Which process of evolution would best explain why closely neighboring sites have very similar beetle populations

Gene flow

Explanation:

Gene flow can be defined as the transfer of genes from one population to another, which may be due to the migration of individuals that reproduce in the new population or due to gamete dispersal (for example, pollen dispersal in plants). Gene flow is a very important mechanism capable of changing allele frequencies in populations. Gene flow is low in species with low dispersal ability and has a huge effect in populations with small sizes. Moreover, high rates of migration among nearby populations are expected to increase their genetic homogeneity (thereby constraining speciation processes).

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds