Wednesday, April 14, 2010
Rapid microevolution in nature
As Coyne stated on pg. 132, humans want to see a natural population meet a natural challenge, and they want to see the population evolve to meet it before their eyes. They want to see it occur without human interferrence and not in bacteria, but in "higher" plants and animals. Since humans have no control in this, it has to be nearly impossible to observe this, yet we have. A finch in the Galapagos Islands was observed to have evolved to a 10 % larger beak size in one generation after a drought made them find nutrution in larger and harder seeds than normal. Coyne states that this is "far larger than anything we see in the fossil record" (134). What other examples exist of microevolution, not man-made, that has been observed in nature in a very short period of time? Also, why is it that there is no past evidence of evolution occuring so quickly (as in one generation)? Could it be that the selective pressures are increasing? What could this mean for future evolution?
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Microevolution is the occurence of small scale changes due to mutation, selection, genetic drift, or gene flow which are four selective pressures that can cause this to occur over a short period of time such as a generation or two. House Sparrows were introduced to North America and once that happened microevolution occured. Sparrows in the north became larger bodied than the sparrows in the south. Also, microevolution has been seen to occur in the evolution of resistance. This is the resistance of such things as pests to pesticides, weeds to herbicides, and pathogens to medicines. When bacteria mutates it occurs very quickly and once it finds an effective mutation to overcome its obstacle, such as a drug, then microevolution occurs and all the bacteria become that strain very quickly. There is past evidence of evolution occurring so quickly; the example given in the question of the finch in the Galapagos Islands is an example of it happening in the past. Also, just because there is not written evidence of the fast evolution happening in the past does not mean that it did not occur it just means that no one notice it or took the time to record it. This is very possible since the changes can be slight and hard to notice and they occur quickly so they can be difficult to track. It is a possibility that selective pressures are increasing however so that microevolution is becoming more common. Selective pressures such as the arms race between viruses and bacteria versus medicine may cause more microevolution to occur. Humans pushing the limits of nature in all kinds of ways adds lots of selective pressures. This means that in the future evolution could all together happen faster.
ReplyDeletehttp://evolution.berkeley.edu/evolibrary/article/_0_0/evoscales_03
http://en.wikipedia.org/wiki/Microevolution
Microevolution is the occurance os small-scale changes in allele frequencies in a population, over a few generation- change within a kind. These changes are generally due to mutation, natural selection, and genetic drift. There are several examples of microevolution other than Darwin's Galapagos Finches. One such case is with the sparrow. "House sparrows were introduced to North America in 1852" (berkelely.edu). These birds have evolved based on their distribution throughout the continent. In the northern region of North America, the sparrow had a larger body and was darker compared to the southern sparrows with smaller and lighter bodies. These changes were most likely due to natural selection and the environmental pressure of climate/temperature. Colder weather in the north would select for larger bodies because those larger-bodied birds can survive better in lower temperatures (more insulation). Smaller birds are more selected for in wamer temperatures because they have less body weight to carry around and therefore expend less energy (which is necessary to conserve in warmer climates). The darker color of the sparrow would attract more sunlight as well which would help the sparrows in the north (colder region) attract heat from the sun. The lighter color on the sparrows in the southern region probably helps the sparrows to stay cool, attracting less sunlight.
ReplyDeleteIt is possible that in the past evidence of evolution did not occur so quickly because the environmental/selective pressures were not extreme enough. In general, evolution is a very slow process and even microevolution generally takes a few generations to display an effect. When the environmental stressors are more intense, it forces a physical alteration much faster because that physical advantage to survive a harsh condition would be that much more valuable. For future evolution, this means that we may see evolution happening faster and faster if effects from events such as global warming cause climate/environmental extremes that pressure nature.
