Wednesday, April 8, 2009
Sexual Selection
On page 109, Carroll mentions Darwin's theory of "sexual selection." Compare and contrast this theory with the theory of natural selection. Explain how the two fit together and give examples of each. Explain why "biologists view sexual selection as one of the most important... forces in the evolution of animals," (109).
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Sexual selection is one of the most important forces in the evolution of animals mainly because of the role of genetic diversity in sexual reproduction. In sexual reproduction, half of the genetic information comes from one parent and the other half from the other parent of the animal. Genetic diversity within a species allows for the appearance of more selective advantages, and the crossing over during meiosis causes a greater amount of mutations, enhancing natural selection. The importance of sexual selection comes from the fact that it causes males with more selective advantages to appear more attractive to females and vice versa. The end result is that the "fittest" of the species mate and produce the most selectively advantageous offspring. Traits that are advantageous survive throughout the offspring, so members of the opposite sex will develop an attraction, on a genetic level, to characteristics of the advantageous traits. These attracted animals are then more likely to mate with the original animals, keeping the advantageous genetic trait and the genetic attraction to this trait within the same geneology. The result after multiple generations is that the attraction survives and stays in the DNA because it occurs more often in the animals that exhibit the initial trait that allows them to survive better. Ultimately, a genetic attraction to selectively advantageous traits in the opposite sex will be passed on through the generations, increasing the overall fitness of the species.
ReplyDeleteThe theory of sexual selection states that certain evolutionary traits can be explained by intraspecific competition, or competition where members of the same species fight for the same resource in an ecosystem. Darwin stated that sexual selection is the "struggle between the individuals of one sex, generally the males, for the possession of the other sex." There are three different types of sexual selection. The first being male to male combat where the males fight each other for the same mate. Traits that are selected for male combat are called secondary sexual characteristics and they are body parts like horns and antlers. The second is called mate choice and this is usually where the female chooses her male mate, and finally there is sexual conflict. This occurs when the two sexes have different levels of fitness and strategies concerning reproduction so there is an evolutionary battle between males and females on who can become the most fit. Females prefer to mate with the male who displays a lot of and large external ornaments, and this does occur through genetic drift where males with the appropriate ornament are selected for.
ReplyDeleteSexual selection can be a subset of natural selection, but in natural selection, traits are promoted through natural selection and not by killing off those traits. Also, natural selection does not search for better ways to do something or for something to exist as, it just ends the production and life of those things that don't work well and are not efficient at a certain time. In natural selection, everything that an organism gains is simply accidental because all natural selection can do is to end the life of a trait. However, sexual selection does search for a good feature that is chosen by the organisms. These choices for strong features are chosen through competition, and this normally occurs between males when they are fighting for mates. Therefore, in natural selection, a slight variation of a trait or in an individual increases that organism's chance and likelihood of surviving and reproducing compared to an organism that does not have that trait. The term fitness simply describes the organism's ability to survive and reproduce, so an organism's sexual selection adds in the component of fitness that relates it to evolution. Compared to natural selection, sexual selection is a struggle between individuals of the same species who are fighting for a mate, while natural selection is not dependent on a struggle between individuals.
An example of sexual selection occurs in the fish family Syngnathidae, where the males are the more selective mate. This family of fish includes seahorses, pipefishes, weedy sea dragons, and leafy sea dragons. Therefore, instead of the female searching and making the male impress her, the male is the one who is looking to be impressed. The females do lay the eggs, but then surprisingly the male fertilizes that egg and then carries the egg during incubation. The men have a ventral pouch to carry the eggs in or they attach it to their tail in order to carry it.
A common Darwin example of natural selection is the changing beak in the finches of the Galapagos Islands. There are thirteen related species that differ in the shape of their beaks. The change in the beak occurred based on what that finch eats, so the different beak shapes evolved through natural selection. When a finch required to eat a different type of food because of the changing climate or location, then through evolution a knew beak would form because of the necessity of a new beak to survive and reproduce. And of course this took thousands of years to occur but it happened to the finches could live in many different parts of the Islands.
