English Naturalist Charles Darwin used his keen observation skills and logic to develop a comprehensive theory that describes the process of evolution. While some controversy surrounds evolution as it applies to human populations, Darwin's theory applies to all organic species. The basic principles of evolution are simple and seem obvious to the modern reader. However, prior to Darwin, no scientist had put all the pieces together.
In every species there is variation. This variability is apparent even within related organisms. Even siblings will vary in color, height, weight, number of offspring and other characteristics. There are other characteristics that do not vary as often, such as number of limbs or eyes. The observer must be careful when making generalizations about a population. Some populations show more variation than others, particularly in areas that are geographically isolated, such as Australia, the Galapagos, Madagascar and so forth. Organisms that live in these areas may be related to those in other parts of the world. However, due to very specific conditions in their surroundings, these species evolve very distinct characteristics.
Each species has traits that are strongly influenced by inheritance. Other characteristics are affected more strongly by environmental factors and are not considered inherited. Inherited traits are passed directly from parent to offspring in a consistent manner. Geneticists work to separate traits according to their heritability. Heritable traits that are related to a particular disease may be manipulated through gene therapy. Those traits related to environmental factors often respond best to behavioral changes, such as dietary restrictions, increasing exercise and quitting smoking. Isolating traits based on their heritability has enormous implications for health care in humans.
Most species produce more offspring each year than the environment can support. This high rate of growth results in competition among the local species for the limited natural resources available. The consequences of the struggle for resources is an increasing mortality rate within a species.
Some individuals will survive the struggle for resources. These individuals will reproduce adding their genes to the succeeding generations. The traits that helped these organisms to survive will be passed on to their offspring. This process is known as “natural selection.” The conditions in the environment result in the survival of individuals with specific traits which are passed through heredity to the next generation. Today we refer to this process as “survival of the fittest.” Darwin used this phrase, but he credited a fellow biologist, Herbert Spencer as its source.