obtain the exact sequence of all of the DNA nucleotides in chromosomes. The Human Genome Project, for example, will produce when completed the entire sequence of the 3 billion nucleotides that make up our genetic inheritance. The complete sequence of the yeast genome (12 million nucleotides) is already known, as are the genomes for numerous species of bacteria (from 0.5 to 5 million nucleotides each, depending on the species). Similar sequencing efforts will soon yield the complete sequences for hundreds of bacteria and other organisms with small genomes.

These molecular studies are powerful evidence for evolution. The exact order of the genes on our chromosomes can be used to predict the order on monkey or even mouse chromosomes, since long stretches of the chromosomes of mammalian species are so similar. Even the parts of our DNA that do not code for proteins and at this point have no known function are similar to the comparable parts of DNA in related organisms.

The confirmation of Darwin's ideas about "descent with modification" by this recent molecular evidence has been one of the most exciting developments in biology in this century. In fact, as the chromosomes of more and more organisms are sequenced over the next few decades, these data will be used to reconstruct much of the missing history of life on earth—thereby compensating for many of the gaps that still remain in the fossil record.

Conclusion

One goal of science is to understand nature. "Understanding" in science means relating one natural phenomenon to another and recognizing the causes and effects of phenomena. Thus, scientists develop explanations for the changing of the seasons, the movements of heavenly bodies, the structure of matter, the shaping of mountains and valleys, the changes in the positions of continents over time, and the diversity of living things.

The statements of science must invoke only natural things and processes. The statements of science are those that emerge from the application of human intelligence to data obtained from observation and experiment. These fundamental characteristics of science have demonstrated remarkable power in allowing us to describe the natural world accurately and to identify the underlying causes of natural phenomena. This understanding has great practical value, in part because it allows us to better predict future events that rely on natural processes.

Progress in science consists of the development of better explanations for the causes of natural phenomena. Scientists can never be sure that a given explanation is complete and final. Yet many scientific explanations have been so thoroughly tested and confirmed that they are held with great confidence.

The theory of evolution is one of these explanations. An enormous amount of scientific investigation has converted what was initially a hypothesis into a theory that is no longer questioned in science. At the same time, evolution remains an extremely active field of research, with an abundance of new discoveries that are continually increasing our understanding of exactly how the evolution of living organisms actually occurred.



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