Nature Versus Nurture

Biological reproduction involves some variability. In other words, when individuals within a species mate, their offspring are not all identical. This variation can be accounted for by genetics, but Darwin had not yet read about Gregor Mendel's research demonstrating some of the basic principles of genetics. Darwin simply knew that variation did occur. When variations in reproduction occurred, some of the offspring would live long enough to mate, but others would die before mating. Therefore, the reproductive variations associated with living long enough to reproduce would be passed along. Those that were less adaptive would reproduce at lower rates because of their reduced survival.

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Some have referred to natural selection as survival of the fittest, although the concept of survival of the fittest has sometimes been used to rationalize or justify social injustice. For example, Nazis attempted to justify their militarism and abuse of others through the notion that they were a "superior race." The exploitation of many people through Western colonialism, invasion, and slavery was also sometimes rationalized with this pseudologic. In Europe, monarchies and aristocracies were said to rule "by the grace of God." Therefore, sometimes, those that are self-serving and aggressive have temporary advantages.

The Process of Natural Selection

Darwin's process of natural selection has four components.

1. Variation. Organisms (within populations) exhibit individual variation in appearance and behavior. These variations may involve body size, hair color, facial markings, voice properties, or number of offspring. On the other hand, some traits show little to no variation among individuals--for example, number of eyes in vertebrates.

2. Inheritance. Some traits are consistently passed on from parent to offspring. Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.

3. High rate of population growth. Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality.

4. Differential survival and reproduction. Individuals possessing traits well suited for the struggle for local resources