(15) Provide an example of an organism in a natural population showing an adaptation for a particular environment, and discuss how that adaptation may shower higher or lower fitness in a new environment. Why is it that only populations, and not individuals, evolve?

A.Cactus adapt to the desert by having smaller leaves, grow compactly and close to the ground, and a non-porous covering on their leaves such as wax. Hair on the leaves on the plant helps to reduce the evaporation of moisture from the surface of leaves by reflecting sunlight and inhibiting air movement. If a cactus is moved to a wet environment it would have lower fitness because it would probability die or not grow well because it’s not adapted for wet environments.
Individual organisms don't evolve. Populations evolve. Because individuals in a population vary, some in the population are better able to survive and reproduce given a particular set of environmental conditions. These individuals generally survive and produce more offspring, thus passing their advantageous traits on to the next generation. So over time, the population changes.

B.Walrus have evolved to exist in the coldest area of the world. It has a thick blubber undercoat to keep it warm on the ice and to keep it fed when food is scarce. It is a really large mammal. It has a torpedo shaped body to help move through water. If the walrus were to move to warmer waters it would have a lower fitness because of competition. The walrus’s harem is too large to exist in areas that are populated by a lot of creatures. Also the blubber would be too warm and internal organs would suffer. Populations evolve because the next generation is where gene changing appears. Individuals in a population vary their offspring will also vary.

C.http://en.wikipedia.org/wiki/Natural_selection

Natural selection is the process by which those heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations.
A well-known example of natural selection in action is the development of antibiotic resistance in microorganisms. Since the discovery of penicillin in 1928 by Alexander Fleming, antibiotics have been used to fight bacterial diseases. Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, primarily as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them slightly less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. This selective elimination of maladapted individuals from a population is natural selection.
These surviving bacteria will then reproduce again, producing the next generation. Due to the elimination of the maladapted individuals in the past generation, this population contains more bacteria that have some resistance against the antibiotic. At the same time, new mutations occur, contributing new genetic variation to the existing genetic variation. Spontaneous mutations are very rare, and advantageous mutations are even rarer. However, populations of bacteria are large enough that a few individuals will have beneficial mutations. If a new mutation reduces their susceptibility to an antibiotic, these individuals are more likely to survive when next confronted with that antibiotic.
Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge. This new changed population of antibiotic-resistant bacteria is optimally adapted to the context it evolved in. At the same time, it is not necessarily optimally adapted any more to the old antibiotic free environment. The end result of natural selection is two populations that are both optimally adapted to their specific environment, while both perform substandard in the other environment.
The widespread use and misuse of antibiotics has resulted in increased microbial resistance to antibiotics in clinical use, to the point that the methicillin-resistant Staphylococcus aureus (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs.[11] Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs.[12]
This is an example of what is known as an evolutionary arms race, in which bacteria continue to develop strains that are less susceptible to antibiotics, while medical researchers continue to develop new antibiotics that can kill them. A similar situation occurs with pesticide resistance in plants and insects. Arms races are not necessarily induced by man; a well-documented example involves the elaboration of the RNA interference pathway in plants as means of innate immunity against viruses.[13]

D.A freshwater water fish is adapted to living in freshwater and peeing out water constantly to maintain a salt concentration. If the freshwater fish was placed in saltwater, the freshwater fish would have a lower fitness in this new environment because it is not physically capable of handling all that salt content. “Only populations evolve because evolution requires a competing force as well as variation in order to evolve. A species is rarely represented by a single continuous interbreeding population. Instead, the population of a species is typically composed of a group of subpopulations-local populations of interbreeding individuals, linked to each other in varying degrees by the movement of individuals.”