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The tangled bank hypothesis is an Evolutionary theory that posits that out of a great variety of offspring produced by the Parent ( Animal or Plant or Bacteria ) at least a few will survive the hardships of their struggle for existence. On 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 a “tangled bank”, in 1982 Graham Bell presented his Tangled Bank Theory in which he suggests that the reason Sex developed was to more effectively prepare the offspring for survival in their environment. With other words, sexual groups may be more able to exploit more living places than 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 deficiencies is that it still does not explain how Asexual species developed sexes. 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 reproduction . In addition, the evidence from fossils suggests that species go for vast periods of [geologic] time without changing much" This statement fully matches the concept of Stasis , the non-changing phenomena of the species for very long time period. Recently a discovery was published in the journal ''Nature'' by Teresa Pawlowska and John Taylor, biologists from the University Of California At Berkeley , about asexual reproduction of Arbuscular mychorrhizal (AM) Fungi . They found evidence that the nuclei in AM fungi are identical with what is typical for asexual reproduction. 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, this seems not to be the case with AM fungi on which no any sex organs were found. According to the fossil records dating back to the Ordovician Period (460 million year s ago) they look just like the modern species. Margulis and Sagan remarked that bacteria supposed to be the origins of sexual production but still remained unchanged for millions of years. 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. Rocks containing bacteria 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 Lake s, Hot Spring s, and Hydrothermal Vent s 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? |
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