In individual selection, some individuals survive to reproduce, and some don't. If the differences in survival and reproduction are the result of particular genes, these genes will increase in frequency in the next generation. Group selection theory adds another evolutionary layer to this tidy little setup. The idea is that in addition to selection at the individual level, selection can also occur at the group level. (You can even think of kin selection as an example of group selection where the group is the family.)
Evolutionary scientists have also become aware that selection can act at the level of the gene, beyond the forces of individual or group selection. Essentially, this theory recognizes that occasionally, certain genes increase in frequency simply because they've got a better-than-average chance of getting into the gamete during reproduction, regardless of the negative effect they may have on the offspring.
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