Biology
Collin Carlton
October 14, 2014
11a

Evolution in Biology is described as “the theory that groups of organisms change with the passage of time, mainly as a result of natural selection, so that descendants differ morphologically and physiologically from their ancestors”1 Biological evolution is a powerful and important process. It is a process which, over time, slowly selects the organisms that are better adapted to their environments, and takes advantage of random mutations to continuously change life and make all living organisms in existence be the way they are. Darwin's Theory of Evolution by Natural Selection is divided into the following three separate modes of selection:
Directional Selection
Stabilizing Selection
Disruptive Selection
As well as two other modes of selection that affect natural selection:
Sexual Selection
Artificial Selection
These modes of selection are what causes a population to first begin to evolve.2
In directional selection, natural selection will occur when a problem in the environment begins to effect a population. When this problem arises the population has no other option but to adapt; survival of the fittest usually being the main factor in direct selection. When there becomes a change in either the weather, the food supply, or the size of the population, there becomes a competition for survival and the individuals with the phenotypes or genotypes that best fit the solution to the problem will survive. The more individuals with these genotypes and phenotypes that survive, the more there are that mate together over generations causing the selected traits to become more prevalent in the population through heredity. Over time the traits begin to become isolated, causing only the individuals with the same genotypes and phenotypes to mate, leaving the other individuals in the population to die out. This is how a new species comes about through directional selection. An example of directional selection is the beaks of the Galapagos finches changing over time due to different food sources. 3
Stabilizing selection occurs when the average individuals of the extremes of the population is favored. This stabilizes the population by the mating of the individuals with the phenotypes that are best adapted to the environment, leaving the individuals with the extreme phenotypes to die out over time. Stabilizing selection is kept from becoming too homogeneous by them having statistically a greater rate of mutations in their DNA. An example of stabilizing selection is the birth weight and size of humans. If the baby is born premature and small it is more susceptible to disease, but if it is born to late and big then there are problems with birth. 4
Disruptive selection is basically the opposite of stabilizing selection. It favors the extremes of the population and favors against the average individuals. Disruptive selection is the rarest mode of selection. It can lead to the creation of two or more different species if there is an extreme change in the environment, usually caused either by parasite or by human interference. An example of disruptive selection is the Peppered Moths during the time of the Industrial Revolution in England. Before the revolution only 0.01% of peppered moths were solid black due to their lack of camouflage, but since the trees were covered in soot the dark moths blended in better and the dark moths rose up to 98% around the industrial parts of Europe, but remained light colored in rural parts. 5
Sexual selection is either caused by certain characteristics being favored by the mate or competition between the same sexes for mates. Sexual selection affects natural selection by either strengthening or weakening certain traits based on the preference of the mates. This usually leads to the strengthening of the better traits and the weakening of the inferior traits due to the individuals with the best traits mating together. An example of this is peacocks having the biggest most