The average female atlantic cod (gadus morhua) produces two million eggs in a single spawning. how does this relate to natural selection?

This is a typical example of R selection life strategy. These species, including the Atlantic cod, produce a large number of offspring with a low chance of surviving to adulthood. This strategy is particularly successful in environments that are unstable. The main advantage of this strategy in changing environments is that the ''cost'' of making offspring is very low, so you the species can produce large numbers offspring, ensuring that at least some of them will survive into adulthood and continue the species.
Natural selection, in this case, favors the individuals that are able to adapt the fastest to the changing conditions.