Final answer:
Hidden genetic variation is the unseen genetic diversity that provides the essential material for evolution, and its discovery was pivotal in substantiating Darwin's theory of natural selection by explaining how advantageous traits are preserved and passed down through generations.
Step-by-step explanation:
Hidden Genetic Variation and Natural Selection
"Hidden genetic variation" refers to the genetic diversity within a population that may not be immediately apparent because it can involve alleles that do not have an observable effect on the phenotype or are only expressed under certain environmental conditions. The discovery of such variation was significant for Darwin's hypothesis about natural selection, as it provides the raw material for evolution. Prior to this understanding, the predominant genetic theory was of blending inheritance, which could not adequately explain how traits that confer a survival advantage are preserved and do not simply blend away. With the rediscovery of Gregor Mendel's work, the concept of particulate inheritance clarified that traits are inherited as discrete units (genes), and this could lead to the stable transmission of advantageous traits that natural selection could act upon.
The integration of genetics and evolution through the modern synthesis reinforced the significance of hidden genetic variation by demonstrating how gradual evolution could occur via changes in allele frequencies over generations, leading to microevolution and macroevolution.