Introduction

The process of natural selection has played an important role in the production of the amazing amount of phenotypic diversity observed among organisms. Natural selection is an agent of microevolutionary change that tends to lead to an population's becoming “adapted” to its environment.

Natural Selection as a Process

Natural selection is a process whereby the environment molds a population's genetic and phenotypic characteristics so that its members become, or remain, suited to that environment.  Natural selection occurs if three conditions are satisfied:

1. There is variation in a trait among individuals in a population,
2. This variation in the trait is at least partially heritable (i.e., it has a genetic basis that is passed from parents to offspring), and
3. This variation in traits affects fitness, that is, survival, fecundity (the number of babies produced), and/or mating ability.

 If these conditions are met, then the frequency of the trait, or its variance, in the population will change between the parent and offspring generations.

Genetic Basis of Phenotypic Variation and Heritability

Both the phenotypic variation and heritability required for natural selection to occur have a genetic basis. Variation in traits among individuals in a population can occur because different individuals have different forms of the same genes or because they are found in different environments and thus experience different ecological pressures. To the extent that a trait is determined by genes, rather than the environment, the trait will be heritable. Offspring tend to resemble their parents because they receive genes from both parents.

Changes in Trait Frequency From One Generation to the Next

Genes get passed on from one generation to the next by reproduction. Genes that produce traits that allow organisms to be better at surviving and reproducing tha other individuals in the population should get passed on more often than genes that produce traits that make the organisms inferior to others at surviving or reproducing. Thus, over time we would expect genes that produce traits that make organisms better at surviving and reproducing to become more common in the population such that organisms become “better adapted” to their environments. Because conditions vary among environments, it is not surprising that the traits that maximize survival and reproduction differ among environments as well.

Limits to Natural Selection

The process of natural selection would be expected to cease if any of the three assumptions are not met. For example, we might expect that genes that code for phenotypes with higher survival or reproduction would get more common in the population until all individuals in the population have these genes (the gene is “fixed” in the population). If this occurred, natural selection should cease because of the lack of a genetic basis for phenotypic variation within a population. However, because environmental conditions are often quite variable in time and space, genetic variation is rarely if ever completely purged to this extent. The process of natural selection would also cease if phenotypic variation no were longer related to differences in survival or reproduction.This also is unlikely.

Even though natural selection should cause traits that increase survival and reproduction in an environment to increase in a population, we should not expect that all organisms will be “perfectly adapted” to their environment, for several reasons. First, it is possible that a genetic mutation that would allow organisms to produce a better adapted trait has not appeared in the population. Second, trade-offs and constraints among different traits may limit the ability of any single trait to become perfectly adapted, either because the same genes influences many other traits, because a single trait is involved in multiple functions, or because optimizing one trait leads to a decline in performance of another genetically linked trait.

Patterns of Natural Selection

Natural selection can lead to three patterns of change in phenotype over time.

1. Directional selection. If the survival and reproduction (fitness) of organisms with one extreme phenotype is higher than that of organisms with the population's mean phenotype, then the trait frequency will move in the direction of the favored extreme phenotype over time. The net result of directional selection is a directional change in the mean trait value over time.
2. Stabilizing selection. If the fitness of organisms with the mean phenotype is greater than the fitness of organisms with either of the extreme phenotypes then over time the mean phenotype should not change but phenotypic variation should be reduced.
3. Disruptive selection. If the fitness of organisms with extreme phenotypes is greater than the fitness of organisms with the mean phenotype, then over time a unimodal trait distribution should be converted into a bimodal trait distribution.

Brief History of the Idea

Although Charles Darwin is the name most associated with the idea of natural selection, Alfred Russel Wallace independently came up with essentially the same idea, and the two naturalists agreed to jointly present their ideas to the Linnean Society in 1858. Darwin applied the term Natural Selection to show its relationship with artificial selection that had been carried out by human animal and crop breeders. However, he thought that the term “survival of the fittest”, which was forwarded by Herbert Spencer was “more accurate and sometimes equally convenient.” Darwin recognized that selection had produced traits that did not aid in survival, but aided in attracting mates; he called the process that produced these traits "sexual selection". Today we consider sexual selection to be part of the process of natural selection.

Further Reading

• Darwin, C. 1979. The Illustrated Origin of Species (abridged and introduced by R.E. Leakey). Hill and Wang, New York. ISBN: 0809057352
• Endler, J.A. 1986. Natural Selection in the Wild. Princeton University Press, Princeton, NJ. ISBN: 0691083878
• Futuyma, D.J. 1998. Evolutionary Biology, 3rd Edition, Sinauer, Sunderland, MA. ISBN: 0878931899