If I remember correctly one definition of Species is that if it can mate and have a fertile offspring it's the same species. The fertile part is because some species can breed which almost always results in a sterile offspring.

 

What is it the makes a species able to breed with another? Do the still share so many common genes involving breeding while others have changed more? Is there something specific Gene-wise that controls when they can.

 

Why do the offspring become sterile?

 

What are some species that can breed. I remember Mule and Liger and some where the species are more similiar then most breeds of dogs ( wolfs/coyotes, polar bear/grizzly) What are the more stranger ones?

 

Also why can dog breeds look so different and still be the same species while things like gray/ red wolf, black/brown bear, white/black rhino look almost identical but are seperate species?

 

Donkey's and Horses ( and zebras ) are quit different looking, so is the Lion and the Tiger. Many species are closer in appearance so I would think more likely to interbreed. Has anyone thought about the possibility of a human/chimp or human/gorilla possibilities ( I'm just talking wether genetics would allow it, not suggesting anything and certainly not volunteering ).

Species is more or less an arbitrary distinction, except the particular definition you gave involving reproduction. However, then you can make a new species with exactly the same genes by changing the number of chromosomes or somesuch.

What is it the makes a species able to breed with another?

 

There are 5 prezygotic barriers, and 3 postzygotic barriers which make it impossible for pieces to breed. They prezygotic barriers are:

 

- Temporal Isolation: The two pieces are reproductively active at different times.

- Habitat Isolation: The two species live in different areas, and therefore do not meet to mate.

- Behavioral Isolation: The two species have different mating rituals/

- Mechanical Isolation: The two species reproduction parts just don't fit together properly.

- Gametic Isolation: The two species gametes are incompatible making fertilization impossible.

 

The three postzygotic barriers are:

 

- Hybrid Inviablity: The hybrid does not mature properly into a function offspring.

- Hybrid Sterility: The hybrid is sterile normally because of gametic abnormalities.

- Hybrid Breakdown: Over generations the hybrids breakdown and become incapable of reproducing.