Biology is all about maintaining a balance; being biological is all about maintaining homeostasis, ecology stabilizes at carrying capacities and climax communities. In our bodies, the endocrine, nervous, etc. systems all help maintain the balance called homeostasis; in the ecological communities, ecological succession and predator/prey relationships maintain the balance of species. Is evolution a struggle to maintain balance? Assess the validity of the following statement:
Evolution is also a balancing act, through the mechanism of natural selection. A large imbalance, such as a great environmental stress, induces "macroevolution," major changes, while small imbalances induces "microevolution," minor changes in species. Even at a relative equilibrium, evolution is still occurring in terms of "stabilizing selection" (133).
Agree or disagree with the statement, and explain and defend your view using evolutionary examples.
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ReplyDeleteEvolution is also a balancing act, through the mechanism of natural selection. A large imbalance, such as a great environmental stress, induces "macroevolution," major changes, while small imbalances induces "microevolution," minor changes in species. Even at a relative equilibrium, evolution is still occurring in terms of "stabilizing selection" (133).
ReplyDeleteI would have to agree with Coyne’s statement, but it isn’t correct for all cases. People could see that the statement is false when an example of bad design is brought up. The flatfish Coyne presents is an example of a fish that has to exert more energy to swim than it ought to because evolution has led it in the wrong direction (81). Coyne presents even more examples of bad design in that section that shows that evolution isn’t all about trying to maintain an animal’s health.
However, that is not to say that many of evolution’s other examples have not facilitated methods for homeostasis. Take a look at Halobacterium. The salty lakes have killed off many of the other bacteria that could not survive the salty waters and instead have left those that can live. With methods of active transport, Halobacterium could compensates for osmotic loss by channeling ions into the bacteria’s cytoplasm (Campbell 556). If these bacterium didn’t have that ability, they would not be able to maintain an osmotic balance with their environment, leading to their death.
Lets look at another example: the wooly mammoth. If it weren’t for macroevolution, then the smooth wooly mammoth would not have been replaced by a shaggy one, leading to the death of the species (11). Another example would be the change from the large sized Carcharodon Megalodon, a giant shark that used to inhabit waters. Natural selection has wiped out the larger sized C. Megalodons because there was less and less food in the ocean for them to eat (Elasmodiver.com). Compared to the ancestral C. Megalodon, macroevolution has promoted genes that made the shark smaller to fit the crowding ocean environment.
There are even very prominent examples for microevolution. A journal article I was reading weeks ago featured weeds that expressed genes for less distributing seeds when placed in a greenhouse where the only patches of soil to grow on were the patches that the weeds were growing on. These weeds surprisingly developed these genes very quickly in a span of only twelve generations. This change allowed the plant from a distributing seed plant to a non-distributing seed plant, optimizing the number of surviving seeds (PNAS).
In conclusion, if we include the examples of those animals that evolved badly, one could come to the conclusion that evolution is, in fact, a process that tries to balance how an organism lives, but evolution does not always come up with the best result. Evolution has no direction, but when it does go somewhere, it is usually for the benefit for the organism to survive and reproduce by evolving an organism into a form that could survive and reproduce more easily.
These examples link to the theme regulation. Most of the organisms explained all have genes turned on and off in response to environmental stress in order to provide a body in working order. Regulation allows an organism to try to pass on these genes to the next generation, changing the trait that will hopefully suit the next generation.
Kevin Malis, I think you're contradicting yourself or have made a few typos somewhere. You make it clear in your first sentence that you think evolution is still macroevolution. However, you bring up Coyne's quote that most evolution today is microevolution, not macroevolution. Can you make your stance more clear?
ReplyDeleteHowever, I understand your analogy of population dynamics and how it relates to the dynamics of evolution. I can link your analogy's assertion with the weed that evolved in twelve generations. At first, dramatic changes in the number of non-distributing seeds occurred, and the number of distributing seeds being released was greatly reduced. Then, if the scientists allowed the plant to continue sprouting new generations, one could assume that the plant will not evolved that dramatically as it initially did. The plant's next generation will most likely have close to the same proportion of non-distributing seeds to distributing seeds as its previous generation. However, if the proportion is dramatically different for one of those weeds, then that weed will most likely not be able to produce have a surviving next generation.
What I mean is that there are still examples of macroevolution going on today. Coyne notes this in the book, but he also notes that the macroevolutionary changes can't be seen by one person in one lifetime. I think this makes it hard to quantify how much macroevolution is actually going on. However, given that most of the species that are on the planet today have already underwent drastic evolutionary changes, all that's left for these species to undergo is microevolutionary changes. That is, the organisms have been molded by natural selection over millions of years into species that are fit enough to survive so only small changes are necessary..
ReplyDeleteAnd yes, I made a typo. My first sentence should read "mostly microevolutionary" not "macroevolutionary".
ReplyDeleteEDITED COMMENT:
ReplyDeleteI would agree with the above quotation that even at a point of evolutionary equilibrium, evolution is still occurring, even if it is mostly microevolution. At evolutionary equilibrium, macroevolution still occurs, but it’s far less common than microevolution. Also, macroevolutionary changes will not be seen in the course of one person’s lifetime. Macroevolution usually occurs when an organism’s environment is drastically changed(if a new predator is introduced, if the organism is introduced to a new biome, etc.). As Coyne states on 133, most species around today are pretty well adapted; otherwise they wouldn’t be around. Given that most species have nothing to adapt to, the only changes that are occurring are minor, microevolutionary changes. At equilibrium, there isn’t a net change in amount of species produced. What most commonly occurs in a well adapted species is stabilizing selection. Stabilizing selection basically functions to eliminate organisms that stray too far from the optimum situation. Coyne cites the example of large and small birds being “cull[ed]” if they are too much bigger or smaller than the ideal bird. So, in many species, drastic changes from natural selection will have stopped occurring, but evolutionary forces will not have stopped acting on these species. Clearly, this relates to the biological theme of regulation. An evolutionary force(in this case, stabilizing selection) regulates the bird population by removing birds that are far too large or small. However, the average bird size of this population doesn’t change. If evolutionary forces such as stabilizing selection didn’t occur, genes that cause both large and small birds would be proliferated in a population, and too large or too small birds would become more common.
This relates to our own study of population dynamics and carrying capacity. Here’s an analogy: In general, if a population is way under carrying capacity, the population will naturally grow up to the carrying capacity unless some outside factors severely and routinely hinder population growth. This is analogous to a maladapted species undergoing natural selection until the species becomes relatively well adapted. So in this analogy, a population at carrying capacity is the equivalent of a “well adapted species”. However, once a population reaches carrying capacity, the population isn’t constant. Organisms will still die and organisms will still reproduce. But, the net growth of a population at the carrying capacity is nothing(provided that there are no extra outside factors like natural disasters). In other words, at carrying capacity, the rate of growth equals the rate of death(Campbell 1183). In a well adapted species, natural selection is still occurring, just that it’s minor changes that occur, not major ones. Small and large birds still die, but the average size of the birds is still the same. So, the natural selection of birds is very similar to population dynamics that we discussed in our first unit.