Everyday Einstein provides an introduction to evolution. Plus, the answer to the age-old question: How could evolution have saved goblins from extinction?
Now suppose that this cycle of long, hard winters lasts for a few more years. Assuming the goblins don't change their hunting habits (which they wouldn’t, as goblins are well known to be relatively slow-witted), whose children are going to eventually be the healthiest and strongest goblins?
Think of the Children!
In our contrived example, Gulrock's fuzzy appearance was caused by a genetic mutation. At some point in his developmental history, the set of goblin-hair-production genes that Gulrock inherited from his parents changed, or mutated, in such a way as to give him his furry coat.
This mutation could have happened while those genes still belonged to one of his ancestors, or during some part of his own development. However it happened, this mutation caused a change in one of the physical "traits" of the goblin. Instead of bare, leathery skin, Gulrock ended up with soft, downy fur covering his body. Now in this particular situation, it turns out that this trait was extremely beneficial for Gulrock. By being able to hunt longer than the other goblins, he made sure that he and the rest of his family was more physically fit than the rest of the tribe. This means that if you were to look at all of the goblins in the next generation, Gulrock's children would be healthy and strong while those outside his family will either have gone to the great goblin hunting grounds in the sky, or be suffering from malnourishment. Assuming that the mutation of Gulrock's hair-related genes can be passed on to his children, (what scientists call a "heritable trait"), this means that all of his healthy goblin children will also carry this mutation and will be hairy goblins themselves.
If the trend of long winters continued, after a while there would be very few non-hairy goblins remaining; the tribe would consist almost entirely of hairy goblins. In scientific terms we say that the hairy goblins evolved from the non-hairy goblins.