Natural selection

Darwin and Wallace proposed two theories:

1. The living organisms we see today are all related by descent (common ancestry)

2. The means by which evolution occurs is a process of "natural selection."

• organisms differ from one another (i.e., there is variation)
• these differences are heritable (passed on from generation to generation)
• many more organisms are born than survive and reproduce (mortality)
• therefore, any variation that makes one offspring more successful than another will have a greater chance of bein passed to the next generation ("survival of the fittest").

In the first lecture we looked at the evidence that evoluion had occurred. now we need a mechanism to explain evolution. Darwin and Wallace took as their starting point an observation by Malthus that there is an imbalance between reproduction and resources.

Thomas Malthus