Examples on Natural Slection

Natural selection is the process by which favorable traits that are heritable become more common in successive generations of a population of reproducing organisms, and unfavorable traits that are heritable become less common.

English peppered moths-
Before the industrial revolution, dark peppered moths were quite rare. In the midst of the revolution, while the air was very sooty, dark moths outnumbered light moths. Once the air became more clear, light colored moths made a comeback. Kettlewell, a scientist who studied these moths in the early 1950's, proposed the following explanation.
Before the revolution the trees where the moths lived were covered in a light lichen the light moths thrived on these trees because they were less visible to predators. The dark moths stood out and were eaten before they were able to reproduce. During the industrial era in England, the soot in the air killed the light colored lichen on the trees and the dark moths were better camouflaged

Bacteria that eat waste nylon-
The ability of a bacterium to consume nylon must be a mutation, as nylon did not even exist until the 1940’s. These bacteria metabolize short nylon oligomers with enzymes in their system. These enzymes have come from a frameshift mutation of a gene which codes for an unrelated enzyme. This has been repeated experimentally to test the validity of the theory. In the experiments, non-nylon-metabolizing strains of the bacterium Pseudomonas were grown in media with only nylon oligomers available for food. Within a few generations, the bacteria were producing the enzyme needed to metabolize the oligomers.

Natural Selection Requires...
For natural selection to occur, two requirements are essential:
There must be heritable variation for some trait. Examples: beak size, color pattern, thickness of skin, fleetness.
There must be differential survival and reproduction associated with the possession of that trait.
Unless both these requirements are met, adaptation by natural selection cannot occur.
Some examples:

If some plants grow taller than others and so are better able to avoid shading by others, they will produce more offspring. However, if the reason they grow tall is because of the soil in which their seeds happened to land, and not because they have the genes to grow tall, than no evolution will occur.
If some individuals are fleeter than others because of differences in their genes, but the predator is so much faster that it does not matter, then no evolution will occur (e.g. if cheetahs ate snails).
In addition, natural selection can only choose among existing varieties in a population. It might be very useful for polar bears to have white noses, and then they wouldn't have to cover their noses with their paws when they stalk their prey. The panda could have a much nicer thumb than the clumsy device that it does have.
When we incorporate genetics into our story, it becomes more obvious why the generation of new variations is a chance process. Variants do not arise because they are needed. They arise by random processes governed by the laws of genetics. For today, the central point is the chance occurrence of variation, some of which is adaptive, and the weeding out by natural selection of the best adapted varieties.

Evidence of Natural Selection
Let's look at an example to help make natural selection clear.
Industrial melanism is a phenomenon that affected over 70 species of moths in England. It has been best studied in the peppered moth, Biston betularia. Prior to 1800, the typical moth of the species had a light pattern (see Figure 3). Dark colored or melanic moths were rare and were therefore collectors' items.
Figure 3. Image of Peppered Moth

During the Industrial Revolution, soot and other industrial wastes darkened tree trunks and killed off lichens. The light-colored morph of the moth became rare and the dark morph became abundant. In 1819, the first melanic morph was seen; by 1886, it was far more common -- illustrating rapid evolutionary change.

Eventually light morphs were common in only a few locales, far from industrial areas. The cause of this change was thought to be selective predation by birds, which favored camouflage coloration in the moth.

In the 1950's, the biologist Kettlewell did release-recapture experiments using both morphs. A brief summary of his results are shown below. By observing bird predation from blinds, he could confirm that conspicuousness of moth greatly influenced the chance it would be eaten.