For hundreds of years, farmers have selected and bred plants and animals to favor, or brin

g out, characteristics they desired. For example, cows that produced large amounts of milk were selected for breeding, while poor milk producers were not allowed to reproduce. In like manner, horses were bred for speed and strength. Those having these desired characteristics were selected for breeding. Over time, these preferred breeds became more common than earlier, less desired types. This selective breeding is called artificial selection.

In this passage, Camp and Arms explain how this same process occurs naturally. The theory of evolution by natural selection was put forward in a joint presentation of the views of Charles Darwin and Alfred Russell Wallace before the Linnaean Society of London in 1858. Darwin and Wallace were not the first to suggest that evolution occurred; but their names are linked with the idea of evolution because they proposed the theory of natural se- lection as the mechanism by which evolution occurs. We are always more likely to believe in a process when people explain how it happens than if they merely assert that it does.

The theory of evolution by means of natural selection is based on three observations. First, as we can see by comparing one cat or human being with another, the members of a species differ from one another; that is, there is variation among individuals of the same species. Second, some (though not all) of the differences between individuals are inherited. (Other differences are not inherited, but are caused by different environments. For in- stance, two plants with identical genes may grow to different sizes if one of them is planted in poor soil. ) Third, more organisms are born than live to grow up and reproduced many organisms die as embryos or seeds, as saplings, nestlings, or larvae.

Inherited characteristics that improve an organism's chances of living and reproducing will be more common in the next generation and those that decrease its chances of reproducing will be less common. Various genes or combinations of genes will be naturally selected for or against, from one generation to the next, depending on how they affect reproductive potential. For natural selection to cause a change in a population from one generation to the next (that is, to cause evolution), it is not necessary that all genes affect survival and reproduction; the same result occurs if just some genes make an individual more likely to grow up and reproduce.

The main difference between natural and artificial selection is that human beings______.

A．control the direction of artificial selection

B．control the direction of natural selection

C．make new genes in artificial selection

D．make new genes in natural selection