Hybrid zygotes between two species might occur, but...
Embryological arrest: hybrid embryos often do not develop properly; no viable offspring is created
Infertility: hybrid offspring might grow to viable adults but these are infertile and cannot produce further offspring ? cannot create a new population (e.g. mules, ?donkey + ?horse)
Natural selection: If hybrids are less adapted (weaker, smaller, etc.), they are removed by natural selection
However: many plants can form viable hybrids
Asexual reproduction can proliferate infertile hybrid populations (plants, fungi, algae, bacteria, invertebrate animals)