[Sethomas]

I would define phenotype as the physical manifestation of an organism's genetic code, id est, its genotype. Two people can have different genotypes and the same phenotype, namely with dominant alleles whether something is heterogeneous or homogeneous.

In the case of the asteroid hitting Earth, a phenotype for small body structure and plenty of insulation (hair, feathers) would be very "convenient", as would the ability to run fast or otherwise avoid predators in rabbits in the case you mentioned. That's the way that punctuated equilibrium works to further evolution.