Reproductive barriers exist between most species living in the same region such that they cannot create hybrid organisms.  Reproductive barriers are divided into 2 categories: pre-zygotic and post-zygotic barriers.

Pre-zygotic (prior to fertilization) Barrier Types.

1. habitat (ecological) isolation:  different location (i.e. water and land)
2. behavioral isolation:  different mating signals or stimuli
3. temporal isolation:  different reproductive cycles
4. mechanical isolation:  physiologically incompatible
5. gametic isolation:  incompatible gametes such that they cannot even fuse to form a zygote

Post-zygotic (after fertilization) Barrier Types.

1. reduced hybrid viability:  incompatible genetic material (i.e. chromosome number)
2. reduced hybrid fertility:  offspring is infertile
3. hybrid breakdown:  hybrids are successful but when hybrids mate, their offspring are feeble or sterile

So what if cross-species reproduction does work?

It is called introgression.  Introgression means a complete success - even with the transfer of inter-species alleles.

Polyploidy.  a form of nondysjunction.

An autopolyploid has more than 2 chromosome sets derived from a single species.  An allopolyploid can be either self-fertilized or created by unreduced gametes plus normal ones.  A mule is an example of an allopolyploid created by inter-species reproduction.  Why are inter-species hybrids usually infertile?  Because their unbalanced haploid set cannot pair correctly during meiosis (gamete formation).  Oats, cotton, tobacco, potatoes are all plant polyploids (but plants can handle the extra chromosomes better than animals).

Next:  "Speciation."