and there are fewer individuals at the extremes of the curve. Without selective pressure, there is no directional preference for one phenotype over another, so extreme phenotypes are neither selected for nor against.
Under Hardy-Weinberg equilibrium, traits not subjected to evolutionary forces maintain consistent allele frequencies over generations, resulting in a stable phenotypic distribution. To test this assumption further, one would monitor the allele frequencies over time to ensure they align with Hardy-Weinberg predictions, and this would involve genetic sampling and counting actual phenotypes in the population.
When analyzing the change in gene frequency of alleles A and a, if the graph shows no preferential increase or decrease over time, it suggests that no specific type of evolution like genetic drift, gene flow, or natural selection is affecting those alleles. The alleles might be considered neutral, as their frequency does not change due to selection pressures.