Answer:
oxygen gas is most abundant in the atmosphere
An ecological pyramid is a graphical representation designed to show the biomass or bio productivity at each trophic level in a given ecosystem. Biomass pyramids show how much biomass is present in the organisms at each trophic level, while productivity pyramids show the procreation or turnover in biomass.
A change in allele frequency can be caused by various reasons. Mutations, recombination, gene drift, gene flow, and hybridization are some of the examples. Basically anything that would cause microevolution to happen would change allele frequencies. The Hardy Weinberg equilibrium might be helpful, too.