Definition of Natural Selection
The simplest definition of natural selection is that it is the mechanism by which an organism that is best suited to its environment will survive and pass on its beneficial traits in increasing numbers to the following generations, whilst those organisms less suited to the environment will be eliminated.
For example, some cheetahs can run faster than other cheetahs and will therefore be more likely to catch their prey. Organisms that blend into their background and remain hidden from their predators will have a better chance of surviving than those members of their species that cannot. Basically if an organism develops a trait that helps it to survive, then it will pass on that trait to its offspring, eventually resulting in the widespread appearance of that trait in a population.
Now members of a species differ in a number of ways and the root cause of this variation is DNA; changes in genes and/or their regulatory elements can generate new beneficial traits. It explains design in nature. It is blind and unconscious and happens because of random genetic mutations throwing up new phenotypes. Although most mutations are harmful, every now and again there are genetic faults that confer some advantage to an organism. We can see examples of this everywhere.
Some Examples of Natural Selection
Antibiotic resistant bacteria - this is the ability of bacteria to survive an attack by an antibiotic and is an excellent example of natural selection at work. Bacteria that develop a mutation that allows them to survive an antibiotic will live long enough to reproduce and so spread the ‘survival’ gene to subsequent generations. Those bacteria with this gene will survive and become more numerous in the bacterial population.
Fast-evolving deer mouse - found in sandy soils in Nebraska. Over a period of several thousand years the deer mouse evolved a pale coat that helped it to evade predators. Deer mice are widespread across North America, but they usually have dark coat, so that they can blend into dark soils and stay hidden from owls. Scientists at Harvard and at the University Of California at Berkeley discovered a single gene called Agouti in light coloured mice which is expressed in higher amounts and for longer than the genes that code for dark hair. The gene emerged about 4,000 years ago, which was only a few thousand years after the dark coated mice colonised the new sandy home. Its spread was rapid. Most animals that quickly evolve new traits do so from variations of existing genes, but in deer mice the trait evolved from a completely new gene. Agouti did not occur before the colonisation of the sandy environment, and when it did appear selection acted on it to confer an advantage, making it more widespread.
Peppered moths - the deer mice are similar to the classic example of natural selection - the peppered moths of northern England. Originally they were light coloured and so blended in with the light bark of trees. However, due to the Industrial Revolution soot and particulate matter covered trees making them darker, and so the light coloured moths stood out like sore thumbs. They became easy pickings for predators. Darker coloured moths were able to avoid predation and so passed on their genes to subsequent generations because they were able to hang around long enough to breed. Dark coloured moths therefore survived. The whole process was completely reversed when cleaner air returned, allowing the trees to revert back to their pale colour; the lighter moths thrived once more.