"Biologists view sexual selection as one of the most important forces in the evolution of animals" because sexual selection occurs by choice in these organisms. It allows for different mates to cross their DNA and to create a new offspring which in turn creates evolutionary change through natural selection, but first through sexual selection a base for fitness must be established and those who don't meet those requirements can no longer survive and reproduce. Sexual selection is the direct competition between organisms of the same species for mates. This force of competition creates the diversity that species show which in the long run creates evolution and the moving forward of organisms so that they can be more fit and able to reproduce.
http://www.biology-online.org/2/10_natural_selection.htm
http://www.redemptionmud.com/crystal/nat_vs_sex.htm
http://www.aquaticape.org/sexualselection.html
http://en.wikipedia.org/wiki/Sexual_selection
Carroll introduces the discussion of sexual selection under the concept of survival being directly related to mating success. He brings up the particular example explored by Darwin of the plumage patterns of male birds evolving as a trait to attract the opposite sex (the most obvious example being the elaborate feathering of the peacock tail). The simplest way to explain sexual selection is that sexual selection is a more specific branch of natural selection- just as kin selection is a particular type of evolving fitness.
ReplyDeleteThere are two general types of sexual selection. In one kind of sexual selection, members of one sex create a reproductive differential among themselves by competing for opportunities to mate. The winners out-reproduce the others, and natural selection occurs if the characteristics that determine winning are inherited. In the other kind of sexual selection, members of one sex create a reproductive differential in the other sex by preferring some individuals as mates. If the ones they prefer are genetically different, then natural selection is occurring.
In birds, the first form of sexual selection occurs when males compete for territories, as is obvious when those territories are on traditional mating grounds. Males that manage to acquire the best territories on a ground are known to get more chances to mate with females.
That second type of sexual selection, in which one sex chooses among potential mates, appears to be the most common type among birds. As evidence that such selection is widespread, consider the reversal of normal sexual differences in the ornamentation of some polyandrous birds. There, the male must choose among females, which, in turn, must be as alluring as possible. Consequently in polyandrous species the female is ordinarily more colorful -- it is her secondary sexual characteristics that are enhanced.
These two types of sexual selection are so strong that they may be the driving forces of speciation. Sexual selection may be the driving force of speciation in allopatry. The main theory has long been that diversification in allopatry is driven by random processes. However, some compelling studies have shown that selective processes based on habitat differences may play an important role. It has also been suggested that speciation through drift is much less common than previously thought. Sexual selection is very important in sympatry, but little is known about its role in allopatry. To shed light on this, scientists are studying diversification in a group of Pacific songbirds, the honeyeaters that exhibit variation in the two cues used in mate attraction, plumage color and song.
Using a combination of extensive fieldwork on Pacific islands and molecular lab work at the Center for Tropical Research (CTR), scientists are studying this hypothesis. They capture honeyeaters throughout their range, take DNA samples, and measure plumage and morphological characteristics, after which they are released unharmed. In addition, they record songs of males that are trying to attract females. Like human language, bird song consists of various elements that can be arranged in different orders to make up a song. These sounds are visualized by means of sonograms, and time and frequency parameters are measured. Plumage color and song, as expounded by both Darwin and Carroll, are products of selection. In contrast, neutral genetic markers, which do not code for any traits, evolve through random processes. These scientists are trying to assess whether the sexual signals are evolving faster than neutral genetic markers, which would imply a role for sexual selection. By relating remotely sensed climatic and environmental factors to divergence in sexual traits, the scientists can also investigate the influence of different habitats on speciation. In a second stage of the study, these scientists will go back to several divergent populations to test whether females prefer local males to foreign ones, which should be the case if sexual selection causes a reproductive barrier. If so, these populations are not only divergent, but new species are possibly evolving. Thus, this study provides a current example of the continual role of sexual selection in enhancing speciation.
In a recent study conducted by scientists of the University of Indiana, the scientists explored how natural and sexual selection cooperate and differentiate in the evolution of dimorphic coloration and ornamentation in agamid lizards. Both sexual selection and natural selection can influence the form of dimorphism in secondary sexual traits. The scientists used a comparative approach to examine the relative roles of sexual selection and natural selection in the evolution of sexually dimorphic coloration, dichromatism, and ornamentation in agamid lizards. Sexual dimorphism in head and body size were used as indirect indicators of sexual selection, and habitat type, or openness, as an index of natural selection. The scientists examined separately the dichromatism of body regions, “exposed to” and “concealed from” visual predators, because these body regions are likely to be subject to different selection pressures. Dichromatism of exposed body regions was significantly associated with habitat type: males were typically more colored than females in closed habitats. By contrast, dichromatism of concealed body regions and ornament dimorphism were positively associated with sexual size dimorphism. When they examined male and female ornamentation separately, however, both were positively associated with habitat openness in addition to snout vent length and head sexual size dimorphism. These results suggest that natural selection constrains the evolution of elaborate ornamentation in both sexes as well as sexual dichromatism of body regions exposed to visual predators. By contrast, dichromatism of concealed body regions and degree of ornament dimorphism appear to be driven to a greater degree by sexual selection. Thus, we see how although sexual selection is a branch of natural selection both work in cooperation/ competition to enhance the development of an organism.
