Breeding between different species, although possible, is often complicated as it results in infertile or non-viable offspring. This usually occurs due to the difference in the karyotype.
The karyotype is the set of chromosomes that an organism possesses, for example, the human has 23 pairs of chromosomes (46 chromosomes).
For reproduction to be successful and give rise to healthy organisms, the parents must be of the same species (have the same number of chromosomes and these chromosomes must be homologous, i.e. have similar genes).
Therefore, unlike organisms of different species, the reproduction of 2 organisms of the same species can result in fertile offspring capable of reproduction.