Both natural and artificial selection produce changes in living organisms over time. However, the selective force of natural selection is the environment, while the agent of change in artificial selection arises from human whim. Here's more on these two evolutionary processes.
* What Is Natural Selection? *
This is the process in nature by which only the organisms best adapted to their environment tend to survive and transmit their genetic characteristics to the next generation, while those less well adapted tend to be eliminated, where environment means the combined biological (ex. disease competition, predation) and physical influences (ex. drought, fire, flood).
* The Power of Natural Selection over Time *
The "goal" of all organisms is to live long enough to produce reproductive offspring. Individuals that can locate, harvest and utilize the resources from their environment the most efficiently, while minimizing the influence of limiting factors acting upon them, will be the most successful in continuing their genes to the next generation.
* Natural Selection Example: The Peppered Moth *
The peppered moth (Biston betularia) is a temperate nocturnal moth species that provides us with a great example of natural selection in action.
1. Pre-industrial Revolution: In pre-industrial England the predominant color type of the peppered moth was light gray. This light color closely matched the lichen-covered trees that were common in their environment. (Lichens are a slow growing life form that you can find on the bark of many trees and on decomposing wood).
In addition to the light gray moths, there was also a rare genetic color variant of very dark moths. Their population numbers remained low because when these dark moths landed on the light gray lichen-covered tress they were easy targets for predators.
2. Advent of Industry: Early coal-based industry was extremely dirty. Around large cities, everything became dusted with soot. This high level of pollution killed the light gray lichens on trees and revealed the much darker bark beneath.
3. A Change in Environment: When the light-colored peppered moths landed on the same trees they had always landed on, they were now extremely visible against the newly exposed dark bark, and easy targets for predators.
4. Dark Moths Now Have Upper Hand: However, the dark colored moth variant was now harder for predators to spot and as a result, these darker moths more often lived 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.
5. More Recent, 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.
* Artificial Selection Example: Dog Breeding *
Artificial selection is when we, humans, act as the "environmental pressure" that shapes populations. An example is dog breeding, when we choose dogs with certain traits and breed them together to accentuate the traits we desire.
All modern domestic dogs, no matter how different they are in appearance, from Chihuahua to Great Dane, all belong to the same species, Canis domesticus.
The originator of today's domestic dog was an ancestral, wolf-like canid. By incrementally selecting for certain traits, we have created a variety of dogs that differ widely in appearance and temperament.
Brown, Bryson (2007) Evolution: A Historical Perspective. Greenwood Press.
Campbell & Reece (2002). Biology, Sixth Edition, Benjamin Cummings.