Sunday, April 4, 2010
Gradualism and Population Ecology
At the very beginning of the book, Coyne mentions gradualism as one of the six components of evolutionary theory. In a previous blog post, students have mentioned environmental pressures as reasons for the speeding up and slowing down of the evolutionary process. Take this idea further and relate gradualism to carrying capacity and the logistic growth model. Try to include an example of a new species in an environment and ecological factors that will make its evolution speed up or slow down.
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When a new species first is introduced into an suitable environment, its population will start growing very fast. When each organism of the species does not need to compete with another for food and water and when space is plentiful, the species' population will grow exponentially. When the population grows to a certain size, organisms of the species will have to compete for food and space will be less plentiful. At this point the growth will start slowing down into a shape called logistical growth. Logistical growth levels out at the carrying capacity. In reality, the carrying capacity is just a level which the population will fluctuate around depending on the availability of food at the moment or the population of predators.
ReplyDeleteOver time, some members of the species may develop abnormalities that give them an advantage over the other members of the species such as a more efficient method of collecting food. These abnormal members will eventually out reproduce the normal members because they survive better than the normal members. This is evolution. But the development of new forms of a species takes a lot of time. According to Jerry Coyne, graduation "requires many thousands of years" (4) for a species with a long generation period to produce a noticeable change.
Now, let's suppose an introduced predatory was inserted into our new species' habitat. This is an example of an increased environmental pressure. The new predator will kill off much of our species except for those who were "abnormal" and had different coloring that allowed them to blend into the surroundings, for example. This increased environmental pressure would increase the evolutionary process for our new species because the pressure only allows those who are "abnormal" to survive versus the regular evolution which just give certain members an advantage to eventually beat out normal members of the species.
Frank's description and relation of gradualism to carrying capacity and the logistics growth model is very thorough. To add an example to his post, introduced in Ch. 5, The Engine of Evolution is the example of the "oldfield" mice experiment (p. 116-117). Ecological factors such as environment, type and number of predators, and availability of food caused a change in the mice. The oldfield mice, previously inland, migrated down to the beaches, probably when fields further inland were no longer able to support that many mice, due to food or space restrictions. Predators in the area then picked off the darker mice in the population, due to their coats showing up better on the lighter surface of the sand. Over time, the mice with darker coats were eaten or migrated back to the fields, while the lighter mice were able to survive better on the beaches, since they were able to blend into their surroundings. These lighter mice then were able to survive and reproduce, producing more light-coated mice. If a new predator that relied on sight had been introduced into the habitat, the mice would have become lighter in colour faster.
ReplyDeleteCoyne says that "The evolution of new features....does not occur in just one or a few generations, but usually over hundreds or thousands - even millions of generations" (4). This is basically gradualism. Jolyn and Frank both highlighted the idea of gradualism quite well. Frank mentioned, "the carrying capacity is just a level which the population will fluctuate around depending on the availability of food at the moment or the population of predators." In my earlier post, I mentioned the theory of dodo birds and how they became extinct in just a few years. Until the time of the foreign migrators, Dodo birds used to reproduce logistically. After the entrance of foreigners, they began to decline at a rapid rate. These foreigners were predators of the birds. They include pigs, dogs, rats and of course, humans. These organisms used the bird as a source of food making it difficult for the bird to survive and thus, eventually resulted in extinction. These birds were declining so rapidly that it made it hard for them to undergo a mutation that would enable these birds to survive.
ReplyDeleteSource: http://www.answersingenesis.org/creation/v17/i4/dodo.asp
According to the logistics growth model, a species when first introduced to an environment will thrive, reproduce, then multiply very rapidly, almost exponentially. This is a result of a plentiful food and shelter in the environment. Frank Wu made excellent connections to this in his response. He also discussed the idea of a carrying capacity. When the environment simply does not have enough food or shelter for a species, the species cannot keep surviving to reproduce because of the lack of food. Only those who are the stronger, maybe smarter, better of the species will be able to find what it needs to reproduce. The species’ population will stay essentially the same (fluctuating in a minor sense) around the carrying capacity, but the ones in the species who are better suited for the environment will keep surviving over the lesser adapted beings. After time, the stronger of the species may become a completely different species and as a result, evolution via natural selection has occurred.
ReplyDeleteJessica and Athena made excellent connections to mice and the dodo birds’ experience with this process of logistics growth and carrying capacity.
I would like to make a connection to one of the experiments we did as a class. Bacteria also experience this process of the S shaped logistics growth model. The thing with bacteria that scientists love is the fact that bacteria have such short life spans, evolution amongst bacteria occurs much faster than a typical species. I’m referencing Lab # 9 where we forced bacteria into brand new environments, thereby making them either accept the new gene (pGlo) to help them survive or die. In this lab, we introduced the bacteria into a hostile environment with ampicillin. Basically, we introduced the bacteria into an environment with ampicillin, which basically meant the carrying capacity for that specific bacteria was zero in a sense of survivable living space. But, if the bacteria accepted the new gene we introduced or “evolved” in a sense, then that bacteria survived the new environment, where the carrying capacity is almost infinite for a short period of time.
The easiest theme this prompt applies to is the evolution theme. When the environment is changed, the organisms will undergo the logistics growth model. When the species’ reaches carrying capacity, only the best of that species will find what it needs to reproduce. Eventually, all of the favorable adaptations will create a new species, and evolution has occurred via natural selection.