|
The thrifty gene hypothesis is an attempt to explain why people from some populations are prone to diabetes. The geneticist James V. Neel proposed the hypothesis, in 1962, to resolve a fundamental problem: diabetes is clearly a very harmful medical condition, yet it is quite common, and it was already evident to Neel that it likely had a strong genetic basis. The problem is to understand how disease with a likely genetic component and with such negative effects may have been favoured by the process of natural selection. Neel suggested the resolution to this problem is that genes which predispose to diabetes (called 'thrifty genes') were historically advantageous, but they became detrimental in the modern world. In his words they were "rendered detrimental by 'progress'". Neel's primary interest was in diabetes, but the idea was soon expanded to also encompass obesity. Thrifty genes are genes which enable individuals to efficiently collect and process food to deposit fat during periods of food abundance in order to provide for periods of food shortage (feast and famine). According to the hypothesis, the 'thrifty' genotype would have been advantageous for hunter-gatherer populations, especially child-bearing women, because it would allow them to fatten more quickly during times of abundance. Fatter individuals carrying the thrifty genes would thus better survive times of food scarcity. However, in modern societies with a constant abundance of food, this genotype efficiently prepares individuals for a famine that never comes. The result of this mismatch between the environment in which the brain evolved and the environment of today is a widespread chronic obesity and related health problems like diabetes. The hypothesis has received various criticisms and several modified or alternative hypotheses have been proposed. ==Hypothesis and research by Neel== James Neel, a professor of Human Genetics at the University of Michigan Medical School, proposed the "thrifty genotype" hypothesis in 1962 in his paper "Diabetes Mellitus: A 'Thrifty' Genotype Rendered Detrimental by 'Progress'?" Neel intended the paper to provoke further contemplation and research on the possible evolutionary and genetic causes of diabetes among populations that had only recently come into regular contact with Westerners. The genetic paradox Neel sought to address was this: diabetes conferred a significant reproductive (and thus evolutionary) disadvantage to anyone who had it; yet the populations Neel studied had diabetes in such high frequencies that a genetic predisposition to develop diabetes seemed plausible. Neel sought to unravel the mystery of why genes that promote diabetes had not been naturally-selected out of the population's gene pool.〔Neel 1962, p.359〕 Neel proposed that a genetic predisposition to develop diabetes was adaptive to the feast and famine cycles of paleolithic human existence, allowing humans to fatten rapidly and profoundly during times of feast in order that they might better survive during times of famine. This would have been advantageous then but not in the current environment.〔Neel 1962〕 The hypothesis was proposed before there was a clear distinction between the different types of diabetes. Neel later stated that the hypothesis applied to non-insulin-dependent diabetes mellitus. In its original form the theory more specifically stated that diabetes may be due to a rapid insulin response which would prevent loss of glucose from the urine. Furthermore, it made use of a then popular theory which was later disproven. This argued that specific insulin antagonists were released in response to insulin with this causing diabetes. In the decades following the publications of his first paper on the "thrifty genotype" hypothesis, Neel researched the frequency of diabetes and (increasingly) obesity in a number of other populations and sought out observations that might disprove or discount his "thrifty gene" hypothesis. Neel's further investigations cast doubt on the "thrifty genotype" hypothesis. If a propensity to develop diabetes were an evolutionary adaptation, then diabetes would have been a disease of long standing in those populations currently experiencing a high frequency of diabetes. However, Neel found no evidence of diabetes among these populations earlier in the century.〔Neel, J.V. 1982. "The Thrifty Genotype Revisited." In ''The Genetics of Diabetes Mellitus'', ed. J. Kobberling and R. Tattersall. New York: Academic Press, 293-93.〕 And when he tested younger members of these populations for glucose intolerance - which might have indicated a predisposition for diabetes - he found none. In 1989, Neel published a review of his further research based on the "thrifty genotype" hypothesis and in the Introduction noted the following: "The data on which that (rather soft) hypothesis was based has now largely collapsed." However, Neel argued that "...the concept of a "thrifty genotype" remains as viable as when first advanced...". He went on to advance that the thrifty genotype concept be thought of in the context of a "compromised" genotype that affects several other metabolically-related diseases. Neel in a 1998 review described an expanded form of the original hypothesis, diabetes being caused by "thrifty genes" adapted specifically for intermittent starvation, to a more complex theory of several related diseases such as diabetes, obesity, and hypertension (see also metabolic syndrome) being caused by physiological systems adapted for an older environment being pushed beyond their limits by environmental changes. Thus, one possible remedy for these diseases is changing diet and exercise activity to more closely reflect that of the ancestral environment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thrifty gene hypothesis」の詳細全文を読む スポンサード リンク
|