Not only are obvious secondary sex characteristics influenced by the force of sexual selection, but also at the molecular level, the refinement of gametes is a product of the play between sexual and natural selection. The large amount of molecular data available for gamete recognition proteins of sea urchins, abalones and other marine invertebrates indicates that male gamete recognition proteins diverge rapidly between species whereas they are under strong stabilizing selection within species. Moreover, the rate of divergence seems not to be constant, but highest between closely related species. These observations can be explained by a single mechanism: intraspecific competition for fertilizations enlarges male gamete recognition protein variation and initiates speciation. After speciation, avoidance of interspecific competition for fertilizations drives the rapid evolution of male gamete recognition proteins. Thus, in order to enhance the fitness of a species in intraspecific competition, natural selection enhances the effectiveness of a gamete from the outward allure of sexual selection evolved characteristics.
Sources:
http://repositories.cdlib.org/postprints/94/
http://www.stanford.edu/group/stanfordbirds/text/essays/Sexual_Selection.html
http://www.environment.ucla.edu/ctr/research/projects/Role-of-sexual-selection-songbirds.pdf
http://www.rug.nl/biologie/onderzoek/onderzoekgroepen/theoreticalbiology/research/sympatricSpeciation
Sexual selection is a big part in the process of evolution. First, we need to distinguish between sexual selection and the natural selection. Natural selection happens as the nature actually picks which traits are going to be passed on to the next generation.
ReplyDeleteAn example of natural selection would be the peppered Moth. There are two forms of the peppered moth in England. One was light and the other one was darker. In the early 1800’s, most moths were the light form. The first dark from was reported in 1848 and the dark from increased drastically by 1895. The dark form of the moth occurred at a time of rapid industrialization which brought pollution which made the trees much darker. This is a classic example of natural selection which the change in environment brought the entire specie to change its traits.
On the other hand, Sexual selection happens as the one sex compete each other to mate with the other sex. This is very important in process of evolution because this ensures that better genes of one male are passed on to the next generation.
There are numerous examples of sexual selection. One example of sexual selection would be peacocks. Male peacocks have really heavily decorated feathers which seem like useless and dangerous because it exposes the peacocks to many different predators. However, male peacocks compete with each other to gain access to the female peacock. The fact that the male peacock had survived with such burdensome feathers, it shows that they are more adapted to the environment.
Natural selection or Sexual selection, they both favor the fittest. They both play important role in passing on the best traits of individual to next generation. Sexual selection is very interesting because no matter how fit an organism is, if he or she cant get a mate, there is no way that they can pass on their genes. Sexual selection is the ultimate test that each organism has to pass in order to pass on their terrific genes :D
source:http://oak.cats.ohiou.edu/~jm703496/es-ssbrd.html
http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/bio%20101/Bio%20101%20Lectures/Natural%20Selection/natural.htm
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ReplyDeleteFirst entering the biology classroom, an ever-present motto seems to be the importance “to survive and reproduce.” As one can clearly see it is not just to survive or to just reproduce, but the answer to almost everything, at least in the Ecology unit, was both to survive AND reproduce. Looking at this, one must take into consideration both natural selection and sexual selection when considering the process of evolution.
ReplyDeleteNatural selection occurs when a chance mutation allows for a slight advantage over the competition to survive in an environment. Examples of these would be the color of the melanic moth on page 53, which adapted to having a darker color in order to survive with the sue of camouflage in a changing environment of decreasing air quality. It is seen again in the white rump feathers of the pigeon, which allows it to distract prey just long enough to twirl into an escape and survive. Through this process of natural selection, these chance mutations that bring forth and advantage are able to survive at a higher rate than those that do not have this mutation. The next step in this process is that of sexual selection. If a carrier of an advantageous mutation were to survive against prey because of this advantage, but was unable to mate for other reasons, the mutation would never be able to be passed down through generations and this mutation may be lost forever stopping the process of evolution of this certain characteristic. This is why sexual selection is so important. The idea of reproducing is an essential part in allowing for the process of evolution to occur. By reproducing the advantageous adaptation is allowed to be passed on and go again through the natural selection, in which survival rate is increased and then again reproduction where the gene is passed on once again. This is why certain species must not only have the ability to survive in the environment they live in, but also have the ability to attract a mate in order to reproduce with. For many birds, such as the peacock, a highly elaborate tail is something that is favorable to mating. The peahen normally is attracted to a tail with a higher number of eyespots that are more colorful and larger in size. This is reasonable at the fact that these traits normally indicate a healthier mate and therefore healthier offspring. Experiments have shown that peacocks with more eyespots generally have offspring that are larger at birth and are better at surviving than those offspring of peacocks with fewer eyespots. In this way the peacock must not only be able to survive in the wild, but also e able to attract a mate through his tail in order to reproduce and pass on his traits to offspring. Although this stops at an extent, such as when the tail becomes too large that it inhibits movement or flight, or when color is too bright that new predators are attracted, the larger and more colorful eyespots is normally and advantage in the area of sexual selection.
