User:Doherty.76

The topic I will be choosing will be questioning if evolutionary processes make disruptive selection result in new phenotypes. I am interested in this topic to get a better understanding on things like gradualism and how the mechanism of natural selection over time would results in a divergence within a population. I would like to gain a better understanding of how an evolutionary process can favor extremes within populations. This relates to evolutionary biology because disruptive selection is the change in phenotypes over time to favor two extremes against the mean in a population. There has to be something going on if evolution is gradually favoring outcomes against the norm for the population. I believe that my topic will be able to add to some areas of evolution on Wikipedia.

Smith T.B. 1993. Disruptive selection and the genetic basis of bill size polymorphism in the Afican finch Pyrenestes. Letters to Nature Volume 363:Pages 618-620. The central question in this article is if a specific type of African finch displays signs of disruptive selection driving their bill size associated with feeding preferences. The method that Smith uses is a long term ethnography field work to study the birds. The most relevant results are that adaptations of bill size through feeding performances begin to occupy two distinct separate groups, supporting disruptive selection. The data suggests that disruptive selection is vital in creating and maintain separate or distinct polymorphisms. This will be a very helpful example aiding me to get a better understanding on gradualism and disruptive selection.

Boam T.B., Thoday J.M. 1959. Effects of disruptive selection: Polymorphism and divergence without isolation. Heredity Volume 13: Pages 205-218. The main question that this article tackles is if disruptive selection can be attributed as a significant cause in creating genetic diversity. The method that was used for this article was positive assortative mating. Two subpopulations diverged drastically as a result of disruptive selection pressures. With only a little time the two subpopulations were unable to diverge steadily. Then after continued selection, gradualism, they were able to do so.

Thoday J.M. 1972. Disruptive Selection. Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 182 No. 1067: pp. 109-143. The main question in study in this article is if disruptive selection is able to increase phenotypic and genetic variance, and causing divergence between subpopulations. The methods used to find results for this study were by using natural populations to specifically test their reaction to certain environments. The articles studies suggest that the effect os disruptive selection is to maintain and create genetic diversity within populations. This is a very useful article for my study topic because it shows ways in which disruptive selection can be seen through natural experiments not artificial selection.

Hendry A.P., Huber S.K., DeLeon L.F., A. Harrel and J Podos. 2009. Disruptive Selection in a Bimodal Population of Darwin’s Finches. Proceedings: Biological Sciences, Vol. 276, No. 1657, pp. 753-759. The main question that this article addresses is if in a population adaptive divergence leading to reproductive isolation can be seen. The researchers did intense fieldwork on a specific type of finch to find out patterns of selection over time in relation to beak sizes with interannual recaptures. The results were very clear cut and showed a distinct divergence between subpopulations over time. There was also a high mortality and emigration rates associated with prolonged droughts. This will be very helpful in guiding my research of and showing examples of disruptive selection.

Van Doorn G.S., Diekmann U., Hansen T. The Long-Term Evolution of Multilocus Traits Under Frequency-Dependent Disruptive Selection. This main question being discussed is if disruptive selection is a large or important contributer to genetic variation. They studied natural population models to see if they could find a realtion between phenotypic and genotypic models for selection. It shows that frequency dependent disruptive selection does in fact introduce and maintain genetic variation. The long term evolution of multilocus traits can diverge over time. This is very relevant to my research that I will be conducting, but also offers some contrasting evidence to my previous articles.

Edit: https://en.wikipedia.org/wiki/Disruptive_selection This introduces the topic of gradualism, which is a slow but continuous accumulation of changes over long periods of time [26] Modeling Modes of Evolution: Comparing Phyletic Gradualism & Punctuated Equilibrium William F. McComas and Brian J. Alters The American Biology Teacher, Vol. 56, No. 6 (Sep., 1994), pp. 354-360 Published by: University of California Press on behalf of the National Association of Biology Teachers

Talks: Disruptive Selection from gradualism or punctuated equilibrium?[edit] Does disruptive selection occur because of gradualism, a slow accumulation of changes over long periods of time; or from punctuated equilibrium, which is rapid bursts of change in small periods of time? Could it also be a combination of both?

Which types of gradualism? Gradualism can occur in biology, as well as in cultural systems. Creating a link between these two will help to better understand gradualism.

Cause of genetic divergence?

Yes genetic divergence can be easily seen with the separation of two populations over time.  But some more information can be added about how this change occurs.  Does change or divergence occur gradually or in contrast during flux periods?  Examples for both would help this page a lot.

