1. From 1984 to 2012, the wing length of cliff swallows in the general population generally decreased.
2. During the same time period, the wing length of cliff swallows killed on the roads generally increased.
3. In most years where data was collected, the wings of cliff swallows killed on the roads were longer than the wings of swallows found in the population at large.
4. Shorter wings allow cliff swallows to take off from the ground more vertically, enabling them to better avoid oncoming cars. As a result, cliff swallows in populations near highways with shorter wings have a survival advantage.
5. The data suggest that, over time, natural selection resulted in the evolution of shorter wings in the cliff swallow population.
6. Shorter wings are a(n) adaptation in this population. Alleles for shorter wings were already present in the general population.