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Evolution of Sexes

Evolution of Sexes


Parthenogenesis An interesting unsolvable puzzle to the evolutionists to explain is the phenomena of parthenogenesis. The origin of the word are the Greek words "parthenos" (virgin) + "genesis" (generation) what all-together means reproduction of usually female gamete (egg) without pollination or fertilization. Parthenogenesis mostly occurs in lower plants and according to statistics of all plants 80% display some form of asexual reproduction and approximately 50% are mainly or exclusively asexual. Amazingly the offspring is genetically or with other words in all inherited respects identical to the mother plant having e.g. egg with an unreduced chromosome number. Now, the puzzle in all this is that how did evolution continued at all from this kind of plants if these species cannot reshuffle genes to produce subspecies variations? With other words, how evolution developed without meiosis or the absence of cell division that results in two daughter cells each with half the chromosome number of the parent cell. If this could not somehow happen that means that practically there was no possibility of development of sexes. Actually, the discussion about how the first fully functional female and the first fully functional male developed to begin the process of reproduction, is not the favorite topic of discussion in the circle of the evolutionists. Graham Bell in his book, The Masterpiece of Nature: The Evolution of Genetics and Sexuality, describes the problem in the following way: "Sex is the queen of problems in evolutionary biology. Perhaps no other natural phenomenon has aroused so much interest; certainly none has sowed as much confusion. The insights of Darwin and Mendel, which have illuminated so many mysteries, have so far failed to shed more than a dim and wavering light on the central mystery of sexuality, emphasizing its obscurity by its very isolation." In this regard Dobzhansky and his colleagues made an interesting remark: "With respect to the origin of sexual reproduction, two challenging questions present themselves. First, in what kinds of organisms did sex first arise? And second, what was the adaptive advantage that caused sexual reproduction to become predominant in higher organisms?" Now in the following section of this essay we would like to describe briefly the how different evolutionary theories attempt to solve the above-mentioned problems but of course ultimately without any success. The Lottery Principle Although this theory doesn't give the explanation how asexual living entities developed into sexual ones, still we like to mention it because of its attempt to explain sexual reproduction as better then asexual. It was in 1975 that George C. Williams an emeritus professor of biology at the Princeton University of New Jersey for the first time suggested an idea that because sexual reproduction results in greater genetic variety it more successfully enables the species to survive in changing novel environments. To describe this idea he compared the asexual breeding to purchasing of many lottery tickets but all with the same numbers; and sexual reproduction to purchasing of a few lottery tickets with different numbers. The point of this analogy is that since asexual reproduction does not produce genetic variations but rather the offspring are the exact duplicates of their parents, there is a little chance for them to quickly adept to the changing environmental circumstances and thus successfully survive. This idea was also described by Carl Zimmer in his book Parasite Rex: "A line of clones might do well enough in a forest, but what if that forest changed over a few centuries to a prairie? Sex brought the variations that could allow organisms to survive change." Looking more closely the species in the variable and non-variable surroundings it is clearly observed that in static environmental conditions e.g. in the tropics sexual reproduction is the most common contrary to the unstable environment where asexual reproduction is prevailing. Conclusively, it turns out that genetic variation gained through sexual reproduction is not necessarily crucial for the survival of the living entity in changing environments. Thus obviously, the lottery theory fails as an explanation for the possibility of sexual reproduction being more beneficial then asexual. The Tangled Bank Hypothesis Briefly this theory assumes that out of a great variety of offspring produced by the parent (animal or plant or bacteria) at least few will survive the hardships in their struggle for existence. About this, Darwin In his book Origin of the Species gives the following description: "It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us" The expression "tangled bank" originated from this last paragraph of Darwin's book. Inspired by this description of many creatures striving to gain sufficient light and food on "tangled bank", in 1982 Graham Bell made up his Tangled Bank theory in which he suggests that the reason sex developed was to more effectively prepare the offspring for the survival of the life's hardships. With other words, sexual groups may be more capable to exploit more living places then the parthogenetic species. This circumstance was also nicely pictured by Carl Zimmer: "In any environment--a tidal flat, a forest canopy, a deep-sea hydrothermal vent--the space is divided into different niches where different skills are needed for survival. A clone specialized for one niche can give birth only to offspring that can also handle the same niche. But sex shuffles the genetic deck and deals the offspring different hands. It's basically spreading out progeny so that they're using different resources." Although this theory looks quite logical, one of it's deficiency is that it still does not explain any steps how asexual species developed sexes. Beyond this there are many other problems this theory cannot solve and answer. In his book, Evolution and Human Behavior, John Cartwright comments: "Although once popular, the tangled bank hypothesis now seems to face many problems, and former adherents are falling away. The theory would predict a greater interest in sex among animals that produce lots of small offspring that compete with each other. In fact, sex is invariably associated with organisms that produce a few large offspring, whereas organisms producing small offspring frequently engage in parthenogenesis [asexual reproduction]. In addition, the evidence from fossils suggests that species go for vast periods of [geologic] time without changing much." [emp. added] This statement fully matches the concept of stasis, which was explained before to be non-changing phenomena of the species for very long time period. There are quite some examples for this and we will mention them in other place but still lets see two interesting ones. Recently an enlightening discovery was published in the journal Nature by Teresa Pawlowska and John Taylor, the biologists of the University of California at Berkeley, about asexual reproduction of Arbuscular mychorrhizal (AM) fungi. They found an evidence that the nuclei in AM fungi are identical what is typical for asexual reproduction. Further, although the present scientific opinion holds that regarding the capability to obtain nutrients and to tolerate diseases, sexual reproduction is more beneficial for long-term survival of species still, this seems not to be the case with (AM) fungi on which no any sex organs were found by any scientists. According to the fossil records dating back to the Ordovician period (460 million year before) they look just like the modern species what means they could successfully survive. Another examples are the bacteria that underwent almost no change. Margulis and Sagan remarked that bacteria supposed to be the origins of sexual production but still remained unchanged for millions of years. Here we can note that asexual reproduction is in many ways more advantageous then sexual e.g. it allows beneficial combinations of characteristics to continue unchanged and eliminates the often-vulnerable stages of early embryonic growth. It is found in most plants, bacteria, protists (free-living or colonial organisms) and the lower invertebrates. But to continue with bacteria, there are rocks containing bacteria that have been dated at 3.5 billion years old and they really defeat the tangled bank theory because e.g. the archaebacteria can survive the very harsh environments such as salt lakes, hot springs, and hydrothermal vents at the bottom of the ocean. Beyond this, there are many other organisms with asexual reproduction system and their existence naturally raise a question, why they did not change to sexual type of reproduction. Especially, don't they need to prepare their offspring for the life's hardships in this world - how it is explained in the Tangled Bank Hypothesis? For more information about similar topics visit "The Hare Krishna Views On Science" website at krishnascience krishnascience
Nitai is the member of the Bhaktivedanta Research Institute Amsterdam - Netherlads. In his research, the main inspiration is from the Vedic literature that was impressive to many thinkers like Einstein, Eugen Wigner, Brian Josepson etc.

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