Reproductive Isolating Mechanisms Essay Example for Free

Reproductive Isolating Mechanisms Essay In the 1940s, Ernst Mayr coined the term Biological Species Concept that was subsequently widely embraced by the scientific community. The definition stated that Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. Certain mechanisms are in place to prevent species from interbreeding with others and these are referred to as reproductive isolating mechanisms, which are biological incompatibilities. There are many mechanisms acting on natural populations and these are broadly grouped into two categories namely prezygotic and postzygotic mechanisms. Prezygotic isolating mechanisms are those mechanisms that isolate species before fertilisation i.e. before a zygote is formed. They include geographical, ecological, temporal, ethological, mechanical, morphological and gametic isolation. These mechanisms lesson the possibility of gametes from different species coming into contact and hence forming a zygote. One prezygotic isolating mechanism is ecological isolation, also referred to as habitat isolation. Ecological isolation prevents different species that live in the same territory but different habitats from interbreeding. These species are referred to as sympatric species since they occur in the same territories. Individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences. An example of ecological isolation occurs within the Rana genus. R. grylio, the pig frog and R. areolata, the gopher frog both occur around New Orleans, Louisiana. The exceptionally aquatic pig frog lives in deep ponds, lakes and marshes amongst surfacing vegetation and breeds in deep water so has no contact with the gopher frog which lives in burrows during the day, and in the swamp margins at night and breeds in shallow water. This way the possibility of interbreeding between the two species is eliminated. Temporal isolation, also known as seasonal isolation is a prezygotic mechanism that prevents interbreeding between species. Temporal isolation is the isolation of species by changing the time at which they release gametes.  A particularly strong example of temporal isolation occurs in three tropical orchid species of the genus Dendrobium. The species only flower for a single day, opening at dawn and withering by nightfall. The flowering of each species is in response to the same environmental stimuli, such as a sudden storm on a hot day, but the lag time between the occurrence of the stimulus and the flowering is eight, nine, and ten or eleven in the different species. Since they flower for a single day, inter-species fertilization is made impossible because when one species has flowered the others are either not yet mature or already withered. Other examples of temporal isolation in natural populations are not as pronounced such as in cases were species are isolated due to breeding during different seasons like Sciurus carolinensis, the gray squirrel which mates in July and August; and Sc. niger, the fox squirrel which breeds in May and June. Another prezygotic isolating mechanism is ethological isolation or behavioural isolation. This mechanism prevents species, mostly animals from interbreeding based on their different behavioural patterns. It affects animals that occur within the same territory and habitat but with different mating behaviours. In most animals, mates are chosen in a species-specific approach often after species-specific mating rituals of some form. Matings follow these rituals which can be ended if at least one of the mating parties decides that the process leading to the mating is not as expected. Ethological isolation can be strong reproductive isolating mechanism in animals also between closely related species. An effective example of ethological isolation occurs in certain frog species of the Hyla species. H. versicolor, the gray tree frog and the closely related H. femoralis, pine wood tree frog often breed in the same ponds. Both species are physically very similar but their male mating calls, which last about three seconds and sound the same to humans can be differentiated by the female tree frogs and thus insures species-specific mating. This is also the case when considering why dogs and wolves dont mate as frequently as expected because of their different behavioural patterns. Mechanical isolation is a mechanism that prevents copulation between different animal species because of incompatible shape and size of the  genitalia. This occurs in species are sympatric and live in a common habitat and have overlapping breeding seasons without any ethological isolating mechanisms. In plants, variation in flower structure can inhibit cross species pollination. In California, two sage species, namely Salvia mellifera and S. apiana exhibit this form of prezygotic isolation. Two- lipped S. mellifera has stamens and style in their upper lip, whereas S. apiana has long stamens and style and a specialized floral configuration. Small and medium sized bees that carry pollen on their backs pollinate the two-lipped flowers, and large carpenter and bumble bees that carry pollen on their wings and other body parts pollinate S. apiana. This mechanism thus ensures that pollen cannot be transferred between the species, as only the corresponding pollinators are able to transfer pollen to the style of each flower. Another prezygotic isolation mechanism is morphological isolation, which in some cases overlaps with mechanical isolation mechanisms. This isolating mechanism prevents mating due to differences in size and shape between species. Morphological isolation prevents the spread of genes between the oak toad, Bufo quercicus and the Gulf Coast toad, Bufo vallicpes due to the size variation between the two species. The female oak toad has a maximum length of approximately 3 centimetres whereas the smallest Gulf Coast males are about twice as long. This way the size differences between the spaces make copulation extremely unlikely as male oak toads are too small to grasp the female Gulf Coast toad and male Gulf Coast toads are large enough to, and generally do, eat the female oak toads. The last prezygotic isolating mechanism is gametic isolation also known as gametic mortality. Gametic isolation is the mechanism in which fertilization cannot occur between species generally due to chemical incompatibilities between their gametes. In animals that practice internal fertilization, sperm may not be able to survive in the females sperm receptacles. Some plant species, pollen grains of one species usually cannot germinate on the stigma of another species thus preventing fertilization between species. Since many aquatic animals disperse their ova and sperm into the water, gametesof different species do not have affinities for each other. This was  demonstrated between the sea urchins Stronglocentrotus purpuratus and S. franciscanus when after induction of simultaneous realease of eggs and sperm, all resultant fertilizations were between eggs and sperms of the same species. Postzygotic isolation mechanisms are those reproductive isolating mechanisms, which are effective after the union of gametes of different species i.e. after fertilisation. Postzygotic isolating mechanisms include hybrid inviability, hybrid sterility and hybrid breakdown. Postzygotic isolating mechanisms reduce the viability or fertility of hybrids or their progeny. These mechanisms come into play when hybridzygotes are formed either naturally or unnaturally. Hybrid inviability is the postzygotic isolating mechanism that occurs between fertilisation and birth. This mechanism prevents the full development of zygotes and thus leads to their subsequent death. An example in animals is the death of artificially created sheep-goat hybrid embryos in early developmental stages before birth. Hybrid inviability also occurs in plants, most commonly resulting in hybrid seeds either failing to germinate or dying shortly after germination. Hybrid sterility or hybrid infertility is a postzygotic reproductive isolating mechanism which affects hybrids that survive to adulthood. This mechanism creates sterile hybrids i.e. hybrids that cannot produce offspring. The most well known example is the case of the mule. When a horse and a donkey mate they are capable of producing offspring called mules. But mules are sterile, leaving them out of the gene pool thus maintaining species integrity. Hybrid breakdown is the last postzygotic isolating mechanism in place to maintain species differentiation. It occurs in cases of crossbreeding were hybrids of inter species matings are viable and fertile. Hybrid breakdown occurs commonly and results in the hybrids offspring or the F2 generation being unviable or significantly weaker and unable to contribute genes to a next generation. Certain cotton plants show this form of reproductive  isolation such as Gossypium barbadense, G. hirsutum and G. tomentosum which produce hybrids that seem viable and fertile, but their offspring die either as seeds, early during development or develop into weaker plants unable to compete and reproduce. Reproductive isolation combines traits that reduce gene flow, such as mate choice or fertilization barriers, with traits that select against genes that have flowed, such as hybrid incompatibility. Reproductive isolating mechanisms are ultimately mechanisms evolved in species to prevent interbreeding with other species and thus allow for scientists to differentiate between species using the biological species concept. Bibliography Futuyma, DJ. 1998. Evolutionary biology, 3rd ed., Sinauer Assoc. Inc., USA Griffiths, A.J.F., Miller, J.H. Suzuki, D.T., Lewontin, R.C. and Gelbart, W.M. 1999. Introduction to Genetic Analysis. W.H Freeman Co. New York. Hale, W. and Margham, J. 1988. Collins Dictionary of Biology. Collins, UK. Mayr, E. 1942 Systematics and the Origin of Species. Columbia University Press, New York, USA. Mayr, E. (1970) Populations, Species, and Evolution. Harvard University Press, Cambridge, Massachusetts, USA. Starr, C. and Taggart, R. (2001). Biology, The Unity and Diversity of Life, 9th ed. Brookes/ Cole, USA. www.abacus.gene.ucl.ac.uk/jim/Sp/isolmech.html www.library.thinkquest.org/ 19926/java/library/article/17a.ht

