They are/were both alive and they are both Animals
The answer is the
population size. The chance has a role
in determining whether a given individual survives and reproduces and the
alleles in the offspring are a sample of those in the parents. it is the change
in the frequency of an existing gene variant (allele) in a population due to
random sampling of organisms. Genetic Drift or another term for allelic drift
or the Sewall Wright effect.
I don't have enough information but it could be natural selection or mutations.