Saturday, March 6, 2010
Smooth v. Erratic Evolution
On pages 29-30, Jerry Coyne uses the example of the marine organism Globorotalia conoidea to explain how tracing a single fossil species can help geologists to see how that species evolved. In this particular example, the number of chambers in the final whorl of the plankton shell decreases gradually and smoothly over time. However, Coyne goes on to say that "evolution, though gradual, need not always proceed smoothly, or at an even pace." What factors determine how smoothly or erratically evolution occurs in a particular species? Provide examples and compare and contrast the environmental conditions in which a certain species can undergo smooth evolution and erratic evolution.
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The progression of evolution and its relative "smoothness" depends on environmental homogeneity. Since the environment may fluctuate, so may the relative strength of the selective pressures that drive evolution. Coyne emphasizes that such uneven patterns in evolution are "understandable" since they may be the result of "environmental fluctuations in climate or salinity" (Coyne, 30)
ReplyDeleteEvolution cannot proceed without genetic variation. In individual populations, size will often affect the range of the gene pool and subsequently the rate of mutation. In fact, "a small population and a stable environment could lead to low rates of evolution for long periods of time". ("Rates of Evolution")
Since genetic drift and inbreeding may further reduce the gene pool and hamper natural selection, the rate of evolution and the ability of a population to adapt in the face of selective pressures often depends on size. Such populations have a "minimum viable population" at which they may avoid extinction and proliferate. (Campbell, 477)
Often, the loss of genetic variation as a result of genetic drift and inbreeding in small populations will determine the fate of the population. Although Natural Selection promotes evolution by imposing "a demographic load... by reducing population growth rate or density" there's a limit at which the "magnitude of selection...can be sustained". (Burger, 151)
The importance of genetic variation and mutation rate is evident in the cambrian explosion. Evidence suggests that the "origin of multicellularity and complex body plans among animals was... dependent on the evolution of Hox genes, their subsequent duplication and divergent change". ("Cambrian Explosion")
Other evidence suggests that the rapid diversification was contigent upon "rising oxygen levels" which "made possible new modes of locomotion", thus suggesting that environmental change drove natural selection and speciation. Thus, "erratic evolution" correlates with rapid, unforeseen changes in environmental conditions. Biotic factors, such as predation, may also affect rate of evolution. In fact, evidence suggests that predator-prey interactions may have spurred the development of shells and mineralized skeletons during the cambrian explosion. ("cambrian explosion, macroevolution, extinction")
Campbell, Neil A. Reece, Jane B.. Biology, AP Edition 8e. 2008.
"Cambrian Explosion" http://faculty.evansville.edu/de3/b39903/PDFs/9_Cambrian.pdf
"cambrian explosion, macroevolution, extinction". www.csun.edu/~dgray/Evol322/Chapter18.pdf
"Rates of Evolution". Plaisted, David A. http://www.cs.unc.edu/~plaisted/ce/rates.html
Richard Burger, Michael Lynch. "
Evolution and Extinction in a Changing Environment: A Quantitative-GeneticAnalysis"
A Beatriz said, the rate of evolution depends on the environment that species are living in. An environment with many unoccupied niches, such as an oceanic island, would be a prime area for speciation and evolution, because very many changes in traits could allow a new species to occupy a niche (http://www.actionbioscience.org/evolution/eldredge.html). Another reason an environment might have unoccupied niches is following a natural disaster, where species were killed off. These disasters allow new species to thrive in that environment. Another factor that affects the rate of evolution is the rate at which mutations occur in species. Although mostly neutral or harmful, mutations do provide a chance for adaptations to arise, giving an organism a selective advantage to survive and reproduce (http://www.detectingdesign.com/dnamutationrates.html#Detrimental). Scientists believe that a very large increase in mutation rates caused the Cambrian explosion about 570 million years ago (http://www.tasc-creationscience.org/other/plaisted/www.cs.unc.edu/_plaisted/ce/rates.html). This 5 to 10 million year 'explosion' produced many new species and made organisms much more complex, leading to the immense species diversity that exists today. Evolution will slow in environments that don't have many disturbances, thus undergoing 'smooth' evolution. However, erratic evolution occurs most often on earth, because there are so many factors that can affect the rate at which evolution occurs in any given place. When a single given factor undergoes a major change, this can alter the rate of evolution drastically, causing new species to arise in that environment.
ReplyDeleteCoyne is correct, it doesn't have to proceed always smoothly. Both Beatriz and Scottie are correct in saying that it depends on the environment. There are so many factors that could drive natural selection which in turn drives evolution. These factors include such things as predation, amount of food, weather, climate, and many others. One that would affect evolution the most would be predation. This would have both smooth and erratic evolutionary patters. This is because animals need to adjust to the predators about them. If an animal were really slow, and a new predator was introduced, then they would have to adjust to be able to escape these predators. Grant it, we know that evolution cannot happen in an instant. These adaptations will happen in the future. Though it would be hard to tell if these evolutionary changes are more smooth or erratic. We would have to determine it by the adaptations that the animal received. If the animals characteristics are completely different, then this helps show that the animal went through a drastic evolutionary change.
ReplyDeleteAnother factor that could affect evolutionary change would be the amount of food. If the animal needs to eat less or eat more, it has to be able to adjust to the amount of food that is available. This could be by in the future, though evolution, it could speed up its metabolism, or slow it down to meet its needs. These species would have to go through some drastic changes, so the evolutionary process would be very erratic. You can't evolve slowly if it involves food because otherwise you would die out. Though we know that evolution is a very slow process, and natural selection would have to ensure that these species stay alive through their adaptations.
Sources:
http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.0030205
http://kevincassell.com/blog/index.php?id=21
Beatriz and Scottie's comment
I agree with Beatriz, that the environment homogeneity controls whether evolution occurs erratically or smoothly. One of the biggest factors in evolution and natural selection is the environment. In an environment with few competitors and predators, evolution of a particular species will occur smoothly. This is because there are fewer environmental pressures and challenges forcing the species to adapt and perhaps evolve. In an environment with many other species, predators and competitors alike, a species is more likely to occur in an erratic fashion. This is more common in the world because there are so many factors and pressures that delay or speed up the rate of evolution. A change in just one factor could be the beginning of an entirely new species. The three main variables are the rate of mutation, the rate of selection and the rate of environmental change (ES). The environmental part (predators, competitors, climate, terrain, availability of nutrients, and food sources) is dependent on whether or not the environmental factors remain constant (evolution will occur smoothly) or whether they fluctuate (causing erratic evolution). During the Cambrian explosion, mutation rates spiked. There would have to have been 100 mutations a year in order to reconstruct a billion base pairs in the 10 million year period (tasc). This doesn’t even account for the harmful mutations. This would only be possible if many mutations could combine in one individual due to sexual reproduction. Sexual reproduction made this mutation rate possible because it would be impossible to reach such fast rates with mutations only accumulating sequentially. After the Cambrian explosion, the life spans of individuals increased, making the mutation rate that had just occurred impossible.
ReplyDeleteSources:
www.es-designs.com/geol105/lectures/lecture10.html
www.tasc-creationscience.org/other/plaisted/www.cs.../rates.html
www.naturalselection.0catch.com/Files/dnamutationrates.html