Why Evolution is True
http://evolution.berkeley.edu/evolibrary/article/_0_0/evoscales_03
http://www.edf.org/page.cfm?tagID=35215&source=ggadgw35215&gclid=CIq49fLwh6ECFQUhDQodpVlhPg
Microevolution, as you stated, can occur very quickly and only over one generation. In the case of the finches, variations in the beak sizes pre drought brought along a relatively large change over a short period of time. However, evolution cannot work much faster than this, simply because stronger environmental pressures would kill all the organisms. During the drought around 75% of the finches died, only to produce a small increase in the next generation’s beak size. Had the drought lasted longer, it is possible that all of the finches would have died (not a selective advantage!) and no evolution would have occurred. If by evolution you meant macroevolution, well this is simply impossible. Think about the amount microevolution that occurs before speciation occurs; trying to compress that into environmental pressures on one generation is like nuking the Galapagos islands (in terms of evolutionary changes) several times to see if any finches survived and were able to give birth to a mutated species. Fossil records are very incomplete and only show tiny glimpses into our evolutionary history, so there may be examples of rapid micro evolution besides the finches. Another problem is that physical structure of organisms one generation apart, even with relatively large changes, is very hard to discern because fossils are never in very good condition.
ReplyDeleteOne example of an increasing pressure is on polar bears and their melting homes. Polar bears are dying in greater numbers due to drowning. This selective pressure may grant better swimming capabilities over a generation but constantly melting ice caps means even less land for polar bears to find and rest on. Other examples would include animals displaced by rainforest destruction; these animals would have to adapt quickly to their new environment, or move to an unfamiliar environment in order to survive. For animals like these environmental pressures are increasing, usually as a direct result of human actions. While it may not mean quicker evolution/speciation human effects certainly are shaping the way some animals are evolving.
http://www.nwf.org/Global-Warming/Effects-on-Wildlife-and-Habitat/Polar-Bears.aspx
http://lanic.utexas.edu/project/etext/llilas/outreach/fulbright05/KalinMiller_Microevolution.pdf
If I understand by your description of microevolution in a short period of time correctly as “not man-made”, then I assume it would be that those species that weren’t physically moved by humans. The reason I say this is because one example I found that was studied in a journal article of a species that is undergoing microevolution that is observable is doing so because of climate change, which is contributed to by humans. From the mid-1970s, the Drosophila subobscura has undergone O chromosomal polymorphisms; specifically, diversity in the genetic code is visible with an estimated 18.3% difference due to microevolution over 16 years (Rodriguez-Trelles, Rodriguez). That means that almost one-fifth of its genetic code has undergo changes; only four-fifths more and it would be a completely different species of Drosophila. While I understand that it took 16 year records to record that change, it may have been possible to view the changes in its genetic code at a rate of 1.14375% change in genetic code per year. Physiologically this could change the features or functions of its organs, which would be the more readily available indicators of such change.
ReplyDeleteIn response to your question of why there is no past evidence on evolution occurring so quickly, there are various answers. I think the most logical reason would be similar to Coyne’s reason for having only a fraction of a percent of the life history on Earth using fossils: the evidence is very hard to find. For instance, in order to actually observe these rapid changes, we would have to have the means for knowing the gene sequences that were changing because a physiological-change-over-time approach to observation would be inefficient and would probably give inaccurate results because of so many variables. But that would mean we would need to know the whole genome of that organism, and as we see from the Human Genome Project, it took 13 years to map out the 20,000-25,000 genes in the human genome (US Department of Energy); the amount of time this would take for every possible species that could undergo microevolution would be unthinkably grandiose. Another the reason that it might be hard to find evidence can be seen when we comparing the average evolutionary rate of genomic sequences amongst different animals. Generally, K-selected species have a much slower microevolutionary rate because they live longer lives; meanwhile, r-selected species have a much faster microevolutionary rate because they live shorter lives and an increase in this rate would increase overall fitness of the species (Campbell, Reece). So in order to find reliable evidence to support an increase in—or at the least the happenings of—microevolution would yield to many smaller organisms, significantly limiting the amount of species we have to test because if the organisms are already small, then by default they have a much lower chance to move between different niches and habitats (except for microorganisms and viruses), so they’re usually not put under evolutionary stresses because they adapt to their environment and don’t move away. Even though there are probably many more reasons as to why it is hard to find such evidence, I believe the above two sufficiently limit our abilities to find organisms to observe such rapid microevolutionary rates.
