Forces of Evolution

Source: Food and Agriculture Organization of the United Nations, 2013, Cary Fowler, Charlotte Lusty and Maria Vinje Dodson, and Global Crop Diversity Trust, "The Youth Guide to Biodiversity" 1st Edition (Chapter 3) Youth and United Nations Global Alliance. Reproduced with permission.

Chapter 3. Verbatim.

Let’s go back to eye colour, only let’s imagine there is a forest that is home to a population of pigeons with different eye colours – let’s say some have green eyes and others grey-eyes. By chance, the green-eyed pigeons are particularly good at seeing in the dark. In this imaginary forest, the pigeons feed all day on delicious figs only available on a small number of fig trees. During the night, the birds rest. But a change in the climate drives a new species of pigeon-hunting eagle into the area. The eagle’s talent for diving from a great height onto pigeons feeding inthe fruit trees depends on good daytime visibility. 

The pigeons soon begin to hide in the daytime and feed at night. The grey eyed pigeons have trouble finding fruit at night and are caught by the eagle during the day. The pigeons with green eyes, however, are able to eat without the threat of the eagle because they can feed at night. The green-eyed pigeons’ green-eyed offspring are more successful! They live longer and have more offspring. As the grey-eyed pigeons disappear, the pigeon population starts to change. Over time, most of the newborn pigeons have the same eye colour – green! 

This story illustrates that where there is variation in a population (e.g. in eye colour), new or existing pressures (e.g. eagles) select specific inheritable traits passed on from one generation to the next (e.g. green eyes) that convey an advantage. Over time (e.g. several generations of pigeons) the population changes and the species evolves. Where there are no pigeon-hunting eagles, the pigeons can still thrive whatever their eye colour. Eventually after much time has passed, the green-eyed pigeons, with their skills at feeding in the night and avoiding eagles,could become a completely independent species.