Because one male is able to fertilize many females, the species will reproduce faster if a higher proportion of females is produced. So the obvious question is "Why not make fewer males?" The reason is one of the key principles of evolution.
Evolution acts most strongly at the level of the individual. The individual isn't concerned with the fate of the population's genes; it's concerned with the fate of its own genes.
An individual's genes are passed down through the generations in this way:
I First, the individual makes children. I Then these children make grandchildren.
The 50-50 sex ratio makes sense when you think about the grandchild generation in the following way. Each grandchild has exactly one mother and one father. When the sex ratio of the children is exactly 50-50, the two sexes have identical fitness. If the sex ratio of the children changes, the fitness of the two sexes is no longer balanced.
Imagine a mutation that changes the sex ratio of the children in favor of the production of females. Now more of the children are female than male. As a consequence, the smaller number of males will average more grandchildren than will the more numerous females. Due to the shortage of males, these males end up producing offspring with multiple females. By producing more children, the male offspring has greater fitness. Because of the bias in the sex ratio, they're in a better position to get their genes into the next generation. If there is a mutation that favors producing more males, it will increase until the sex ratio is again 50-50 and the two sexes have identical fitness.
Changing sexes: Sequential hermaphrodites
An interesting example of the trade-offs inherent in being different sexes is the special case of sequential hermaphrodites: species that began life as one sex but then changed into the other sex. In some species, these organisms start out as females and change to males; in other species, they start out as males and change to females. This lifestyle appears in several fish species.
In the case of female-to-male, these species have mating systems whereby males compete for access to females. Large males are more successful than small males and, perhaps as a result of this selection, have favored a system in which small fish avoid being males. All the fish are born females. They mature, mate with the larger males, and reproduce. When they become large enough to compete for mates as males, they simply change sex. Voilà! They grow male sex organs (female organs get reabsorbed) and begin battling to fertilize eggs.
¿¡tJABEfl Whenever the sex ratio deviates from 50-50, the rare sex has a fitness advantage, and selection responds by favoring individuals that produce more of the rare sex until the numbers balance out again. As a result, the sex ratio never strays very far from 50-50.
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