Continued below..
However, you suggest another reason that this may be going on and that would be increased selective pressures. To an extent, I would have to disagree because the point of selective pressures is that it facilitates the mechanism of evolution: natural selection. With an increased selective pressure, the organisms in a given a species that survive would undergo macroevolution (Coyne). On the other hand, there is some evidence as to microevolutionary pressures in the study of epigenetics. Epigenes are molecules that are part of the secondary structure of the genome and act as “middlemen” between our genes and environmental stresses (University of Utah). Basically, the ‘epigenetic system could serve as the basis of non-darwinian evolutionary strategies by means of "memorization of rather unsuccessfully steps of evolution’”. Say, for instance, an organism is placed in a pressurized situation in their lives, then the epigenes may react respectfully and cause the genetic code to alter, even if it’s just a bit. During reproduction, these changes have a good chance of being passed on, thus acting as a microevolutionary mechanism.
ReplyDeleteIn terms of your last question, if selective advantages were to increase, then it would be safe to predict that this would be a macroevolutionary event. For example, we can see the simple evolution of human processes has led to a proportional increase in weight. “In evolutionary terms, it's good to put on weight. We need energy stores to carry out activities that use lots of energy, like growing and repairing muscle, as well as carrying pregnancies, and raising children — and historically, food supplies weren't always a given” (Professor Michael Cowley). So we see this selective pressure is caused by the diversification of function in the human body, causing the demand for more energy. In terms of other organisms, if they underwent a selective pressure—need it not be food, but anything for that matter—then they would evolve as a species because of the demand, causing macroevolution, not just microevolution.
http://www.springerlink.com/content/pj5508tv31402101/
http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml
http://learn.genetics.utah.edu/content/epigenetics/intro/
http://www.ncbi.nlm.nih.gov/pubmed/15859448
http://www.abc.net.au/science/articles/2010/01/21/2798024.htm
Jerry Coyne: Why Evolution is True
Niel Campbell, Jane Reece: Biology: AP Edition, 8e
Although microevolution, and evolution for that matter, are not considered to be a fact, there is compelling evidence for the case. For example, scientists introduced a single species of lizard to fourteen small, lizard-free Caribbean islands near the Bahamas. The lizards were left there for fourteen years. There was a pre-existing correlation between vegetation and leg size, where lizards that inhabit trees tend to have longer legs. Therefore, they hypothesized that the long-legged lizards they introduced would evolve to have shorter legs because the Carribean islands are almost treeless. Their hypothesis was correct. But, what is really amazing is that these lizards evolved in a little over a decade. It showed that the lizards were receptive to environmental pressures and adapted accordingly.
ReplyDeleteEven though there isn’t any past evidence fo evolution occurring so quickly, I think that the amount of time it takes for evolution to occur has slowly been decreasing. For example, “the evolution of whales from land animals [. . .] took place within only 10 million years” (51). Relatively speaking, this is not very much time considering how much they evolved.. Especially compared to previous evolutionary process. Therefore, I think that rapid evolution is due to two factors. First of all, just as Corey mentioned selective pressures are increasing. This increases the stress a specific animal feels to evolve. In turn, this facilitates natural selection where animals that are a better fit for the changing environment continue to survive. The second factor is that the degree of evolution is not as great. Over time, organisms have evolved and as result, modern organisms already have a majority of necessary structures that they can work off of and improve upon. Therefore, when they evolve do not have to make as large of an evolutionary leap. For example, the long evolutionary process in which reptiles evolved into birds, required the development of feathers and a major shift from arms to wings. Comparatively, whales took much less time. This is because they already had the necessary structures in their arms, and all they had to do was shorten them and connect them with tissues to create fins. Currently, many of the major evolutionary processes, from water to land and from land to air have already occurred and now all organisms are in a perfect position to fix what they already have in order to fit the needs of their environment.
SOURCES:
http://www.dinosauria.com/jdp/evol/lizard.html
Why Evolution is True
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