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Religions of India India is the second largest populated country in the world, and for being only the seventh largest country India populates over 1.2 billion people. Holding 1/7th of the world’s population, India has quickly become the largest populous country in the world. India therefore is home to many different religions with the most popular including: Buddhism, Jainism and Islam. The development of these three religions has shaped India into a wonderful multi-cultural nation. Buddhism is now considered a world religion, with excess of over 708 million followers. Buddha, which can also be translated to the enlightened one, was known as a man named Gautama Sidhatha, and was Son of Suddhodana. He lived in a small village on the border of India and Nepal around the time period of 563 BCE. The facts and knowledge of Buddha’s life is represented in a book known as Book of Great Decease. This book was written about 200 years after Buddha’s death and has been passed down over ma ny generations. The Book of Great Decease was written with both fact and fiction mixed together. When his mother became pregnant she experienced a dream where an elephant and a lotus flower were lying by her side. She had this dream interpreted and the results came back explaining that she was to have a baby boy and that he would become a universal emperor or a universal teacher. As a child Buddha grew up learning all of the skills appropriate for a prince and later became a Kshatriya. Gautama (Buddha) showed off his excellent skills and qualities including his intelligence and strength. He later married but his father, however, was concerned that he would not become a universal teacher if he underwent any kind of misery. Gautama led a normal life but o... ...gious freedom to all Hindus and Buddhists under his control.† This source signifies that the Muslim invasion was not one of making people convert, it was a decision that people could make if they chose. Nothing was forced upon the people of India. As a result the spread for Islam throughout India happened very quickly and easily. Many of the people form India saw Islam as a way of becoming a part of a bigger organization and many Muslims were able to achieve higher social status than those of Buddhists or Jains. Subsequently, India became a dominant Muslim country with the help of political leaders such as Mahmud of Ghanzi and Muhammad Tughaluq who helped drive the Islamic expansion in India without altering, changing, or conflicting with the Indian culture. Due to the expansion of Islam through India other religions quickly declined, including the fall of Buddhism

