Artifical Selection Doesn't Necessarily Make a Species More Fit for Survival
Since humans formed cultures based upon agricultural and the raising of animals for food, man has gradually understood that he could selectively breed organisms in order to enhance certain traits that were beneficial for man. However, such traits may not necessarily be beneficial for a species fitness in nature. An example of this would be in the current breeding of bulldogs.They are being selected by man to have large heads, which requires them to be born by cesarean section. This would obviously not be a trait selected for in nature, as it would decrease species fitness. Artificial selection actually can reduce the natural variation of traits in a population.
Natural Selection Relies on Trait Variation and Selects Traits That Enhance Fitness
Natural selection doesn't decide on traits, rather it is a process of species having varying traits and the traits that get passed on are the ones that increase an individual's ability to survive and reproduce. If a giraffe with a slightly longer neck is able to reach food in high treetops when supplies are low, he or she will have a greater chance to survive and reproduce than one with a shorter neck.The shorter necked giraffes may die that season or not have the energy resources to produce offspring.Therefore, the trait of a longer neck may be passed on to the offspring and the gene pool of giraffe will gradually have more individuals with long necks. In order for natural selection to operate there must be a variation in traits in the population.
Artificial Selection Can Be Trendy and Even Dangerous
When man selects organisms to breed for specific traits, many times he selects related individuals to enhance that trait. Unfortunately, this inbreeding can cause the expression of genes that are dangerous. An example is the inbreeding that occurred during ancient times and more recently with European royals. In order to preserve royal lineages, many times relatives were allowed to marry and produce children. Many of these families had children who suffered from genetic disorders, such as hemophilia.
Natural Selection Can Be Affected By Small Populations
Inbreeding can occur in natural populations also and this is a serious problem in today's world. Populations of wild cheetahs are very diminished and located in small areas and there are low levels of genetic diversity. Natural selection will still select traits that enhance fitness, but due to this type of forced inbreeding, even natural populations are faced with decreased variation in traits. This concerns scientists and conservationists because the cheetahs could lack the diversity needed to survive disease outbreaks or rapid environmental changes. (Reference 5)