Natural selection is one of the main processes by which organisms evolve across time (the other is genetic drift; see activity 2). The theory of natural selection was first put forward by Charles Darwin and Alfred Russel Wallace (1858). Natural selection is the gradual, non-random process by which biological traits become either more or less common in a population as a function of differential reproduction of their bearers. It will occur if three conditions are met:
1. Heredity: Individuals resemble their parents: they are in general more like their parents than are other members of the population.
2. Variation: Individuals vary in their characteristics
3. Fitness differences: There is a relationship between some of the characteristics of an individual and its ability to survive and reproduce (its fitness).
Although these prerequisites are still widely accepted today, for a long time after Darwin’s theory was proposed, a theory of inheritance was lacking. The first step in resolving this issue was taken in 1900 when Hugo de Vries and Carl Correns rediscovered the work of Gregor Mendel (the “father of modern genetics”) whose work on plant hybridisation identified a number of laws of inheritance. For the next 40 years, there was then a long struggle to incorporate genetics into evolutionary theory, culminating in what is generally known as the Modern Synthesis (ca 1940-50). The theory of natural selection could now be stated in terms of genes: what actually changes during biological evolution is the relative frequency of genes. Natural selection is thus the differential survival of alternative alleles.
You can read more about the modern synthesis and natural selection in general on the website of Christs College (the college Darwin attended in Cambridge – link below). You may think that a historical perspective is not useful, but understanding the process by which theories change and develop is an important skill in modern science.
Before you progress to the next section, make sure that you understand the ways in which variation arises in the genome. A good video to show this is found on this page.