In Miltons paradise lost, god Essay

In Milton’s Paradise Lost, God is portrayed as having limited influence and contact with our world. This is perhaps a result of his respect for free will/conscience. This lack of contact is supported by one; God’s passiveness, there are several situations in the book in which God seems like he should be able to influence events but he simply doesn’t act. When he does act, he acts indirectly. God seems to execute his plans through either his angels or his son. Finally, perhaps the best indication of God’s limited connections is in the cases where God uses complicated, elaborate plans to do things that if he really had 100% power he would perform simply and immaculately. In the book Paradise Lost, God plays a relatively passive role considering that he is by far the most significant character in this book. He seems to sit up on his heavenly throne and observes rather than interact with his creations. A good case of this is in Book three lines 80-90, when God watches Satan ascending from hell. It would seem that when he was alerted by Uriel, the archangel would have been a good time to intervene and smite down Satan. It almost seems like Milton’s God wants the events of Paradise Lost to transpire because he yields so many times at so many opportunities to stop Satan. Satan should have been stopped at the very beginning. God must have foreseen this incident (the partaking of the forbidden fruit,) after all, does he not have sight of the future, past and present? (Book Three lines 75-80) Sadly, no-one will ever know what God was planning when he allowed Satan to run rampant in the garden. Or then again, maybe God wasn’t planning anything at all but rather leaving events to unfold without divine intervention, thus his seeming respect for free will. The only problem with that theory though, is that God punished Adam and Eve for making a decision with the free will that he gave them. Two other instances make cases against God’s absolute power. Why was hell so easily escaped by Satan? One would think that a Purgatory created by God himself would be impenetrable. Even stranger is the case with the Angelic War. Although for the most part symbolic, God either was not capable or at least unwilling to strike down Satan’s attempted conquest before it began. Instead, God chose once again to remain passive and allow things to go along for awhile. A good question to ask at this point is just what are God’s intentions? If he truly wanted a perfect heaven with conformist angels, what is stopping him from taking their free will? That leads to the point that possibly the reason why God’s influence is limited is his own conscience, based on his respect of free will. When God does act in the story, it is almost exclusively indirectly through his Son, (as in Book Nine,) or through his Archangels. The most well-known case where God acts through his Son is when God sent him down to be sacrificed upon the cross. Although this specific event does not occur within this story, the reasoning behind it is lengthily discussed especially in Book Three. This however, only supports the thesis if one believes that Jesus is the son of God rather than the Christian view that God IS Jesus. (John, 8:58) Based on how Milton writes, it shows that he is using the interpretation of the Bible in which Jesus was created by God. Jesus plays a sympathetic role when it comes to mankind and often influences God’s decisions on what to do about Adam and Eve. He persuades God to allow him to go down to heaven to inform Adam about his state of sin. In this case, Jesus actually influences God rather than the usual case with God giving orders and Jesus acting carrying them out. God also acts a lot through his throng of angels. The archangels are his main instruments of manipulation. Of the seven archangels, Raphael, Michael, and Gabriel are the three most influential. God acts through Raphael most often. Raphael plays a large role in starting in Book Five lines 246-249 â€Å"So spake th’Etetnal Father, and fulfill’d All Justice: nor delay’d the winged Saint (Raphael)after his charge receiv’d. † Raphael then proceeds down to the Garden to warn Adam and Eve of the impending danger posed by Satan. Raphael also spends Book Six and Book Seven informing Adam of the war in heaven and then telling him the nature of his own creation. Michael and Gabriel have slightly smaller roles than Raphael does; but they do get a chance to enact God’s will when he orders them to lead the faithful Angels in the war against Satan. The final case to prove God’s limited interactions and influence is when God seems to have to work around rules that he must have created himself. A good example of this is when he prepares for the redemption of man. It simply doesn’t make sense that if God desires to redeem his creations that he simply doesn’t grant them redemption from their sins. Instead he schemes up the elaborate plan to send his son down to receive punishment in place of man. Once again, this could be explained by God having to act within the parameters of what his conscience will allow, (regarding free will). Maybe God has to do these elaborate things so that he can justify to himself the redemption of man. Maybe he thinks that it is only right that somebody receives punishment. Cases similar in nature occur when God didn’t keep Satan from entering the Garden of Eden and when he had to send the great flood. If not for his respect for free will, God wouldn’t have had to allow mankind to sink so low. In Paradise Lost, Milton presents a God that is strangely limited in his actions and influence with his own creations. Whether through passiveness, indirectness, or a conscious â€Å"distancing of himself† God seems to allow many things to happen without direct intervention. However, this is not really a novel concept; people throughout history have questioned the concept of an all-powerful God in a very imperfect world.

Importance Of Project Risk Management Project Management

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