This is where I will begin adding information for multilevel selection
Multilevel selection 1 (MLS1)
Multilevel selection 1 is when individuals combine to form a group, and non-additive properties that arise from individual interactions leads to trait variation at the group level. While fitness variation associated with trait variation arises at the level of the group, the response to selection is determined by the reproduction of individual organisms that make up the groups. An example of a MLS1 model would be David Sloan Wilson's trait-group model. The occurrence of MLS1 is not disputed. The usual response to claims of MSL1 is to state that fitness variation that arises at the level of the groups can be ascribed to the individuals that compose groups, which is true.
Multilevel selection 2 (MLS2)
Multilevel selection 2 is when individuals combine to form a group, and non-additive properties that arise from individual interactions leads to trait variation at the group level, similar to MLS1. The difference between MLS1 and MLS2 is how the evolutionary response to selection is generated. Fitness variation at the level of the group in MLS2 models is due to differential reproduction at the level of the groups themselves. This can be achieved through mechanisms of group fissioning.