Peter and Rosemary Grant's Finches

Question

Explain the following statement: "Selection occurs within generations; evolution occurs between generations."

Answer 1

Selection occurs amongst individuals within a single generation whereas evolution occurs over a longer period of time, with several generations of selection for a specific trait within each generation. For example, if a cheetah were faster than other cheetahs, it would have an evolutionary advantage over its peers because it might be better at hunting. This is a selection within a single generation. Then let's say that cheetah reproduces and its offspring are as fast as it is. These second-generation cheetahs reproduce and their offspring (third-generation) who inherit this trait for speed are more successful at hunting prey. Over time, this trait becomes more widespread as the cheetahs reproduce. Cheetahs who do not receive this trait may end up eating less or may be less able to escape from predators. Thus, evolutionary success is based on individual selection within a single generation.

Answer 2

Selection suggests small changes to the actions or bodies of the birds over a generation - in other words, their life span or the life span of their offspring. So, if a finch lives between 10 - 15 years, then during that time their feeding habits might change because of a change to habitat or their flight paths might change because of something that has been built that obstructs their path. But it's the finches who are able to adapt to these changes who survive. And it takes many, many generations to change the bird itself. This is especially true for, let's say, the wing span of the bird or the shape of the beak or the strength of the flying abilities of the bird. For such major changes to occur, there has to be more than adaptation happening in a certain moment in time, there also has to be survival of the fittest. The adaptations and behaviors of the finch have to occur over several generations for evolutionary changes to occur in the entire species. So it's not just a change in behavior, but a change that becomes inherited, so it is passed through the genes of the bird to the next generation. The idea of "selection" is the strongest survive the changes/adaptations that occur within a generation, so that the species evolves and survives between generations.