FINAL DRAFT STARTS HERE

Disruptive Selection in Ecology Natural selection is known to be one of the most important biological processes behind evolution. There are many variations of traits, and some cause greater or lesser reproductive success of the individual. Selection’s purpose is to promote certain alleles, traits, and individuals that have a higher chance to survive and reproduce in their specific environment. Since the environment has a carrying capacity, nature acts on this mode of selection on individuals to let only the most fit offspring survive and reproduce to their full potential. The more advantageous the trait is the more common it will become in the population. Disruptive selection is a specific type of natural selection that actively selects against the average in a population, favoring both extremes of the spectrum. Disruptive selection is inferred to often times lead to sympatric speciation through a phyletic gradualism mode of evolution. This is particularly relevant and important because it shows neither a physical barrier nor the favoring of the average have to be present for speciation to occur. Intraspecific competition in a population can be seen to drive disruptive selection. Interspecific competition is competition between a population of the same species for resources such as mates, food or territory. An example of this would be two flowers of the same species that are growing right next to one another and competing for resources such as food, sunlight, and water. Another would be male animals that grow antlers to fight with other males of their species for territory or mates. Daniel Bolnick conducted experiments on a species of stickleback fish called Gasterosteus aculeatus, to see if frequency dependent resource competition causing intraspecific competition will result in disruptive selection. Using the traits of body size and relative gonad mass as a way to indirectly measure fitness, he showed that gill raker length was subject to disruptive selection, which could then lead to niche diversification, sexual dimorphism, and sympatric speciation. To see if competition could have been causing this disruptive selection he manipulated the densities of five populations in lakes so that some had high densities while others had low, but he made sure the populations had the same natural phenotypic average distributions. The results showed that in all of the high density populations there was constantly stronger competition and divergence than in the lower densities. This supports the idea that intraspecific competitions within a population can drive disruptive selection, and also be a vital agent in how ecological variation evolves (Bolnick 2004). Another example of how competition can drive disruptive selection can be seen in competitions for resources like food in nature. Initially, individuals that select for the most common resource, such a moderate sized prey, will have a direct fitness advantage. However, as competition increases for that most common resource it will cause the fitness of those individuals to decrease due to the intense competition. As long as there is a wide range wide range of resources available the individuals that favor both extremes, small or large prey, will have higher fitness with less competition. Disruptive selection can cause a favoring of less common, more extreme phenotypes (Kingsolver and Pfenning 2007). Competition over resources is caused by large densities or frequency dependent competition within a population can result in disruptive selection, and over time it can eventually lead to speciation. Human interaction with the environment can cause disruptive selection to occur. This can be seen by a study of peppered moths conducted by Michael Majerus, which showed that human pollution had an effect on the phenotypes of this species of moth. In more rural areas the moths were commonly a lighter color while the moths were a darker color in more industrialized areas. Birds are a predator of the moths and were used as selective agent. The study found that lighter moths were seen more easily by the birds in the industrial areas and eaten more frequently. In contrary, as in the more rural areas the darker colored moths were seen more easily in that specific environment, causing them to have a lower fitness outcome. The medium colored moths were seen very easily in both the locations and hence selected against (Cook et al. 2012). In this example, human caused pollution that created the mechanism for disruptive selection to occur. Suddenly after the industrial revolution there was an increased fitness advantage to favor against the intermediate and for either of the extremes; light or dark colored depending on environment. Disruptive selection often times occurs in along with predator-prey coevolution. They create corresponding phenotypes in relation to their counterpart. (Abrams et al. 2006). Finches on the Galapagos Islands are a great example in showing disruptive selection acting in nature. These finches show adaptive radiation with beak size and shape. Their beak is determined by their diet and environment: If they eat soft nuts they have a small beak, and if they eat large nuts they have a larger beak. Thomas Smith studied the African finch Pyrenestes to show that polymorphisms can be maintained by disruptive selection. Disruptive selection existed in four of the six traits that Smits studied: In the lower mandible width and length, upper mandible depth and tarsus length. They all showed two extremes of a trait, with a low frequency of the intermediate form between them. Differences in feeding preferences and both interspecific and intraspecific competition during droughts lead to the survival of polymorphisms through the biological process of disruptive selection (Smith 1993). A similar study of a different species of finch, Geospiza fortis, also of the Galapagos Islands showed selection by relating beak size to yearly recaptures during droughts. Disruptive selection was seen to act on the beak size through adaptations to their environment and reproductive barriers. Ecological factors can promote eventual speciation through disruptive selection but in certain cases constraining factors can halt or reverse speciation. There is a sort of tug of war that occurs within an environment between factors that promote and restrain speciation. There is an evolutionary tug-of-war in that there are promoting factors in the environment that can lead to speciation, but there are also constraining factors that can take the upper hand in reducing diversification (DeLeon et al. 2009). Positive assortative mating can also influence disruptive selection. Individuals that chose to mate with people similar to them can promote and keep polymorphisms within an environment. There doesn’t have to be an isolation barrier that limits gene flow for a population to diverge. Individuals just have to adapt to something specific in their local environment. It takes a long amount of time and many mutations to build up for two species to become genetically diverged from one another and thus not being able to reproduce viable offspring. This introduces the topic of gradualism, which are slow constant changes accumulating over time, as opposed to punctuated equilibrium. Disruptive selection that leads to speciation occurs over long periods of selection (Boam and Thoday 1959). Disruptive selection in positive assortative mating selects for a polymorphic trait which can result in reproductive isolation, causing speciation (Rice and Salt 1988). In many of the experiments that have been done there has simply not been enough time or generations to support full reproductive isolation. It is assumed that eventually the populations will diverge, which supports the idea of gradualism. To the counter the idea of gradualism, disruptive selection can also be shown to cause rapid speciation. Haplochromine cichlids have been extensively studied in East African lakes studies have found that disruptive sexual selection acts on color polymorphisms that can lead to sympatric speciation. Elevated speciation rates can be caused by mating choice, especially when choice is based on an individual to emit a signal, like coloration. Speciation can also occur allopatricly, due to geological isolation of a population. This is opposite to sympatric speciation which relies on the evolution of behavioral or other social isolation influences within certain populations in the absence of a barrier. The sympatric mode of speciation has a lot to do with how much gene flow is occurring between populations. There are many intrinsic factors that influence sympatric speciation within a population that can cause a more rapid speciation initially than allopatric. It helps explain the radiation of different species of cichlids as well as coexistence without niche partitioning of coloration in the recently diverged species. Having a different local environment, creating a subpopulation and allowing for speciation to occur is not the only way that speciation can occur. There can be the same ecological conditions between populations and disruptive selection can still be seen (Seehausen and Van Alphen 1999). Disruptive selection directly can affect allele frequencies as well as genotype frequencies. These changes can occur relatively quickly because they do not require new mutations to occur; it can take advantage of genetic variation that already exists. Even further when there is a heterozygote disadvantage or selection favors the homozygous extremes of traits. This can be exemplified by morphological differences in the way species specialize in gaining access to resources like food. But if there is a broad spectrum of resources available in a population the effect of disruptive selection is not very strong because there is no longer a disadvantage for the intermediate. With clinal variation in environments, disruptive selection does not pull a species toward speciation. When there is not morphological specialization in phenotypes or a lot of variation within their environment, disruptive selection is not seen to have as much of an impact (Abrams et al. 2006). Disruptive selection can be caused or influenced by multiple factors and also have multiple outcomes, in addition to speciation. Sympatric speciation, though it might not be able to be seen as clearly as allopatric speciation, plays an important role in disruptive selection. Individuals within the same environment can develop a preference for extremes of a trait, against the intermediate. Selection can act on having divergent body morphologies in accessing food, such as beak and dental structure. It is seen that often this is more prevalent in environments where there is not a wide clinal range of resources, causing heterozygote disadvantage or selection against the average. Niche partitioning allows for selection of differential patterns of resource usage, which can drive speciation. To the contrast, niche conservation pulls individuals toward ancestral ecological traits in an evolutionary tug-of-war. Also, nature tends to have a ‘jump on the band wagon’ perspective when something beneficial is found. This can lead to the opposite occurring with disruptive selection eventually selecting against the average when everyone starts taking advantage of that resource it will become depleted and the extremes will be favored. Furthermore, gradualism is a more realistic view when looking at speciation. Although disruptive selection can initially rapidly intensify divergence, this is because it is only manipulating alleles that already exist. Often it is not creating new ones by mutation which takes a long time. Usually complete reproductive isolation does not occur until many generations, but behavioral or morphological differences separate the species from reproducing generally. Defining a species can also be unstable ground because it can differ across time and space, making speciation individually constructed by certain influences. Disruptive selection is very important because of the ways in which it operates are not researched extensively causing ambiguity. This mode of selection is rare but still very important in the evolution of traits and speciation.

