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The evolution of sex ratios has produced, in most plants and animals with separate sexes, approximately equal numbers of males and females. Why should this be so Two main kinds of answers have been offered. One is couched in terms of advantage to population. It is argued that the sex ratio will evolve so as to maximize the number of meetings between individuals of the opposite sex. This is essentially a "group selection" argument. The other, and in my view correct, type of answer was first put forward by Fisher in 1930. This "genetic" argument starts from the assumption that genes can influence the relative numbers of male and female offspring produced by an individual carrying the genes. That sex ratio will be favored which maximizes the number of descendants an individual will have and hence the number of gene copies transmitted. Suppose that the population consisted mostly of females, then an individual who produced sons only would have more grandchildren. In contrast, if the population consisted mostly of males, it would pay to have daughters. If, however, the population consisted of equal numbers of males and females, sons and daughters would be equally valuable. Thus a one-to-one sex ratio is the only stable ratio; it is an "evolutionarily stable strategy." Although Fisher wrote before the mathematical theory of games had been developed, his theory incorporates the essential feature of a game that the best strategy to adopt depends on what others are doing. Since Fisher’s time, it has been realized that genes can sometimes influence the chromosome or gamete in which they find themselves so that the gamete will be more likely to participate in fertilization. If such a gene occurs on a sex-determining (X or Y) chromosome, then highly aberrant sex ratios can occur. But more immediately relevant to game theory are the sex ratios in certain parasitic wasp species that have a large excess of females. In these species, fertilized eggs develop into females and unfertilized eggs into males. A female stores sperm and can determine the sex of each egg she lays by fertilizing it or leaving it unfertilized. By Fisher’s argument, it should still pay a female to produce equal numbers of sons and daughters. Hamilton, noting that the eggs develop within their host—the larva of another insect—and that the newly emerged adult wasps mate immediately and disperse, offered a remarkably cogent analysis. Since only one female usually eggs in a given larva, it would pay her to produce one male only, because this one could fertilize all his sisters on emergence. Like Fisher, Hamilton looked for an evolutionarily stable strategy, but he went a step further in recognizing that he was looking for a strategy. According to the passage, successful game strategy depends on ______.
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As one works with color in a practical or experimental way, one is impressed by two apparently unrelated facts. Color as seen is a mobile changeable thing depending to a large extent on the relationship of the color to other colors seen simultaneously. It is not fixed in its relation to the direct stimulus which creates it. On the other hand, the properties of surfaces that give rise to color do not seem to change greatly under a wide variety of illumination colors, usually (but not always) looking much the same in artificial light as in daylight. Both of these effects seem to be due in large part to the mechanism of color adaptation mentioned earlier. When the eye is fixed on a colored area, there is an immediate readjustment of the sensitivity of the eye to color in and around the area viewed. This readjustment does not immediately affect the color seen but usually does affect the next area to which the gaze is shifted. The longer the time of viewing, the higher the intensity, and the larger the area, the greater the effect will be in terms of its persistence in the succeeding viewing situation. As indicated by the work of Wright and Shouted, it appears that, at least for a first approximation, full adaptation takes place over a very brief time if the adapting source is moderately bright and the eye has been in relative darkness just previously. As the stimulus is allowed to act, however, the effect becomes more persistent in the sense that it takes the eye longer to regain its sensitivity to lower intensities. The net result is that, if the eye is so exposed and then the gaze is transferred to an area of lower intensity, the loss of sensitivity produced by the first area will still be present and appear as an "afterimage" superimposed on the second. The effect not only is present over the actual area causing the "local adaptation" but also spreads with decreasing strength to adjoining areas of the eye to produce "lateral adaptation." Also, because of the persistence of the effect if the eye is shifted around from one object to another, all of which are at similar brightness or have similar colors, the adaptation will tend to become uniform over the whole eye. If a person’s eye has been looking at an object in bright sunlight for some time, and then shifts to an object not well lit, we can expect ______.
现代人口再生产类型的特征是( )。
在市场经济条件下，社会的经济权利不再垄断在国家手中，而是被分解为经济主体的权利，每个商品生产的经营者都是独立的经济主体，都享有充分的自主权利。他们通过平等、自主的协商或讨价还价，共同决定他们之间的互利有偿、互相制约的关系。由此可见( )。
按民事权利是否已具备实现的现实可能性，可将其区分为( )
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