How has human behavior and actions caused certain traits to be selected for in a given population of organisms? Are these traits necesarily a selective advantage in a natural, non-disturbed environment? (Use the Bighorn ram and the Atlantic cod as examples)
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There are three levels of biodiversity: genetic diversity, species diversity, and ecosystem diversity. The loss of species and genetic diversity is caused by "unnatural selection" - selection by humans in this case.
ReplyDeleteIn the Bighorn Ram example that Carroll presents, there has been a selective hunting of trophy rams. Hunters have been trying to capture rams with the largest horns. However, horns are also a sign of mating success and female rams desire males with the largest horns, "Mating success increases with dominance rank and horn length, and increases from about age six onward. But most rams are shot before reaching age eight, and some are taken as young as age four," (Carroll 254). The results of this "unnatural selection" causes a decline in body size and horn length of future generations. With selection for smaller-bodied and shorter-horned rams by removing larger-bodied and longer-horned rams, the "form and genetic makeup of the sheep population are evolving away from their naturally selected optima," (Carroll 255). These traits, as stated, are not a selective advantage in a natural, non-disturbed environment because they cause a disturbance in tropic food webs. With smaller-bodied and shorter-horned rams, there will be a decrease in food intake with a decreased metabolism (based on body size). With a decrease in food consumption, the prey of rams will increase and change the ecosystem by being able to have less-checked growth. This may cause an ecological disturbance and would cause a reordering of trophic food webs/levels.
In the example of Atlantic Cod, as presented by Carroll, humans' over-exploitation of the cod has caused the decrease in the cod population. "Unnatural selection" has also taken place in limiting the amount of largest/oldest fish who matured at a larger size and greater age. This fostered the growth of the smaller-maturing codfish, "with extensive fishing, as the population size declined, fish matured earlier and at smaller sizes," (Carroll 257). The traits were not selective advantages in a natural, non-disturbed environment because the codfish went from being predators to prey. These fish were not as threatening and could not check the growth of the bottom-dwelling crustaceans and pelagic fish. With the removal of a dominant species, the top-down model approach has taken place where a cascade of effects has occurred. There has been an increase in the bottom-dwelling organisms, most likely a decrease in herbivores, an increase in phytoplankton, and a decrease in mineral nutrients (Campbell 1185). This has caused the cod to not only decline but in some areas vanish as their small-bodies are treated as food for once their own prey.
Both Atlantic Cod and the Bighorn Ram have been drawn into a vicious population vortex. Ecologists will need to find new mechanisms to ensure their survival - as to make sure other food webs are not affected, like ours. Both the cod and the ram have been affected by the biological themes of regulation, evolution, and interaction with the environment.
As Danyal has already summed up the Atlantic Cod and Bighorn Ram, I will discuss other examples. When buying a new horse, the owner wants several things: good breed, good parents, longevity, etc. Because most owners want the same things from their horses, many breeders breed the same types of horses over and over again, increasing the probability of having a “good” horse. When this happens, the biodiversity within the horse species decreases because all of the “bad” genes have more or less been eliminated from the gene pool. Selection of animals in this nature occurs not only for horses, but also for dogs and anything else that can be used in competition.
ReplyDeleteSuch limitations in the gene pool cannot be considered entirely negative or entirely positive. Obviously, the relatively immediate result of such genetic selection is going to be positive because the horses become stronger and faster (or whatever the owners want). However, the long-term results might be positive or negative. Having a large amount of biodiversity helps ensure that horses will survive under a number of unexpected situations. Under our current selection process, the horses might not be able to survive if the environment suddenly changes due to the lack of biodiversity. Removing the “bad” genes from the gene pool ensures that no mutations of the “bad” genes will occur. Some of these now-impossible mutations could be positive, but now we will never know. In the long run, it is safer to have somewhat mediocre horses now with a large biodiversity than having near-perfect horses with limited biodiversity.
Another example of how human behavior can “select” a certain trait for a population is with the use of DDT. DDT is a pesticide that has many uses but is primarily used to kill off the many various insects that thrive on farmer’s crops. When DDT was first used on the crops, a relatively small amount of the pesticide would kill almost 100% of the insects. However, “subsequent spraying are less and less effective” (Campbell 437). The reason behind the fact that more spraying s are less effective lies in the humans role innatural selection. The insects that survive the DDT sprayings are insects that have genes that resist DDT. Also, since all other insects that don’t posses that specific residence gene die, only resistant insects are left. Those insects then reproduce and soon the offspring have the favorable gene. Our involvement, as humans, has created a new strain of insects that are more resistant to pesticides and therefore require new techniques for removal.
ReplyDeleteBecause the environment in which these insects live in is disturbed, due to the spraying, the mutated gene for resistance is favorable, for the insects. If, however, no spraying were ever to take place, then all insects would thrive off the crops and there would be no “need” for a genetic mutation. This idea ties in with the evolutionary theme from our biology books. Due to the DDT spraying, subsequent insects will now have a higher chance of containing a resistant gene. The second, third, fourth and onward generations of insects will now have to be combated with either higher and higher dosages of pesticides or a new means of insect removal will need to be though of.