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other challenges to the Modern Synthesis were by no means accepted by all evolutionists, and, as in the early days of Darwinism, debates among rival views often became rancorous. In 1980, Gould wrote that “[T]he synthetic theory…is effectively dead, despite its persistence as textbook orthodoxy.” Going even further, Niles Eldredge and Ian Tattersall contended that the view of evolution due to the Modern Synthesis “is one of the greatest myths of twentieth-century biology.” On the other side, the eminent evolutionary biologists Ernst Mayr and Richard Dawkins strongly defended the tenets of the Synthesis. Mayr wrote, “I am of the opinion that nothing is seriously wrong with the achievements of the evolutionary synthesis and that it does not need to be replaced.” Dawkins wrote, “The theory of evolution by cumulative natural selection is the only theory we know of that is in principle capable of explaining the existence of organized complexity.” Many people still hold to this view, but, as I describe in chapter 18, the idea that gradual change via natural selection is the major, if not the only force in shaping life is coming under increasing skepticism as new technologies have allowed the field of genetics to explode with unexpected discoveries, profoundly changing how people think about evolution.

It must be said that although Gould, Eldredge, and others have challenged the tenets of the Modern Synthesis, they, like virtually all biologists, still strongly embrace the basic ideas of Darwinism: that evolution has occurred over the last four billion years of life and continues to occur; that all modern species have originated from a single ancestor; that natural selection has played an important role in evolution; and that there is no “intelligent” force directing evolution or the design of organisms.

CHAPTER 6

Genetics, Simplified

SOME OF THE CHALLENGES to the Modern Synthesis have found support in the last several decades in results coming from molecular biology, which have changed most biologists’ views of how evolution takes place.

In chapter 18, I describe some of these results and their impact on genetics and evolutionary theory. As background for this and other discussions throughout the book, I give here a brief review of the basics of genetics. If you are already familiar with this subject, this chapter can be skipped.

It has been known since the early 1800s that all living organisms are composed of tiny cells. In the later 1800s, it was discovered that the nucleus of every cell contains large, elongated molecules that were dubbed chromosomes (“colored bodies,” since they could be stained so easily in experiments), but their function was not known. It also was discovered that an individual cell reproduces itself by dividing into two identical cells, during which process (dubbed mitosis) the chromosomes make identical copies of themselves. Many cells in our bodies undergo mitosis every few hours or so—it is an integral process of growth, repair, and general maintenance of the body.

Meiosis, discovered about the same time, is the process in diploid organisms by which eggs and sperm are created. Diploid organisms, including most mammals and many other classes of organisms, are those in which chromosomes in all cells (except sperm and egg, or germ cells) are found in pairs (twenty-three pairs in humans). During meiosis, one diploid cell becomes four germ cells, each of which has half the number of chromosomes as the original cell. Each chromosome pair in the original cell is cut into parts, which recombine to form chromosomes for the four new germ cells. During fertilization, the chromosomes in two germ cells fuse together to create the correct number of chromosome pairs.

The result is that the genes on a child’s chromosome are a mixed-up version of its parents’ chromosomes. This is a major source of variation in organisms with sexual reproduction. In organisms with no sexual reproduction the child looks pretty identical to the parent.

All this is quite complicated, so it is no surprise that biologists took