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 lays eggs in a given larva, it would pay her to produce one male only, because this one male 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.
　　21. The author suggests that the work of Fisher and Hamilton was similar in that both scientists
　　(A) conducted their research at approximately the same time
　　(B) sought to manipulate the sex ratios of some of the animals they studied
　　(C) sought an explanation of why certain sex ratios exist and remain stable
　　(D) studied game theory, thereby providing important groundwork for the later development of strategy theory
　　(E) studied reproduction in the same animal species
　　22. It can be inferred from the passage that the author considers Fisher’s work to be
　　(A) fallacious and unprofessional
　　(B) definitive and thorough
　　(C) inaccurate but popular, compared with Hamilton’s work
　　(D) admirable, but not as up-to-date as Hamilton’s work
　　(E) accurate, but trivial compared with Hamilton’s work
　　23. The passage contains information that would answer which of the following questions about wasps?
　　I. How many eggs does the female wasp usually lay in a single host larva?
　　II. Can some species of wasp determine sex ratios among their offspring?
　　III. What is the approximate sex ratio among the offspring of parasitic wasps?
　　(A) I only
　　(B) II only
　　(C) III only
　　(D) I and II only
　　(E) II and III only
　　24. It can be inferred that the author discusses the genetic theory in greater detail than the group selection theory primarily because he believes that the genetic theory is more
　　(A) complicated
　　(B) accurate
　　(C) popular
　　(D) comprehensive
　　(E) accessible
　　25. According to the passage, successful game strategy depends on
　　(A) the ability to adjust one’s behavior in light of the behavior of others
　　(B) one’s awareness that there is safety in numbers
　　(C) the degree of stability one can create in one’s immediate environment
　　(D) the accuracy with which one can predict future events
　　(E) the success one achieves in conserving and storing one’s resources
　　26. It can be inferred from the passage that the mathematical theory of games has been
　　(A) developed by scientists with an interest in genetics
　　(B) adopted by Hamilton in his research
　　(C) helpful in explaining how genes can sometimes influence gametes
　　(D) based on animals studies conducted prior to 1930
　　(E) useful in explaining some biological phenomena
　　27. Which of the following is NOT true of the species of parasitic wasps discussed in the passage?
　　(A) Adult female wasps are capable of storing sperm.
　　(B) Female wasps lay their eggs in the larvae of other insects.
　　(C) The adult female wasp can be fertilized by a male that was hatched in the same larva as herself.
　　(D) So few male wasps are produced that extinction is almost certain.
　　(E) Male wasps do not emerge from their hosts until they reach sexual maturity.
　　C D B B A E D

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