As we would expect based on evolutionary theory, populations evolve in response to their surroundings. Much of this evolution happened long ago and was not observed by humans – and consequently, we have to investigate many lines of evidence to reconstruct this history.  However, in some cases evolution has occurred in the wild over timescales and in places that we can make direct observations of – that is, we humans know what a species was like at one point in time, and then later observe that it has changed in ways that can only be attributed to evolution.

For example, house sparrows were brought to North America from Europe in the nineteenth century. Since then, genetic variation within the species, and the different selective pressures present in different habitats have allowed them to adapt to different parts of the continent.  Thus, modern 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 different populations, producing micro-evolution on a continental scale. And it is one that humans have been around to observe firsthand. We were there when the sparrows were intentionally released in the 1800s, and today we can directly observe that sparrows from different parts of the continent are different from one another, as shown by this map.