Complex sentence: Selection pressure in the environment selects for antigenic changes in the antigen determining spots of HA, that includes places undergoing adaptive evolution and in antigenic locations undergoing substitutions, which ultimately results in changes in the antigenicity of the virus.
Split into simpler sentences: Selection pressure in the environment selects for antigenic changes in the antigen determining spots of HA. Sites undergoing adaptive evolution and substitution are ultimately changed which results in the antigenicity of the virus.