how do you know that a horse and a donkey are two diffrent species?

Answer 1

They are separate species because their offspring are infertile. There are several definitions of species, but they all have a similar idea. They are naturally occurring populations that are genetically isolated from other such populations. Naturally occuring is added to seprate them from laboratory conditions. Just because we can make animals from two different species breed with each other in the lab doesn't mean that they would do so in their natural habitat.

The genetic isolation may be spatial, temporal, or physical, with physical being the most obvious. The populations are unable to interbreed with viable offspring. Temporal separation poses more difficulties in definition. If you have two similar populations that overlap but that breed at different times of the year, they would be different species because there would be no gene flow between the two populations. Spatial is the most difficult to define because if the spatial component were removed, the two species would commonly be able to interbreed.

Generally, except in some rare specific circumstances, species develop through spatial isolation at first. Mutation and selective pressures change the isolated populations over time, pssibly leading to chenges in breeding time, perhaps depending on climate differences between the two (or more) populations. As the changes build up more, they become physically unable to breed. This progress can be very slow in stable environments and where there are large numbers in each population or relatively quickly in unstable environments with high selective pressures or where one of the isolated populations is very small.

Answer 2

Species is just a definition. It's more convenient to define them different.

Defining a species is based on various characteristics: morphological (historically, extremely important), breeding habits, hybridization, and nowadays, genetic sequencing. However, even genetic sequencing doesn't necessarily simplify the problem - do you place a greater emphasis on certain genes over others? What's the "threshold" for a different species?

So really, it's pretty arbitrary.
And to answer your question, it's either "it's just the way it is" or "it's based on various characteristics that make them quite different."

Answer 3

Horses and donkeys are NOT separate species, they are sub-species. This means that when they breed, their offspring are infertile (can't have babies), with only extremely rare exceptions. If they were separate species, their mating would not produce any offspring at all. This is also true for lions and tigers.

Answer 4

when animals from the same species mate their offspring are fertile if the offspring are sterile the parents are not the same species

Answer 5

Because the offspring which are hybrids, cannot reproduce.