How does Natural Selection work?

Natural selection requires four conditions in order to operate:

1. Reproduction: Organisms must reproduce themselves, creating a new generation of individuals.
2. Heredity: The characteristics of the parents must tend to be present in the offspring.
3. Variation: Different individuals in the population must be different in at least some characteristics from each other.
4. Differential fitness: Some individuals must be more fit (that is, more likely to survive long enough to reproduce) than others, based on their varying characteristics.

By definition, living things must reproduce themselves. Thus condition #1 is fulfilled.

As Mendel first proved in the 19th century, parental characteristics are passed to the offspring. While the lack of a known mechanism for heredity was a primary weakness of Darwin's theory when it was first proposed, today the mechanisms of gene transfer and expression are well understood. Thus condition #2 is fulfilled.

Darwin recognized that different individuals of the same species vary from each other in their characteristics. Today we can find variation not just at the morphologic (i.e., regarding physical shape) level, but at the chromosomal, biochemical, and DNA levels. The sources of this variaton are understood; they include chromosomal recombination, migration (influx of new material from different populations), and ultimately mutation. Thus condition #3 is fulfilled.

Empirical observations show that the fitness of individuals is differential: in any population, some individuals are more likely to survive to reproduce than others, based on their inherited characteristics. This condition is intuitive, given condition #3. Thus condition #4 is fulfilled.

When all four conditions are fulfilled, then natural selection is a logical deduction. An individual is most likely to survive to reproduce if its physical characteristics are such that it has high fitness. Those individuals who are less fit are less likely to reproduce. The offspring of that generation all have characteristics of their parents, including those which determine fitness. There are more offspring of the more fit individuals than of the less fit individuals, and since the characteristics of the parents are transfered to the offspring, the offspring on average are more fit than the parents on average (the less fit have been weeded out). Left to run indefinitely, the system will tend toward maximum fitness of all individuals.

The theory of natural selection itself is very simple. Its implications, however, are extraordinarily complex.