DEPARTMENT OF BIOLOGY
The Basics of Evolution
The Fundamental Process
Biological evolution occurs when individual organisms with particular characteristics replace other individuals with other characteristics. It is not that the characteristics of any single individual change, or evolve, during its lifetime. Rather, each individual lives its life in its own way, producing offspring if and when it can.
The key to evolution is the degree of success with which each individual produces offspring. Any genetically-heritable characteristics can be passed on to offspring. Consequently, those individuals that have the most offspring contribute the most to future generations. Those individuals that have the least offspring contribute the least to future generations. Over the course of many generations, some genetically-heritable characteristics are lost from the population overall, while others become common.
Evolution can occur only if there are different genetically-heritable characteristics in a population of organisms. There must be genetic diversity. In nearly all populations of nearly all species, there is genetic diversity. Partly, this is because the genetic information is re-assorted at each generation. It is also because mutations occasionally occur, causing changes in genetic information. Once a mutation occurs, and is passed on to the offspring of the first individual to carry that mutation, then the mutation becomes a part of the genetic diversity of the population.
Evolution can occur rapidly or very slowly. It occurs slowly, if at all, when the particular characteristics of a population make it well-suited to the environment in which it lives. Under such conditions, most mutational changes make individuals less suited to the environment. As a result, the more common characteristics remain the norm.
Rapid evolution can occur during times of environmental change. A population's environment can change if the individuals in that population migrate to a new location. Or, environmental conditions can change for numerous reasons, including long-term climate change, the introduction of a new species, or the loss of a previously-common species. In a new environment, genetic variations that were previously uncommon may be advantageous. Individuals with these variations may now out-compete their fellows, and their genetic variations may become the new norm for that population.
The fundamental principles of evolution are these:
1. Evolution depends upon genetic variation.
2. Evolution occurs as some genetic variations become common in populations, and others become rare.
3. The source of genetic variation is mutation. Genetic variation can be augmented by genetic reassortment during the production of offspring.
4. Individual organisms do not change or evolve. Evolution is the replacement, over the course of numerous generations, of some genetic variations by other genetic variations.
5. It is not possible to mutate in anticipation of environmental change, or to direct mutations to specific characteristics. Mutation is not a conscious process.
Below, we explore some of the details.
last updated: Dec. 21 2008