References Cited Abrams, P.A., Leimar, O., Rueffler, C., Van Dooren, J.M. 2006. Disruptive selection and then what? Trends in Ecology & Evolution Vol. 21 Issue 5:238-245. Boam, T.B., Thoday, J.M. 1959. Effects of disruptive selection: Polymorphism and divergence without isolation. Heredity Vol. 13:205-218. Bolnick, D.I. 2004. Can Intraspecific competition drive disruptive Selection? An experimental test in natural population of sticklebacks. Evolution 58(3):608-618 Cook, L.M., Grant, B.S., J. Mallet, J., Saccheri, I.J. 2012. Selective bird predation on the peppered moth: the last experiment of Michael Majerus. Biology Letters 6:609-612. DeLeon, L.F., Harrel, A., Hendry, A.P., Huber S.K., Podos, J. 2009. Disruptive Selection in a Bimodal Population of Darwin’s Finches. Proceedings: Biological Sciences, Vol. 276, No. 1657:753-759. Kingsolver, J.G., David W. Pfenning, D.W. 2007. Patterns and Power of Phenotypic Selection in Nature. BioScience Vol. 57 Issue 7:561-572. Rice, W.R., Salt, G.W. 1988. Speciation Via Disruptive Selection on Habitat Preference: Experimental Evidence. The American Naturalist Vol. 131 No. 6:911-917. Seehausen, M. E., Van Alphen, J.J.M. 1999. Can sympatric speciation by disruptive sexual selection explain rapid evolution of cichlid diversity in Lake Victoria? Ecology Letters Vol. 2 Issue 4: 262-271. Smith, T.B. 1993. Disruptive selection and the genetic basis of bill size polymorphism in the African finch Pyrenestes. Letters to Nature Volume 363:618-620.

Edit on Wiki Page: Disruptive Selection Page: https://en.wikipedia.org/wiki/Disruptive_selection (Overview section)