I didn't know this but there is a way to calculate the minimum population required for a certain species to survive over the next 100 to 1000 years. This is called the "minimum viable population".
Genesis states that 2 (or 7) of each "kind" went into the ark. Supposing that each "kind" means each "species", and they all could fit in the ark, and they came out safely, most of the species would have died, because it is impossible to survive, long-term, with a starting population of 2.
This is interesting:
"An MVP of 500 to 1,000 has often been given as an average for terrestrial vertebrates when inbreeding or genetic variability is ignored. When inbreeding effects are included, estimates of MVP for many species are in the thousands. Based on a meta-analysis of reported values in the literature for many species, Traill et al. reported a median MVP of 4,169 individuals."
4169 individuals PER SPECIES are required to have the species survive. Otherwise there is not enough genetic variation to survive, there would be massive extinction the moment the species encountered anything that would be harmful to its genetic pool (which would be very limited).
If you want to read more, here's the article on Wikipedia: