The seal species, live on a glacier and very close to a volcano that is bound to erupt any day from now.
Some seals were born being able to swim faster than others. Once the volcano does erupt, some seals will be able to flee faster than others, as they are better swimmers than others.
Once the grey seals were to notice this volcano, they could flee faster than others due to their speed. But, some were born with different colors are not able to get away faster than others are able to.
The other seals swim slower than the grey seals due to the traits they were born with. Making it more difficult for them to leave once the volvano erupts.
Because of this, grey seals are able to live on longer and be killed less. The rest of the seals will have to worry because they can not swim away faster than the grey seals.
Since surviving seals are able to get a chance to reproduce, mostly grey seals will be able to pass on their traits due to surviving the volcano. So, a higher percentage of grey seals will be in the next generation.