Self-replication without a self-assembly code--论文代写范文精选

由临界阈值现象,虽然不同聚合物的数量成指数增加,反应的数量可以增加转化的速度。随着它们的多样性增加,一个子集的概率达到临界阈值。下面的paper代写范文进行论述。

Abstract
  If organisms evolve because they are self-replicating automata, and socially transmitted elements of culture are not self-replicating automata, must it be concluded that culture does not evolve? No, because, von Neumann self-replicating automata are not the only kind of self-replicating structure. Let us investigate a second, more primitive kind, and then see if there is something in culture that possesses it. Examining the earliest stages in the evolution of organic life can constrain the development of a realistic theory of the evolution of culture. Thus the topic of how life began is not a detour but a key step forward to a deeper understanding of evolution in general and culture in particular.

  Present day life replicates using a template, a coded set of instructions encoded in DNA or RNA for how to make a copy of itself. The probability of such a structure arising spontaneously is exceedingly small; Hoyle infamously compared it to the probability that a tornado blowing through a junkyard would assemble a Boeing 747 (Hoyle, 1981). The implausibility of the spontaneous appearance of a self-assembly code has led to skepticism toward replication-first theories of the origin of life, and the wide-spread acceptance of metabolism-first theories. According to the metabolism-first view, life began with a chemically isolated, self-organized set of simple collectively replicating molecules—for example through autocatalytic closure[7] of catalytic polymers—and genetically mediated template replication came afterward (Bollobas, 2001; Bollobas & Rasmussen, 1989; Dyson, 1982, 1985; Gabora, 2006; Kauffman, 1986, 1993; Morowitz, 1992; Wechtersheuser, 1992; Weber, 1998, 2000; Williams & Frausto da Silva, 1999, 2002; Vetsigian et al., 2006). Polymers catalyze reactions that generate other polymers, increasing their joint complexity, until together as a whole they form a protocell that can more or less replicate itself. 

  Due to criticality and threshold phenomena in random graphs, though the number of different polymers increases exponentially, the number of reactions by which they can interconvert increases faster than their total number (Cohen, 1988; Erds & Rnyi, 1960). Thus, as their diversity increases, so does the probability that some subset of the total reaches a critical threshold where there is a catalytic pathway to every member. The set may be said to be autocatalytically closed because although no polymer catalyzes its own replication, each catalyzes the replication of another member of the set. So long as each polymer is getting duplicated somewhere in the set, eventually multiple copies of all polymers exist.

  At least some subset of the polymers spontaneously adhere to one another, forming a spherical vesicle that encloses the polymer set. Such a structure is prone to fission or budding, where part of the vesicle pinches off and it divides in two. Replication is far from perfect, thus offspring are unlikely to be identical to parent. But so long as there is at least one copy of each polymer in each of the two resulting vesicles, they can self-replicate, and continue to do so indefinitely, or until their structure changes drastically enough that self-replication capacity breaks down. Replication is sloppier and more haphazard than a von Neumann self-replicating automaton, but nonetheless gets the job done.

  Note that the resulting structure is autopoietic because the parts collectively reconstitute the whole, but it happens in a piecemeal manner, through bottom-up interactions rather than top-down interpretation of a genetic code. Therefore replication occurs with low fidelity, and there is nothing to prohibit the inheritance of acquired characteristics. A change to one polymer persists after fission occurs, and this could cause other changes that have a significant effect on the lineage further downstream. Evolution of these early life forms has been described as a non-Darwinian process taking place through horizontal exchange (i.e. not restricted to vertical transmission from parent to offspring) of  innovation-sharing protocols (Vetsigian et al., 2006). It was not until the genetic code came along—and the process in which self-assembly instructions are copied (meiosis) became distinct from developmental processes—that acquired characteristics could no longer be passed on to the next generation (Gabora, 2006).(paper代写)

  Thus we have two kinds of self-replication (Gabora, 2004). Coded self-replication, such as is seen in present-day organisms, uses self-assembly instructions as proposed by von Neumann. This ensures they replicate with high fidelity, and acquired characteristics are not inherited. Uncoded self-replication, such as is seen in protocells, involves autopoiesis. This is a low fidelity means of replication, and there is nothing to prohibit inheritance of acquired characteristics. Note that it is often said that because acquired traits are inherited in culture, culture cannot be described in evolutionary terms. It is ironic that this is also true of the earliest stage of biological life itself.(paper代写)

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