DNA needs to be copied when cells divide, so that each new cell has a complete set of genetic instructions. The cellular process of copying DNA is full of mechanisms that check and double check the construction of a new DNA molecule. But when changes, or mistakes do occur, it is usually harmful or at best has no effect on the organism.
Very, very rarely a mutation results in a change that is advantageous to an organism; providing some characteristic that confers an edge to the organism, some type of survival advantage that ultimately results in the mutated organism leaving more offspring (copies of its genes) that do others in the population.
Mutations & Natural Selection
Natural selection is the mechanism of evolution, the process in nature by which only the organisms that are best adapted to their environment tend to survive and transmit their genetic characteristics to the next generation. Individuals less well adapted to their environment tend to be eliminated, where environment represents the combined biological and physical influences.
The meaningful difference between the organisms of a population is genetic; differences in the genome of one member of a population compared to the others. These differences arise through mutation.
The Power Is in Natural Selection over Time: Reproductive Isolation & Extinction
Over time, natural selection can even result in sub-populations within a species being so genetically different that they are no longer able to reproduce with each other. They become separate species (reproductive isolation). But natural selection does not always result in new forms or species. Natural selection may also result in the elimination of species from the environment (extinction).
Natural Selection Example: The Peppered Moth
The peppered moth (Biston betularia) is a temperate nocturnal moth species that provides a great example of natural selection.
The Peppered Moth Pre-industrial Revolution
In England, the common color of peppered moths was light gray (Biston betularia f. typica), prior to the industrial revolution. The moth’s light gray color closely matched the lichen-covered trees in their environment.
There was also a genetic color variant in the moth population that resulted in some, relatively rare, very dark colored moths (Biston betularia f. carbonaria). Their numbers remained low because when these dark moths landed on the light grey lichen-covered tress, they were easy targets for predators.
The Peppered Moth after the Advent of Industry
Early coal-based industry was extremely dirty, and around large cities, everything was essentially covered in soot. This high level of pollution killed the light gray lichens on trees and the bark became much darker in appearance.
When the light-colored peppered moths landed on the same trees they had always landed on, they were extremely visible against the dark bark, and easy targets for predators. The dark colored moth variant now had the advantage, being harder for predators to spot and more often living long enough to reproduce.
Over generations, the polluted environment continued to favor darker moths, and they progressively became more common. By the late 19th century, 98% of the moths near cities were black.
Biston betularia in an Era of Modern, Cleaner Industry
Modern air pollution controls have cleaned up the environment compared to the early days of the industrial revolution. The cleaner environment has allowed the lichens to grown back, and the trees have returned to a lighter in color. Now, natural selection favors lighter moth varieties so they have become the most common and the dark-colored variant is again rare.
More Information
For more information on evolution and general biology see the science website Science Prof Online.
Sources
Brown, Bryson (2007) Evolution: A Historical Perspective. Greenwood Press.
Campbell & Reece (2005) Biology, 7th Edition. Pearson
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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