It is known that mules can not have offspring; for a long time people only knew the fact, not the reason until a few decades ago.
Information to determine biological nature is DNA and genes, which align on chromosomes. Horses have 64 chromosomes, whereas donkey 62, the mules chromosome 63. With cross-mating of different species, the fertilized egg would not have developed, or the fetus would die in uterus, no new baby will not come into the world, the mule is an exceptional case.
Mules are able to develop into a mature adult in first-generation, but it can not produce offspring by mating among themselves or mate with either horses or donkeys, as mules either have problems to form reproductive cells, or form zygotes when mating with horses or donkeys, their chromosomes does not match, for either reasons mule does not have offspring.
Natural watermelon is a diploid with 11 pairs of chromosomes (2n = 22). With colchicines treatment of the seeds, its chromosome would double to have 22 pairs (2n = 44). Tetraploids could become a tetraploid watermelon. Tetraploid has seed of 22 chromosomes which receives diploid pollen, the offspring are triploid (33 chromosomes). This triploid chromosomes fails meiosis when pairing, its fruit does not the seed or seeds are very small without proper function.
Chromosomal change in the species is not a simple mathematical addition and subtraction. Mating between two mules would neither reproduce donkeys with 62 chromosomes, nor the 64 horse with 62 chromosomes, they do not have normal sperms and eggs.
Similarly, mating of two Down syndrome, who has 47 chromosomes, would neither produce monkeys with 48 chromosomes, nor human with 46 chromosomes, as sperm, eggs and reproductive organs are abnormal.
More than 95% of organism have has a unique chromosomal structure under the microscope, since the resolution of microscope is 1-2 one million base pairs, there are some organisms that had no the structural difference under a microscope, in fact there are the structural difference under a microscope with the improvement of technology.
These differences could be from change of chromosome number, multiplication of the chromosome (polyploidy), chromosomal translocation, amplification, deletion, and so on. It can be said that all the different species has unique structure on chromosomes, in other words as long as organisms have same chromosomal structures, they belong to same species, no matter how different they look or behavior, they can mate with reproductive offspring if their germs cells can meet, naturally or by artificially insemination.
What I emphasize here is that the of different species never have the same chromosomal structures, I am not to say that one species cam only have one type of chromosomal structure, some organisms have more than one type of chromosome, as they could still mate, and give birth to healthy fertile offspring, they are same species.
Infertile mules tell us for a bisexual-organisms, if evolution involving chromosome number of changes, that the process can not be involved any animals with chromosomes in odds number , such as mule with 63 chromosomes, or Down syndrome with 47 chromosomes , they must have the even (2,4,6 ..) number chromosomes.
For example, if one animal has 2A chromosomes, chromosomes in its gamete has A chromosomes. By mutation, another animals with 2A+2 chromosomes was born, who survive and become adult. Its gamete has A + 1 chromosomes. If the new-born mutant were only one in the world, mating between two gametes would have A+(A+1) chromosomes, the organism with the combined gametes would either die, or born without fertility, just like mules.
The new-born mutant (2A+2 chromosomes) must have another organism with the same chromosomes (2A+2) at the same time and place to mate.
The next question ones will naturally ask is: how can that happen? Only solution to this dilemma is that new species of animal come in form of bisexual twins. They have same or similar chromosomal structure, their gametes are matched, and they are born and raised in same time and place.
Sterile mule teaches us a lesson: “Adam” and “Eve” was an inevitable initiated in one-generation at the same time, they must be twin brothers and sisters by a parent.