The easiest example is of Pentadactyl limbs. This is the 5 bone structure of the 'arm' of different animals being similar, contributing to the explanation of common ancestors. Animals that have this are whales, horses, dogs, cows, birds and even humans. We each share the same five bones in similar places but with some of them changed to suit certain functions ie birds are more spread to support flying, while humans are made for throwing, picking up and holding things.
The horse and donkey, though closely related by sharing a common ancestor, are different species. They have different number of chromosomes hence pairing of homologous chromosomes during fusion of gametes becomes complicated. The mule will have an extra chromosome from the horse hence will have abnormalities such as sterility. A Mule is unable to reproduce due to this same phenomenon. Homologous chromosomes are not well paired for meiosis I.
This is because of the populations.
Less water would make a large population dehydrate.
The answer is a slim layer, Flagella, and a Fimbriae
Hope this helps!!
