For every organism in every ecosystem, there are selective pressures that influence what traits are passed on and what traits die out based on selective advantages/disadvantages. Sometimes this selective pressure is simply due to competition for resources; one trait gives an individual an advantage in collecting food or water, for example. More often, the selective pressure is applied by "agents of natural selection," organisms that act to literally determine which trait is the fittest for survival; these agents are usually predators that consume less well adapted prey.
Cite an instance of selective pressure; state the nature of the pressures; discuss the effects of this pressure in the context of the fifth theme of biology, interaction with the environment, or the sixth theme of biology, regulation.
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Natural selection works very differently than the human mind. Natural selection doesn’t always preserve the genes that appear useful to humans, such as color vision or a great olfactory system. Instead, selection pressure only applies to those genes that are vital to the survival of the organism. Once a gene is no longer necessary, “then natural selection, which would normally preserve the text, is relaxed” and “mutations can accumulate in them” (123). For example, blindness and albinism is normally selected against in the surface cousins of the fish Astyanax mexisicanus. However in cave population of the fish, the selection pressure on these two traits is relaxed and consequently, these cave dwellers are blind as well as albino. Populations of Astyanax mexisicanus that habit in the Malino and Pachon cave live in a pitch dark environment. In such a niche, a fish that is albino or blind wouldn't be at a selective disadvantage as if they had dwelled on the surface. As Sean Carroll puts it “who cares what you look like” in the dark (149). Abiotic factors such as light probably limit the distribution of the cousins of Astyanax mexisicanus to these caves at first. But when the fishes finally populated the caves, they evolved traits that allowed them to adapt to living in the dark without the use of eyes. As a result, cave dwellers no longer need some of traits that were essential when they lived on the surface. Selection pressure is relaxed on these traits which “have become dispensable” (123).
ReplyDeleteA good example of selective pressure is the instance of the peppered moth during the Industrial Revolution. Before the Industrial Revolution, the light-colored moth was more prevalent. However, afterward, the dark-colored moth was more prevalent. Due to the Industrial Revolution, pollution darkened the lichen growth on trees. This development made it easier for dark-colored moths to camouflage in the darker-colored trees than the light-colored moths, giving them a survival advantage. The "agents" of selective pressure were birds that preyed on the moths. Before the pollution, lighter-colored moths were able to camouflage better than darker-colored moths in the lighter trees. Therefore, the birds consumed more of the dark-colored moths, who were easier to find. However, after the pollution darkened the trees, it was easier for the dark-colored moths to camouflage than the light-colored moths, so the birds consumed more of the lighter moths. THis led to a decrease in the population of lighter moths and an increase in the population of darker moths. Having a darker color became a selective advantage due to the pollution from the Industrial Revolution, and the birds acted as agents of natural selection by preying on those moths with a selective disadvantage.
ReplyDeleteI also think that this example is very good because it shows that selective advantages are only good for certain types of environments. We tend to think of a selective advantage as being necessarily better and 'always' better in all situations. Yet this is not true. Just like a deer freezing in the headlights of an oncoming car, what is a selective advantage in one environment might be a disadvantage elsewhere. Before the Industrial Revolution, having a lighter color was a selective advantage, because it was easier to camouflage, and having a darker color was a disadvantage. However, following the Industrial Revolution, having a lighter color made a moth stand out more, so it became a disadvantage, while having a darker color made it easier to camouflage and was an advantage. Thus,in a span of fifty years, from 1848 to 1896 (Carroll 52), the overall population of moths changed from predominantly light-colored to predominantly dark-colored, as much as 98 percent dark-colored in some areas (Carroll 52). The moth example relates to the theme of interaction with the environment because the ability to camouflage is an advantage only because it enhances the chances for survival from the predation of the birds.
One of the most important selective pressures is the immediate environment that the organism lives in. In the case of the threespine stickleback fish, some fish underwent a habitat change from the oceans to lakes and streams at the end of the last ice age. The original oceanic form of the sticklebacks had more than “30 armor plates running from head to tail” (56). However, the fish in the lakes and streams experienced a reduction in armor plates. A regular sampling of the Loberg Lake in Alaska revealed the real time adaptation of the stickleback fish. In a course of twelve years, the oceanic form of the stickleback fish dropped from 100% to 11%, and the “form with low plate number rose 75%” (57). The stickleback fish in the lakes usually exhibited 0-9 plates. The reduction of armor plates could be attributed to the facility of movement and greater flexibility when swimming. The oceanic form of the stickleback would be more selected for in an ocean environment. In the ecology unit, we have learned that animals have developed adaptations that protect them or allow them to fight their predators. For example, porcupines have quills that pierce the body of their predators if the predators try to eat them. In the ocean, the stickleback would encounter more predators, and the armor plates would provide the stickleback with greater protection. The selection for more armored plates would relate to the theme of interaction of the environment, as the theme deals with the exchange of materials and energy with their surroundings. In the case of the stickleback fish, the predators that target the fish would be part of their environment, and would affect their development. However, in the lake and stream environment, the armor plates would be selected against because the armor plates inhibit movement. Also, the lake could contain less predators, which would select for a reduced number of armored plates. Because of the differences of the environments the stickleback fish have lived in, different adaptations were selected for when the fish moved from the ocean environment to a lake or stream environment.
ReplyDeleteGreat dialogue here, kids!!
ReplyDeleteRemember the 'armadillo story' i told you on the second day of class? The reason for the story centers around the concept of each environment presents a unique set of selective pressures on a population of organisms. Armadillo jump up and curl into a ball successfully startles an fox or coyote so the armadillo can survive. However, the same behavior possess no selective advantage in the 'interstate highway' environment! Eighteen-wheelers don't care what the armadillo does; the trucks crash into the animal, especially if it jumps up really high!