Ecology

The environment affects the evolution of living things.
As predicted by evolutionary theory, populations evolve in response to their surroundings. In any ecosystem there are finite opportunities to make a living. Organisms either have the genetic tools to take advantage of those opportunities or they do not.

House sparrows arrived in North America from Europe in the nineteenth century. Since then, genetic variation within the population, and selection in various habitats, have allowed them to inhabit most of the continent. House sparrows in the north are larger and darker colored than those in the south. Darker colors absorb sunlight better than light colors and larger size allows less surface area per unit volume, thus reducing heat loss—both advantages in a cold climate. This is an example of natural selection acting upon a population, producing micro-evolution on a continental scale.