here's a notion. what if natural selection works on an eco-system basis (as opposed to, or in addition to, the level of genes, or individuals, or species)? earth's climate has been highly volatile throughout. so, eco-systems that are very specifically dependent on very specific climate conditions collapse as a whole, including more or less all the individuals, genes and species that are exclusive to it. the system is mutually interdependent, so on some occasions the whole system more or less has to stand or fall, adapt or die together. say if there are no trees the squirrels die (oh and stipulate that somehow without the squirrels the trees die; this seems unlikely, but if you took it through a few species steps...): well, then, the two are one evolutionary unit, engaged in mutual cooperative selection; they stand or fall as species together, or we might say for the purposes of natural selection they are one thing, a primary individual as fundamental as any particular squirrel or tree, or all the squirrels together or all the trees together. but climate is only one factor: we might think of invasive species, for example, and in general what happens when successful ecosystems meet or touch or overlap. that is, we might say the unit of evolution is all the species in a system insofar as they are mutually interdependent for life.