Oh no, SPECIATION has occurred! The land has separated us with a river, and now we can't meet.
This scene is an example of geographic isolation between two goats. They are unable to swim across this river to mate and have babies.
Red Scarf Misses Purple Scarf
I sure wish I could interbreed with the purple goats, but GEOGRAPHIC ISOLATION stopped us from doing so.
These two goats cannot swim. Therefore, they can't mate with each other. The red scarf and the purple scarf are representing two traits that the goats like about each other. That is why they want to mate.
Purple Scarf Misses Red Scarf
Since we can't continue the population, I'll have to start a new one from scratch!
This river prevents two types of goat from mating, causing the gene pool to become less varied for the goats. This is something called SPECIATION, which (in this example) means that the river divided two land chunks, causing the start of two new species.
New Populations Are Formed
Yes and so does yours! Goodbye old friend!
Now that the two goats are separated, they can no longer breed into their normal species.
At this point, it has been too long to start/continue the regular goat species so they all must start a new one.
The Red and Purple Goats have now started their own new population again. The cycle of GEOGRAPHIC ISOLATION and SPECIATION will continue over the following years.
I see you've started a new population. It looks great!