A key difference in sexual selection and that of natural selection is that natural selection is the fitness relative to the entire population of the species while sexual selection is that fitness relative to those of the same sex of the species. Most animals of the same sex fight for a mate and with certain advantages they are able to pass on their traits. This is the case for the bighorn ram on page 255 whose long horns were used in attracting mates. The larger the horns the more likely that the male would attract a mate and have offspring. Although this has changed due to human interference, many other animals have certain distinguishable features that are used to attract mates and increase their chances for reproduction.
Apart from the fact that mating allows for reproduction and passing of certain traits, it also is the fundamental grounds where mutations may occur in an organism. When sexual intercourse occurs genes from two different sources come together to form one organism. In this time different combination of genes and characteristics can be expressed, offspring being completely different. This variation in a species allows for the chance of survival of at least a few organisms if endangering factors may arise in their ecosystem. This is helpful in the process of natural selection where a chance mutation allows for advantage over the species as a whole.
Both natural selection and sexual selection work hand in hand in order to allow evolution to take place. Natural selection brings forth characteristics that allow an organism to have advantage over the entire species for survival. The sexual selection allows for certain traits to be passed that increase chances of reproduction. In both cases, the traits that are passed along increase the chances that an organism will “survive and reproduce.”
http://www.pbs.org/wgbh/evolution/library/01/6/l_016_09.html
http://www.pbs.org/wgbh/evolution/library/glossary/glossary.html#sexual_selection
http://evolution.berkeley.edu/evosite/evo101/IIIE3Sexualselection.shtml
Sexual selection is one of the most important forces in the evolution of animals mainly because of the role of genetic diversity in sexual reproduction. In sexual reproduction, half of the genetic information comes from one parent and the other half from the other parent of the animal. Genetic diversity within a species allows for the appearance of more selective advantages, and the crossing over during meiosis causes a greater amount of mutations, enhancing natural selection. The importance of sexual selection comes from the fact that it causes males with more selective advantages to appear more attractive to females and vice versa. The end result is that the "fittest" of the species mate and produce the most selectively advantageous offspring. Traits that are advantageous survive throughout the offspring, so members of the opposite sex will develop an attraction, on a genetic level, to characteristics of the advantageous traits. These attracted animals are then more likely to mate with the original animals, keeping the advantageous genetic trait and the genetic attraction to this trait within the same geneology. The result after multiple generations is that the attraction survives and stays in the DNA because it occurs more often in the animals that exhibit the initial trait that allows them to survive better. Ultimately, a genetic attraction to selectively advantageous traits in the opposite sex will be passed on through the generations, increasing the overall fitness of the species.
ReplyDeleteA crucial aspect of sexual selection is that it involves members of the same gender engaging in a genetic competition for the opportunity to mate. In a sense, sexual selection can be seen as a subset of natural selection, since it increases the chance of procreation in animals that display signs of evolutionary fitness. However, the typical view of natural selection is that the unfit are removed from the gene pool because of being relatively unfit, thus limiting future generations to the genetic information of the survivors - the fit. Sexual selection, on the other hand, only affects the animal's likelihood of procreating. Therefore, traits selected for through sexual selection may not affect an animal's chance of survival. The courtship among animals studied during the Ecology unit also ties in to sexual selection. In both, the animal tries to outwardly display its genetic fitness to the member of the opposite sex in the hopes of being chosen as a suitable mate. Because of this, it is quite probable that courtship rituals developed through the process of sexual selection, as opposed to the survival aspect of typical natural selection. This is important because courtship allows an animal to reveal its fitness, as a reflection of traits chosen through natural selection, but can also involve certain traits that have no affect on how well the animal can survive.
http://en.wikipedia.org/wiki/Sexual_selection
http://dienekes.50webs.com/blog/archives/000338.html
http://oak.cats.ohiou.edu/~jm703496/es-ssbrd.html