why cant different species breed with each other? If they both have sperm and ovum?


why cant different species breed with each other? If they both have sperm and ovum?



Because that’s the (most widely agreed upon) definition of a species: two animals are of the same species if they can breed together and *generally* produce viable (that is, able to sire/carry children of their own) offspring. I say “generally” because there are rare exceptions; I want to say something like 1 mule in 100k+ will be fertile, but that isn’t enough to consider horses and donkeys the same

For instance: All dogs are the same species, despite there being wild size, coloration, and texture differences throughout the different breeds of dog. However, if you take two really disparate breeds of dog, say Chihuahua and St. Bernard, and get them to (somehow) mate, [their offspring](https://www.shopforyourcause.com/dog-breed/chihuahua-saint-bernard-mix) will be able to produce offspring of its own.

On the other extreme, take a horse and a donkey: not the same, but certainly more similar in size/general shape than a St. Bernard and a Chihuahua seem to be at first glance, and they can breed, but you get a mule (or a hinny), which is infertile in the vast majority of cases.

Different proteins on the sperm and ova that need to be able to bind to each other in order for them to fuse. Those proteins will be different enough that the two will not fuse and become an embryo. In fact that’s one way to determine that one species has diverged into two, because they can no longer interbreed.

In other cases, like with horses and donkeys, they can breed and produce an offspring, a mule. Due to differences in their chromosomes, they are typically unable to reproduce, although it isn’t impossible.

First, if the sperm cell surface markers are too different it won’t even get into the egg. The key doesn’t match the lock.

Provided you beat that, you have a half set of chromosomes and an entirely different half set with a different number of chromosomes and different gene placement. Nothing matches, the chromosomes can’t pair off, and gene expression is catastrophically wrong. The fertilized egg fails to divide and dies.

You’re shuffling half a deck of Uno cards into half a deck of Pokemon cards and trying to play a game.

The sperm is unable to fertilize the egg because the genomes (the entirety of DNA of the species’) are not compatible. This means that the typical genetic recombination (the rearrangement of the DNA between the sperm and the egg) cannot occur properly and therefore not allowing for an offspring to develop. In order for breeding to occur the genetic material from the father and mother need to combine together and that cannot happen if animals from different species try to mate.

However, there are hybrids. This is when two animals from different, but similar, species mate. The hybrid offspring is often times sterile. This means that the offspring won’t be able to mate at all, even with the same hybrid offspring. A Liger is an example of a hybrid between a Lion and a Tiger. It’s basically a huge cat, but the Liger is sterile and therefore infertile and unable to produce offspring of its own.

TLDR: Sperm can’t fertilize egg because DNA is too different. Hybrids are possible from different yet similar species like the Lion and Tiger because DNA is close enough.