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The more that is discovered about the intricate organization of the nervous system, the more it seems remarkable that genes can successfully specify the development of that system. Human genes contain too little information even to specify which hemisphere of the brain each of a human's 10'' neurons should occupy, let alone the hundreds of connections that each neuron makes.For such reasons, we can assume that there must be an important random factor in neural development, and in particular, that errors must and do occur in the development of all normal brains.
The most vivid expression of such errors occurs in genetically identical (isogenic) organisms. Even when reared under the same conditions, isogenic organisms are rarely exact copies of one another, and their differences have revealed much about the random variations that result from an organism's limited supply of genetic information. In isogenic Daphniae, for example, even though the position, size, and branching pattern of each optic neuron are remarkably constant, there is some variability in connectivity, and the number of synapses varies greatly. This variability is probably the result of random scatter beyond the resolution of genetic control and is best termed "imprecision," since its converse, the degree of clustering about a mean, is conventionally, called "precision."
Imprecision should be distinguished from developmental mistakes: wrongly migrated neurons, incorrect connections, and the like. To use a computer analogy, minor, rounding-off errors occur universally and are analogous to imprecision, but occasionally a binary digit is incorrectly transmitted, perhaps ruining a calculation, and this incorrect transmission is analogous to a developmental mistake. Thus, imprecision is a form of inaccuracy inherent within the limits of design, but mistakes are forms of gross fallibility.
Both impression and gross fallibility can plausibly be blamed on the insufficiency of genetic information, since either could be reduced by adding more information. It is universally accepted among information theorists that codes and languages can be made mistake-resistant by incorporating redundancy. However, since the amount of space available in any information system is limited, increased redundancy results in decreased precision. For example, π when written incorrectly in English, "three point oen four two, "can be understood correctly even though a typographical error has occurred. More precision could be gained, however, if those 24 spaces were filled with Arabic numerals; then π could be expressed to 23 significant digits, although any error would significantly change the meaning. There exists a trade-off, the more precisely a system is specified, using a given limited amount of information, the greater the danger of gross mistakes. The overall scheme by which genetic information is rationed out in organisms, therefore, must involve a compromise between two conflicting priorities: precision and the avoidance of gross mistakes.
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[阅读&逻辑 ]新版冲分救命题 -11894
Select the sentence that presents a conventional requirement that is rejected by the author of the passage.
[阅读&逻辑 ]精选题 -11861
Which of the following, if true of the cave, most weakens the argument?
[阅读&逻辑 ]精选题 -11860
It can be inferred that the “sediments
[阅读&逻辑 ]精选题 -11859
Which of the following best describes the issue raised by the question in the highlighted portion of the passage?
[阅读&逻辑 ]精选题 -11858
It can be inferred that which of the following is true of the “geologic evidence”?
[阅读&逻辑 ]精选题 -11857
Which of the following elements in the debate over the origin of Varanus is NOT provided in the passage?
[阅读&逻辑 ]精选题 -11856
It can be inferred from the passage that the geographical distribution of the Komodo dragon is
[阅读&逻辑 ]精选题 -11855
Which of the following, if discovered to be true, would most tend to suggest that the “correlation” mentioned in the passage held true for the people of the West Branch site?
[阅读&逻辑 ]精选题 -11854
The final sentence of the passage primarily serves to do which of the following?
[阅读&逻辑 ]精选题 -11853
The passage implies which of the following about beavers?
[阅读&逻辑 ]精选题 -11852
The passage provides support for which of the following statements about beaver dams?
[阅读&逻辑 ]精选题 -11851
According to the passage, which of the following best accounts for the apparent bias in antelope-migration direction?
[阅读&逻辑 ]精选题 -11850
The author of the passage implies that during the
[阅读&逻辑 ]精选题 -11849
According to the passage, which of the following is true of an Eurasia-Africa land bridge during ice ages?
[填空 ]精选题 -11843
The political upheaval caught most people by surprise: despite the ______ warning of some commentators, it had never seemed that imminent.
[填空 ]精选题 -11842
Even if the story now seems a surprisingly innocuous overture to the author’s later, more fully developed narrations, it ____ some of the key traits of those bleaker tales.
[填空 ]精选题 -11841
William Perkins, his ______ speaking style notwithstanding, has long been seen as the moderate face of his political party.
[填空 ]精选题 -11840
Just because, as a photographer, Friedlander (i)____ places that most people consider ugly does not mean that he is out to prove they are beautiful. Instead, his work suggests that the photographer simply cannot ignore so much of the built American landscape but is obligated to (ii)_____ what we pass through day in and day out, regardless of (iii)_____.
[填空 ]精选题 -11839
In the search for truth, the knowledge gained by scientists consists of approximations with varying degrees of certainty. Such (i)_____ truth can be highly (ii)_____, as the rapid and relatively steady progress of medical knowledge well exemplifies.
[填空 ]精选题 -11838
History teaches us that science is not _________ enterprise, indeed, it is quite the opposite, a motley assortment of tools designed to safeguard researchers against their own biases.
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