Biology - Speciation

Directional selection: The types are stabilizing, directional, and disruptive. It's directional because if you think about it, the lighter the bones are the better adapted the bird will be, so natural selection will just keep selecting for the lightest bones out of every generation. This will cause the average bone weight to get progressively lighter over generations.

Evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms; for example, the allele for black color in a population of moths becoming more common. So if the resources of a forest are breaking up, and group of a species of plant or animal is naturally selected by the environment, and those surviving species interbreed and evolution would be affected that way. Small horned moose might be naturally selected over small horned moose. A worm that survives in warm soil might be naturally selected over a worm that survives in cold soil.